Tool Engg Notes By Dvshirbhate

by a great deal of friction & heat generation is governed by definite laws. Metal cutting operation involves three basic requirements. (1) There must be a cutting tool that is harder and wear resistant than the work piece material, (2) there must be interference between the tool & the work piece as designated by the feed and depth of cut, and (3) There must be relative motion or cutting velocity between the tool & the work piece with sufficient force and power to overcome the resistance of work piece material. As long as above three conditions exist, the portion of the material being machined that interferes with free passage of the tool will be displaced to create a chip. 1.2 Classification of production process: The metals are given different usable forms by various processes. processes may be classified as under.

These

Metal Forming Chip-forming Process (Metal Cutting) Continuous-contact Cutting

Single-edge Cutting (Turning, Shaping, Boring)

Double edged cutting (Drilling)

Chip-less Process

Intermittent cutting

Continuous (Rolling, Spinning Etc.)

Sizable Swarf (Milling)

Ground Chips (Honing, Grinding, etc.)

Impact or Intermittent Contact (Forgoing, Drop-stamping)

In chip removal processes the desired shape and dimensions are obtained by separating a layer from the parent work piece in the form of chips. During the process of metal cutting there is a relative motion between the work piece & cutting tool. Such a relative motion is produced by a combination of rotary and translatory movements either of the work piece or of cutting tool or of both. These relative motions depend upon the type of metal cutting operation. The following table indicates the nature of relative motion for various cutting processes.

Center less) In chipless processes the metal is given the desired shape without removing any material from the parent work piece. 1.3 Basic elements of cutting tools: The cutting tool consists of three basic elements (1) cutting element or Principle element – This is the element, which is actually fed into the material of work piece to cut the chips ex. In drilling lips (or cutting edges) are cutting elements. (2) Sizing element – The part, which serves to make up any deficiencies of cutting element after sharpening, is sizing element. It imparts final shape to the machined surface and also provides guidance in tool operation ex. In drill sizing element; (flute portion) immediately follows the lips). (3) Mounting element – It serves for securing the tool in machine or holding it in hand of worker ex. In the twist drill the shank is mounting element. The cutting & sizing element taken together is referred as working element of the tool. 1.4 Machining parameters: 1.4.1 Cutting Speed (V) – It is the travel of a point on cutting edge relative to surface of cut in unit time in process of accomplishing the primary cutting motion. It is denoted by ‘V’. The unit of cutting speed is m/min. In lathe work for turning a blank of diameter ‘D’ mm, (The diameter of machined surface is ‘Do’ mm.) rotating at a speed ‘N’ (rpm) the cutting speed at periphery (maximum) is given by. V = π D N /1000, m/min ........………………………….. 1.41 Fig. 1.1 Elements of cutting process in turning SPEED

FEED

DEPTH OF CUT

Fig. 1.2 Sketches Showing V, f and d

From this formula it is easy to find rotational speed N = 1000 V / Π D ................... 1.42 From figure 1.1. it is evident that the cutting speed varies along the cutting edge from maximum at point ‘m’ to minimum at point ‘K’ though the rotational speed is same. In drilling a work piece with a drill of diameter ‘D’ mm., rotating at a speed ‘N’ (rpm) the cutting speed will vary from zero at center to maximum at periphery given by eqn 1.41.

V=

πDN , m/min 1000

Similarly in facing the cutting speed varies from zero at center to maximum at periphery. 1.4.2 Feed (Feed rate) (f, fm) It is the travel of the cutting edge in the direction of feed motion relative to the machined surface in unit time. The feed may be expressed as distance traveled by the tool in one minute (fm) or distance traveled by the tool in one revolution (f). The terms ‘f’ and fm are related by . . . . . . . . . 1.43 f = fm / N, mm/rev In lathe work, distinction is made between longitudinal feed, when tool travels in a direction parallel to work axis, cross feed when tool travels in a direction perpendicular to the work axis, and angular feed when tool travels at an angle to work axis (for example, in turning tapered surface.) 1.4.3 Depth of cut: (d) It is the thickness of the layer of metal removed in one cut or pass; measured in direction perpendicular to machined surface. The depth of cut is always perpendicular to the direction of feed motion and, in external longitudinal turning; it is half the difference between the work diameter and the diameter of machined surface obtained after one pass. d = (D – Do)/2 mm ............ 1.4.4

To reduce machining cost machining time should be less i.e. the metal removal rate should be high. To achieve this following facts should be considered. 1) Proper cutting tool material should be selected. 2) Correct tool (angle) geometry should be produced or ground on tool 3) The tool should be rigidly held to avoid vibrations. 4) Depending on the rigidity of machine – tool system maximum values of speed & feed should be selected. A process, which removes metal at a faster rate, may not be the most economical process, since the power consumed & cost factors must be taken into account. Due to this, to compare two processes, the amount of metal removed per unit of power consumed in unit time is determined. This is called “ Specific metal removal rate” and is expressed as, mm3/w/min, if the power is measured in watts. 1.5 Basic shape of cutting tools: Wedge. Almost all cutting tools used in metal cutting operations consist of basic form of a wedge, which is defined as one form of inclined plane in shape of a triangular prism. Assume that a wedge under the action of force P is penetrating into another body at a constant speed as shown in Fig.1.3. N β M

K

β

N

N

N

P P L

Fig. 1.2 Force acting on an indenting wedge

Fig. 1.4 Force triangle at the wedge

check

in fig.1.3. The body resists the motion of the wedge. The reaction N.N. appear at the cheeks of the wedge. The forces N.N. are perpendicular to the cheeks in absence of friction. From the equilibrium of forces (fig.1.4) N KM 1 1 = = = P KL  KL / 2  2 sin β 2  2  KM 

Thus, the mechanical advantage in force is dependent on the wedge angle B . The smaller the angle of wedge, the greater will be the gain in force. In other words, the wedge angle 'β' determines the resisting force of the cutting edge. The cutting edge must be oriented at certain required angles with the work surface depending on nature of operation to be performed. Fig.1.5 shows that the wedge must be set at right angles to the work surface, so that the driving force "P" is in the direction of parting. Fig.1.6 shows during chipping the wedge must be set at an angle inclined to work surface so that separation of chip can be done. Thus for the wedge two geometric parameters can be defined i.e. (1) The wedge angle 'β' and (2) the axis of symmetry along which 'P' acts. In addition to above, two more parameters are introduced to confirm conditions of chipping action. These parameters are set with respect to velocity Vector, 'V' and are defined as (3) cutting angle 'δ' and (4) clearance angle;, as shown in fig.1.7. The sign convention for describing these angles are set wr.t. left handed cork screw rule with "Z" axis coinciding with the direction of the velocity vector, V, and the cutting edge lying along 'Y' axis. Hence, 'δ' & 'α' are measured positive, when moving from 'Z' to 'X' axis as shown in fig.1.7. The parameter ' γ ' defines the indination of the top face of the wedge (called Rake face) w.r.t. velocity vector V, while the parameter 'α' describes the relief provided from the bottom face of the wedge (called flank), often another derived parameter, called (5) Rake angle 'γ', is used to describe the indination of the top face of the wedge. This is derived parameter given by γ = 900 - δ . However if δ > 90 , then ' γ ' is negative. Thus from this equation it may be seen that while 'δ' is always positive the rake angle can become positive or negative depending an value of angle 'δ'. v

δ

β

δ

β

β < 90 0

β > 90 0

1.6 Types of metal cutting processes: The metal cutting processes are classified in to two types, on the basis of angular relationship between cutting velocity vector V, & the cutting edge of the tool. (1) Orthogonal cutting process (two dimensional cutting) (2) Oblique cutting process (three dimensional cutting) In orthogonal cutting the cutting edge of the tool is perpendicular to cutting speed direction. In oblique cutting, the angle between the cutting edge & cutting velocity vector is different from 900. fig 1.9 & fig.1.10

Fig1.9

Point 1. Definition -

Orthogonal Cutting The cutting edge of tool perpendicular to cutting speed V; 2. Alternative name Two dimensional cutting 3. Volume of metal Less metal removal due removal for a cutting to square cutting condition. condition. 4. Tool life Shorter 5. Friction & More Chatter

Fig1.10

Oblique Cutting The Cutting edge of the tool is inclined at an angle other than 900 to V. Three dimensional cutting More metal removal, as greater area of chip is removal for same depth of cut & other conditions. Longer Less, as small amount of heat developed due to friction at the

material, 4) The above relative values are affected by changes in cutting, conditions & in properties of the material to be machined to give chip that range from small lumps to long continuous ribbons. These observations indicates that the process of chip formation is one of deformation or plastic flow of the material with the degree of deformation dictating the type of chip that will be produced. Fig. 1.11 shows progressive formation of a chip using a wedge shaped (single point) tool. At “a” tool contacts the work piece material. At “b” compression of material takes place at point of contact. At “c” the cutting force overcomes the resistance of penetration of tool is begins to deform by plastic flow. As the cutting force increase, either a rupture or plastic flow in direction generally perpendicular to face of the tool occurs & the chip is formed as shown at “d”. tool

tool

a

b

Fig. 1.11 Progressive formation of a metal chip.

Fig1.12

tool

c

tool

d

whole chip slides up the tool face. The distorted layers now by means of phenomenon of slip & the layers are called slip planes. The number of slip planes depends upon the lattice structure of parent workplace material. The distortion of layers tends to strengthen them (work hardening or strain hardening) & therefore the hardness of chip is much greater than the hardness of the parent material. Thus in simple language the mechanism of chip formation in any machining operation is a rapid series of plastic flow & slip movements ahead of the cutting edge. The degree of plastic flow ahead of the cutting tool determines the type of chip that will be produced. If the w/p material is brittle & has little capacity for deformation before fracture the chip will separate along the shear plane to form what is known as a discontinuous segmental chip. Material that are more ductile & have capacity for plastic flow will deform along the shear plane without rupture. The planes tend to slip & weld to successive shear planes, & the result is a chip that flows in a continuous ribbon along the face of tool. This is known as a continuous chip & is usually much harder than the parent material because of its strain hardened conditions. 1.8. Types of Chips: The tool engineer's handbook lists four different types of chips viz. 1) Segmental chips or Discontinuous chips 2) Continuous chips 3) Continuous chip with BUE or BUE chips. 4) Inhomogeneous chips. 1) Discontinuous Chips: These chips are in the form of small individual segments, which may adhere loosely to each other to form a loose chip. These chips are formed as result of machining of a brittle material such as gray cast iron or brass castings, etc. These chips are produced by actual rupture or fracture of metal ahead of the tool in brittle manner. Since the chips break up into small segments and also shorter chips have no interference with work surface. The friction between chip & tool reduces resulting in better surface finish. These chips are convenient to collect, handle & dispose of during production runs. The conditions favorable for formation of discontinuous chips are: 1) Brittle & non ductile metals (like cast iron brass castings Beryllium, titanium etc.) 2) Low cutting speed. 3) Small rake angle of the tool. 4) Large chip thickness. 2) Continuous Chips: These chips are in the form of long coils having uniform thickness throughout. These chips are formed as result of machining of relatively

4) Small chip thickness. 5) Sharp cutting edge. 6) Efficient cutting fluid. 7) Low friction between chip tool interfaces. 3) BUE Chip (or continuous Chip with BUE): These chips are also produced in the form of long coils like continuous chips, but they are not as smooth as continuous chips. These chips are characterized by formation of built up edge on the nose of the tool owing to welding of chip material on to tool face because of high friction between chip tool interfaces. Presence of this welded material further increases the friction leading to building up of the edge, layer by layer. As the built-up edge continuous to grow, the chip flow breaks a portion of it into fragments. Some of them are deposited on the work piece material while the rest are carried away by the chips. The hardness of this BUE is two to three times higher than the work piece material. This is the reason why the cutting edge remains active even when it is covered with built-up edge. The only point in favor of BUE is that it protects the cutting edge from wear due to moving chips and the action of heat. This brings about an increase in tool life. These chips normally occur while cutting ductile materials with HSS tools with low cutting speeds. Chips with BUE are under desirable as they result in higher power consumption poor surface finish and higher tool wear. Generally speaking any change in cutting conditions that will eliminate or reduce BUE is desirable, since high friction between chip & tool face is major cause of BUE. Any means of reduction of friction such as use of lubricant & adhesion preventing agent is often effective to reduce BUE, especially when it is necessary to operate at low cutting speeds. Tool material with inherent low coefficient of friction or a high polish on tool face can also reduce friction & hence BUE. The conditions favorable for BUE chip are. 1) Ductile material 2) Low cutting speed. 3) Small rake angle of tool. 4) Dull cutting edge. 5) Coarse feed. 6) Insufficient cutting fluid. 7) High friction at chip tool interface. 4) Inhomogeneous Chip: These chips are produced owing to non uniform strain set up in material during chip formation and they are characterized by notches on the free side of chip, while the side adjoining the tool face is smooth. The shear deformation which occurs during chip formation causes temperatures on shear plane to rise which in turn may decrease the strength of material & cause further strain if the material is poor

Table 1.1. : Factors responsible for the formation of different types of chips. Factors Types of chips Discontinuous Continuous With BUE Inhomogeneous 1. Material Brittle Ductile Ductile Which Shows decreased in Yield Strength with temp. & Thermal conductivity medium. 2. Cutting speed Low High Low 3. Tool Small rake Large rake Small geometry 4. Friction Lower Higher 5. Chip Large Small Small thickness 6. Cutting fluid Efficient Poor 7. Feed Coarse 8. Cutting edge Sharp Blunt -

Thus, Cutting ratio, r = t/tc Where t = undeformed chip thickness (i.e. before cutting) and tc = mean thickness of chip ( i.e., after cutting ) Chip reduction coefficient K = 1/r The following methods can be used to determine cutting ratio 1) The cutting ratio "r" can be obtained by direct measurement of "t" & "tc". However since underside of chip is rough the correct value of "tc" is difficult to obtain and hence tc can be calculated by measuring length of chip (1c) and weight of piece of chip "W". tc = W/ (bc .1c. ρ ) Where, bc = length of chip 1c = width of chip ρ = Density of material assumed to be unchanged during chip formation. 2) Alternatively, the length of chip (1c) & length of work (l) can be determined. The length of work can be determined by using a work piece with slot, which will break the chip for each revolution of work piece. The length of chip can be measured by string. It can be shown that r = 1/1c as under. When metal is cut there is no change in volume of metal cut. Hence volume of chip before cutting is equal to volume of chip after cutting i.e. 1.b.t. = 1c.b.tc or l.t. = 1c.tc (assuming b = bc) l/lc = t/tc = r 3) Cutting ratio can also be determined by finding chip velocity (Vc) and cutting speed (V). The chip velocity (Vc) can be accurately determined by determining length of chip with a string for a particular cutting time measured with the help of a stopwatch. It can be shown that r = Vc/V, as under. From the continuity equation, we know that volume of metal flowing per unit time before cutting is equal to volume of metal flowing per unit time after cutting. i.e. V.b.t. = Vc .b.tc or Vc/V = t/tc = r (assuming b = bc) 1.10 Shear Angle: The shear angle is the angle made by shear plane with the direction of tool travel. In fig 1.7a it is the angle made by the line AB with direction of tool travel. The value of this angle depends on cutting conditions, tool geometry, tool material & work material. If the shear angle is small, the plane of shear is larger, the chip is thicker and therefore higher fore is required to remove the chip. On the other hand, if the angle is

where γ = rake angle the derivation of the above equation is as follows. from fig 1.7 a

t1 = AB sin φ t2 = AB sin cos (φ - γ ) t2 1 cos (φ − γ ) = = t 1 rc sin φ =

cos φ cos γ + sin φ sin γ sin φ

1 = cot φ cos γ + sin γ rc 1   − sin γ  r  1 − r sin γ  c  c cot φ = = cos γ rc cos γ tan φ =

rc cos γ 1 − rc sin γ

1.11 Velocity relationships in orthogonal cutting There are three velocities in orthogonal cutting process, namely (i) Velocity of chip (Vf) which is defined as the velocity with which the chip moves over the rake face of the cutting tool. (ii) Velocity of shear (Vs) is the velocity with which the work piece metal shears along the shear plane.

sin φ cos (φ − α) cos α Vs = Vc, cos (φ − α) Vf = Vc ,

where α is the rake angle, φ is the shear angle. From the principle of kinematics, the relative velocity of two bodies (tool and chip) is equal to the vector difference between their velocities relative to the reference body (here the work piece). The vectors of these three velocities - Vc, Vs and Vf should form a close velocity diagram (Fig.30.15) and Thus Vc = Vs + Vf Refer Fig. 30.15(b) From right-angled ∆ ACE AC = sin φ or AC = AE. sin φ = Vc . sin φ AE

From right -angled ∆ ABC

AC = cos (φ − α) or AC = AB, cos (φ − α) = Vf , cos (φ − α) AB

From Eqs. (a) and (b) or

Vf . cos (φ − α) = Vc . sin φ sin φ Vf = Vc . cos (φ − α)

Consider ∆ ADE DE = cos α or DE = Vc . cos α AE

Consider ∆ BDE

DE = cos (φ − α) BE

rake angle "γ" by the following equation: ε=

∆s ∆x cot φ + ∆x tan(φ − γ ) = ∆x ∆x

= Cot φ + tan (φ - γ)

or

ε=

cos γ sin φ cos(φ − γ )

This relation can be obtained from the pack of inclined cards model suggested by Prof. Pushpanen. In this model the formation of chip and its motion along the tool face can be visualized from an idealized model in which a stack of inclined (playing) cards is pushed against the tool (fig.1.16 a). As the tool advances, segments, which had been part of the work place, become part of the chip. From this figure it can be seen that card closest to the tool point slips to a finite distance relative to the uncut material as tool point slips to a finite distance relative to the uncut material as tool advances. When the tool point reaches the next card, the previously lipped card moves up along the tool face as a part of the chip.

BA = BE + AE BA = ∆x cot φ +∆x cot {90 - (φ -γ)}

ε=

BA = cot φ + cot (90 − (φ − γ ) CE = cot φ + Tan (φ − γ )

)

t= f sin φp

b=

sinφ p

p

φp

φp=900

can be evaluated as R = (Px + Py + Pz )1/2 =

PX 2 + PY 2 + PZ 2

........ 1.14.1

This three-dimensional force system can be reduced to a two-dimensional force system if in orthogonal plane π 0 the forces are considered in such a way that the entire force system is contained in the considered state, when R= Pxy =

Pz2 + Px2 y

..... . . . 1.14.2

Px2 + Py2

..... . . . 1.14.3

This is possible only when Pxy is contained in plane π0 which is possible only under conditions of free orthogonal cutting. This corresponds to 'orthogonal system of first kind' for which conditions are: i) 0<φ< 90 ii) λ=0 iii) The chip flow direction lies on the plane π0. Fig. 4.10 shows the cutting forces for the case of orthogonal system of the first kind. An orthogonal two-dimensional system of second kind can be obtained by choosing λ and φ in such a manner that either Px or Py can be made zero. For the orthogonal system of second kind either i) "Py" is made zero by having λ = 0 and ii) φ = 90 when two dimensional force system is ... . . . . 1.14.3 R = Pz2 + Px2 Fig. 4.11 shows the disposition of cutting forces in plane orthogonal turning with λ = 0 and φ = 90.

However out of all the above cases shown in fig 4.10 4.11 and 4.12 the cutting in the first two cases is "non free" or 'restricted" type where the auxiliary cutting edge is also active in causing deviation of chip flow direction from the orthogonal plane. The contribution of auxiliary cutting edge is to deviate Pxy from the orthogonal plane. This deviation is small & neglected if the depth of cut is very large compared to feed, such process is called "Restricted Orthogonal cutting. However during cutting of a thin pipe with a cutting edge whose length is considered to be very large compared to the width of cut, a "pure" orthogonal cut of first or second kind could be obtained. The principal schemes of metal cutting shall be based on pure orthogonal cutting from which schemes for oblique or other continuous and intermittent cutting processes like drilling, milling, etc., can be derived by similarly principles.

FIG1.22 FORCES IN METAL CUTTING

1) The chip behaves as a free body in stable equilibrium under the action of two equal, opposite and collinear resultant forces viz. R & R. 2) The tool edge is sharp. 3) The work material suffers deformation across a thin shear plane. 4) This is no side spread (or the deformation is two-dimensional). 5) There is uniform distribution of normal & shear forces on the shear plane & 6) The work material is rigid, perfectly plastic (or behaves like ideal plastic) 7) As, (shear plane area). Ts (shear stress) & "B" (Friction angle), are constant & are independent of shear angle 'φ' Forces on the chip (Merchant’s Analysis, theory) From the concept of chip formation and measuring force Ft and Ff with a cutting tool dynamometer, Merchant was able to build up a picture of forces acting in the region of cutting which give rise to plastic deformation and sliding of the chip down the tool rake face. See fig 30.16(a) The forces exerted by the work piece on the chip are Fc - Compressive force on the shear plane. Fs - Shear force on the shear plane. The force exerted by the tool on the chips are N - Normal force at the rake face of tool. F - Frictional force along the rake face of tool. The forces acting on the tool and measured by dynamometer are Ft - tangential or cutting force Ff - feed force Angle α is tool rake angle, φ is shear plane angle and β is the angle of friction

(a) Graphical Treatment Using the concept explained in fig. 30.16(a) it is now possible to find graphically the magnitude of force Fc , Fs, N and F. The vector diagram of forces is constructed as follows [fig 30.16(b)] Draw Ff and Ft to some convenient scale and joint AB to obtain their resultant. Bisect AB and draw a circle having the resultant force as its diameter. Set off BE, making angle φ with force Ft, to cut circle at E. Join EA. The magnitudes of Fs and Fc are now known. Set off a line BG at an angle (90-α) with Ft (Ft is vertical and BB' is horizontal, ∠DBB' is 900) Join GA. The magnitude of force N and F are thus known, as also the coefficient of friction at the chip tool interface (F/N). Angle BAG is the angle of friction between chip and tool. Tan β = F/N (b) Analytical Treatment [See fig 30.16 (b)] F = GH + HB = AI + HB Or F = Ff. cos α + Ft. sin α N = AG = DH - DI = Ft.cos α - Ft. sin α Now

………….30.17 …………..30.18

F Ff cos α + Ft sin α = N Ft cos α − Ff sin α

Dividing R.H.S. by cos α F Ff + Ft tan α = N Ft − Ff tan α

…………..30.19

s

t

f

Fc = AO + OE Fc = Ff. cos φ + Ft. sin φ In ∆ AGB, ∠GBA = 180 – 90 - β = 90 - β Hence ∠ABD = 90 – α - (90 - β) = 90 – α - 90 + β = β - α Now Ft = BD From ∆ ABD

BD (or Ft) = cos (β − α ) AB (or R)

Thus

Ft = R. cos (β - α) Ft = R. cos (β - α)

Also from ∆ ABE

………… 30.24 ………… 30.25

Fs = cos (φ + β − α ) R

Now from Eqs. (30.24) and 30.26) Ft R cos(β − α) = Fs R cos(φ + β − α)

or

Ft = Fs.

cos(β − α) cos(φ + β − α)

………….30.27

Assumption Made The above derived relations (eqs) are based upon the following assumptions 1. The tool is perfectly sharp and it does not make any flank contact with the job. 2. The cutting velocity remains constant. 3. A continuous chip with no built up edge is produced 4. The chip does not flow to either side 5. Chip shears continuously across the shear plane as the shear stress reaches the value of shear flow stress. The earlier discussed theoretical analysis of mechanics of metal is of use only if the value of the shear angle φ is known. Machining is an unconstrained process and the shear angle (or chip thickness) has no obvious value as, for example, has the exit thickness in rolling. Apart from the time involved in determining the magnitude of the shear angle experimentally, the understanding of the machining process is clearly incomplete if one cannot formulate a satisfactory criterion for the orientation of the shear plane. 1. Theory of Ernst and Merchant According to this hypothesis, the shear plane orientates itself so that (a) the work done in cutting is a minimum, or (b) the maximum shear stress occurs on the shear plane.

F1 = R . cos (β -α)

(30.37)

Now from equation (30.36) and (30.37) F1 =

τ s A1 cos (β − α ) × sin φ cos (φ + β − α)

(30.38)

Eq. (30.38) may be differentiated w.r.t. φ and equated to zero to find the value of shear angle, φ for which F1 is a minimum.  cos φ. cos (φ + β − α) − sin (φ + β − α )  d F1 = − τ s A 1 cos (β − α).  = zero (0). dφ sin 2 φ. cos 2 (φ + β − α )  

or or

cos φ .cos (φ +β - α ) – sin (φ + β - α ) = 0 cos ( φ + φ + β - α ) = 0 cos (2 φ + β + α ) = 0 2φ+β-α=

or

π 2

π β α π 1 − + = − (β − α) 4 2 2 4 2 π 1 ∴ Shear angle, φ = − (β − α) 4 2

(30.39)

φ=

(30.40)

-Merchant found that the above theory agreed well with experimental results obtained when cutting synthetic plastics but agreed poorly with experimental results obtained for steel machined with a sintered carbide tool. -It should be noted that in differentiating equation (30.38) with respect to φ, it was assumed that A1, α and ι should be independent of φ. On reconsidering these assumptions, Merchant decided to include in a new theory the relationship. τ s = τ so + k σ s (30.41) 1.16 Power and energy Relationship: The power or the total energy per unit time or the rate of energy consumption is the product of cutting speed "V" and cutting force Fc i.e. E = Fc x V, K.g. mm/min. The energy consumed during cutting process is primarily utilized at the shear plane, where plastic deformation takes place and at chip tool interface where friction resists the flow of chip. The total energy per unit time (E) is approximately equal to the sum of shear energy (Es), Friction energy (Ef) and negligible amount of energy required

Similarly specific shear energy (es) & specific friction energy (ef) can be defined by the following relations. eS = ES/b.t.v. = FS . VS/b.t.v. = FS . cosν/b.t.cos (φ-ν), Kg/mm2 and ef = Ef/b.t.v. = FS . VS/b.t.v. = F/b.tc ,Kg/mm2 SOLVED PROBLEMS : Example : 1) In an orthogonal cutting operation, following date have been observed : Uncut chip thickness, t = 0.125 mm. chip thickness, tc = 0.250 mm Width of cut, b = 6,500 mm. V = 100 m/min. Rake angle, ν = 100 Cutting force, Fz = 70 Kg. Trust force, Ft = 25 kg. Determine : Shear angle, the friction angle, shear and normal stress on shear plane, shear strain, shear strain rate, cutting power, specific shear energy, friction energy, cutting energy. Solution : (i) Shear angle : φ

(ii) Friction angle

(iii) Shear and normal stress on shear plane

Thus, shear rate = 103.75/0.026 = 3938.755-1 (vi) Cutting power E = Fe V/4500 = 70.000/4500 = 1.55 H.P. vii) Specific shear energy E's viii) Specific cutting energy 'e' = Fc x c/b.t.v. (70)/(6x5x0.125) = 86.15 kg/mm2 ix) Specific friction energy = E - es = 86.15 - 63.65 = 22.5 kg/mm2 Example 2 : During machining of a C-30 steel with 0-10-6-7-8-80-0.5 mm (ORS) shaped tungsten carbide tool, the following observations, have been made, depth of cut, d = 2 mm, feed f = 0.2 mm/rev. speed V = 200 m/min. chip thickness tc 0.40 mm. Calculate shear angle width of chip Solution : d = 2mm, 0p = 800, tc = 0.40, V = 10 Now, thickness of uncut chip t = f.sin 0P = 0.197 mm Chip thickness ratio, r = t/tc = 0.49 Shear angle, 0 = tan-1 (r.cos v/(1-r sin v) = 23.820 Width of Chip, b = d/sin 0P = 2/Sin 80 = 2.03 mm Example 3. During machining of C-20, steel with a triple carbide cutting tool 0-8-710-70-1mm(ORS) shape the following data was obtained. Feed = 0.18 mm/rev., Depth of cut = 2.0 mm. Cutting speed = 120 mpm, Chip thickness = 0.4 mm. Determine chip reduction coefficient & shear angle. Solution : = 8, = 70 Uncut chip. thickness = sin = 0.18 sin 70 = 0.169 mm. chip reduction coefficient k = = 0.42 Now = tan-1 ( r.cos / ( 1 - r sin ), = 23.980 Example 4 : In orthogonal turning process the feed is 0.25 mm/rev. at 50 rpm. The thickness of chip removed is 0.5 mm. (a) What is the cip thickness ratio ? (b) If the wok diameter is 50 mm before the cut is taken what is the approximate length of chip removed in the minute. Assume a continuous chip is produced in process. Solution : Uncut chip thickness, t = f = 0.25 mm & ctc = 0.5 mm

8. What is cutting ratio (or chip thickness ratio) and chip compression factor (or chip reduction coefficient) ? 9. What are the various methods of estimating cutting ratio ? 10.What is shear angle ? How it can be measured ? 11. Prove that tan φ = r cos ν/(1 - r sin ν) Where, φ = Shear angle. r = Cutting ratio. ν = Rake angle. 12. Prove that Vc = Vsin φ/cos (φ - ν ) and Vs = Vcosν/cos (φ - ν ) Where V, Vc, Vs are cutting, chip & shear velocities respt. "φ" is shear angle & ν is rake angle. 13. Prove that shear strain "∈" in orthogonal cutting is given by ∈= tan (φ - ν ) + cosφ, where φ is the shear angle and ν is the rake angle. 14. How is the thickness & width of undeformed chip estimated in turning operation 15. What is metal removal rate ? What is specific metal removal rate ? How can "MRR" be increased ? 16. What is meant by the orthogonal cutting system of first & second kind ? Illustrate with neat sketchs. 17. What are the components of resultant force in an oblique cutting operation ? 18. What are the assumptions of Merchant's theory ? 19 Prove that φ = π/4 + ν/2 - β/2 where φ is shear angle, ν is rake angle & β is friction angle. 20. What is the modified Merchant's theory of metal cutting ? 21. What is meant by power consumed in metal cutting ? What are it's various components ? 22. What is total specific energy, specific shear energy, and specific friction energy 23. How can the resultant force in orthogonal cutting be estimated by graphical method ? 24. What is metal cutting ? What are the basic requirements for metal cutting ? How are the metal cutting processes classified. 25. What is the effect of setting up of the cutting edge on rake & clearance angle ? 26. In an orthogonal cutting operation the following data is obtained. 1) Cutting force = 180 Kg. 2) Feed force = 100 Kg. 3) Chip thickness ratio = 0.32 Kg. Find graphically or otherwise shear force on shear plane, normal force on shear plane, Frictional force, Normal force, and resultant Force.

Determine : shear angle, friction angle, shear stress along shear plane, chip velocity, shear strain in chip, cutting power and specific cutting power. 29. A tool making an orthogonal cut has a rake angle of - 100. The feed is 0.10 mm, the width of cut 6.5 mm. the speed 160 mpm, and a dynamometer measures the cutting force to be 180 kg and normal thrust force to be 140 kg. A high speed photograph shows a shear angle of 200. Estimate, (a) Chip thickness (b) coefficient of friction. (c) Shear and normal stress on shear plane (d) shearing strain, (e) H.P. to shear the metal (f) H.P. lost in friction. -000-

either replaced or reground resulting in loss of production due to machine down time. Thus, study of tool wear is important from standpoint of satisfactory performance & economics. However it is very difficult to find out exact cause and nature of tool wear, the phenomenon being very complex & dependent on many aspects, viz, tool work pair, environment, temperature of interfaces etc. 2.2.

Wear Mechanism or Causes:

Tool wear causes the tool to lose its original shape. So that in time the tool ceases to cut efficiently or even fails completely. After a certain degree of wear, the tool has to be resharpened for further use. The following basic causes, which can operate singly or in various combinations, produce tool wear. 2.2.1 Attrition Wear (Adhesive Wear) : At low cutting speeds the flow of material past the cutting edge is irregular or less laminar & contact between the two becomes less continuous due to built up edge formation. Under such condition fragments of tool are torn intermittently from the tool surface. This phenomenon is called Attrition or Adhesion. This wear progresses, slowly in continuous cutting, but rapidly in interrupted cutting or in cutting where vibrations are severe due to lack of rigidity. As the cutting speed is increased, the flow of metal becomes uniform & attrition disappears. Such wear is prominent in cutting with carbides at low cutting speeds where B.U.E. is likely to form. 2.2.2. Diffusion Wear: Diffusion wear occurs because of the diffusion of metal and carbon atoms from tool surface into work material & chips. It is due to high temperature and pressures developed at the contact surfaces in metal cutting & rapid flow of chip on tool surfaces. The rate of diffusion wear depends upon the metallurgical relationship between the tool & work material. It is one of the major causes of wear and is of special significance in the case of carbide tools diffusion is a phenomenon strongly dependent upon temperature. For example diffusion rate is approximately doubled for an increment of the order of 200 C. in the case of machining steels with HSS tools. 2.2.3. Abrasive Wear: The abrasive action of the work on tool is basically due to two principal effects.

tool reducing the relief or clearance angle. This is a deformation rather than a wear process but it accelerates other wear processes, which reduce life of the tool. The deformation leads to sudden failure of the tool by fracture or localized heating. The occurrence of plastic deformation is in itself an indication of the overstressing of the tool material. 2.2.5 Fatigue Wear: In tension

In compression In compression Stress distribution around the interlocking asperities

In tension Fatigue wear

When two surfaces slide in contact with each other under pressure, asperities on one surface interlock with those on other. Due to the frictional stresses compressive stress is produced on one side of each interlocking asperity and tensile stress on the other side. After given pair of asperities have moved over or through each other, the above stresses are relieved. New pair of asperities is, however, soon formed and the stress cycle is repeated. Thus the material of the hard metal near the surface undergoes cyclic stress. This phenomenon causes surface cracks, which ultimately combine with one another & lead to crumbing of the hard metal. Further the hard metal may also be subjected to variable thermal stress owing to temperature changes brought about by cutting fluid, chip breakage & variable dimensions of cut, again contributing to fatigue wear. 2.2.6. Electrochemical Effect: This type of wear may occur when ions are passed between the tool & work piece-causing (an oxidation of the tool surface & consequent) breakdown of the tool material in the region of the chip-tool interface. It has been argued that since sufficiently high temperatures exist on the chip tool interface, a thermoelectric E.M.F. is set up in the closed circuit due to the formation of a hot junction at chip tool interface between dissimilar tool & work materials. This current may assist the wear process at

2.3.

Types (Geometry) of Tool Wear:

The progressive wear of cutting tools can take two forms: i) Wear on the tool flank characterized by the formation of wear land as result of the newly cut surface rubbing against tool flank; and ii) Tool wear on rake face characterized by the formation of a crater or a depression, as a result of chip flowing over the tool rake face. 2.3.1. Flank Wear: These wear produces wear lands on the side & end flanks of the tool on account of the rubbing action. In the beginning, the tool is sharp & the wear land on the flank has zero width. However very soon, the wear land develops & grows in size on account of abrasion adhesion & shear. A typical case of flank wear development is shown in fig. 2.3.a. This figure can be divided into their definite regions A, B & C. In the region A, the wear grows rapidly within a short period of time because during the initial contact of sharp cutting edge with work piece; the peaks of the micro-unevenness at the cutting edge are rapidly broken away. In the region C, the wear rate is rapid and may lead to catastrophic failure of the tool. In general it has been found that the most economical wear land at which to remove the tool & re sharpen is just before the start of rapidly increasing portion of the

The wear land on flank will not be generally uniform along the entire cutting length of cutting edge. Depending on machining conditions, the following types of wear lands of combinations of these are generally observed. 1. Excessive wear at the nose end of the flank (fig. 2.4a) is brought about by plastic deformation, which reduces relief in the area, thus increasing the rate of wear. This can also be brought about if the crater on the rake face breaks through the nose area. 2. Irregularities in the wear along the whole cutting edge length due to minute chipping or attrition of the cutting edge (Fig. 2.4 b). 3. Excessive wear at the line of depth of cut (Fig.2.4 c). This can be either due to the work hardened surface by the previous cut or heat treat scales or by abrasive materials on the work-piece.

2.3.2 Crater Wear: It occurs on the rake face of the tool in the form of a pit called as crater. The crater is formed at some distance from the cutting edge. As the cutting speed is increased the tendency of the cutting tool to fail by cratering is increased. The tool chip interface temperature increases with cutting speed & at these higher temperatures the rate of material removed from the tool increases. The careful measurements have shown that the location of maximum cratering & maximum chip tool interfacial temperature coincide with each other. It may there fore be assumed that cratering is a temperature dependent phenomenon caused by diffusion & adhesion etc. fig.2.5 shows how the radius of curvature 'Rc', the depth of crater 'dc', the width of crater and the distance of start of crater from the tool tip 'a', change with time. The crater significantly reduces the strength of the tool & may lead to its total failure.

a

Rc

b

Rake face dc

Tool

The time for which a cutting edge or a cutting tool can be usefully employed without regrinding ( eq HSS) or replacement (eg. Throw away carbides tip) is called the tool life. It is not economical to continue to use the tool beyond its useful life. This is because increased bluntness of cutting edge causes increase in cutting forces & as a result tool temperature also increases. Consequently affecting the dimensional accuracy & quality of machined surface, ultimately leading to rejection. Also, the rate of flank wear-after certain critical value increases rapidly. The progress of crater wear is also of similar nature. Continued use of worn out tool would ultimately cause catastrophic failure or total loss of tool & even damage of the component. If tool is ground or replaced prior to catastrophic failure; the volume of material ground off the tool (therefore the regrinding cost) would not be excessive. Hence certain tool failure criteria have been devised to specify maximum wear of the tool that can be tolerated before regrinding or changing it. The tool failure criteria (or tool life criteria) can be classified as direct & indirect. nose portion

middle portion

rear portion

hf hf max

TH flank FLANK WEAR LAND ELEVATION A

Dc RC CROSS SECTION OF AA

B

A

B

FIG 2.6 WIDTH OF WEAR LAND AND MAXIMUM DEPTH OF CRATER

2.4.1. Direct criteria: These depend upon measurement of tool wear or direct visual examination of cutting edge (fig.2.6)

recommends the value of dc = 0.06 + 0.3f & as a tool failure criteria where f is the feed per revolution. Also Opitz & weber has suggested the ratio dc/hc value between 0.2 & 0.4 as tool failure criteria. iii) Limiting extent of chipping & crack formation: Faulty cutting conditions may lead to appearance of fine cracks near the cutting edge shortly after the tool is put to operation. This can be detected by visual examination. The situation may be remedied by correcting the cutting conditions such as selection of a tougher tool material, a more rigid machine; a stiffer tool material, proper tool angles, proper machining parameters such as speed, feed & depth of cut etc. 2.4.2

Indirect criteria:

These depends upon the measurement of effects produced by tool wear and chipping etc. 1) Limiting value of surface roughness: The roughness of a machined piece increases in proportion to the damage suffered by the cutting edge & unevenness of the flank wear. Monitoring of surface roughness can help to keep a control on limiting value of wear land. But surface roughness measurement requires costlier equipments than measurement of width of wear land, which can be measured by a microscope. 2) Limiting value of change in machined dimensions: In this method, the dimensions of each machined component are measured. When the tool is new, the dimensional accuracy is satisfactory & deteriorates as the tool progressively wears out. When the dimensional accuracy falls below a prescribed level, the tool is said to have failed. 3) Limiting value of increase in cutting forces: With increase in wear the tool forces increases (However, cutting forces tend to decrease some - what with increase in crater wear on account of increase in effective rake angle). A tool dynamometer or a power meter can therefore be used to monitor changes in cutting forces or rate of power consumption. When their increase exceeds predetermined amount, the tool life is said to have been exhausted. 4) Limiting value of Volume of metal removed: If the cutting conditions are kept constant (eg feed, speed; depth of cut) the progress of tool wear is directly proportional to actual machining time or volume of metal removed. Thus limiting value of volume of metal removed can be related to limiting value of width of wear land on flank & can, therefore be used as tool failure criterion.

The tool life equation is an empirical relationship between the tool life and one or more variables of cutting process, e.g. cutting speed (V), feed (f), and depth of cut (d) etc. The most famous tool life equation is due to F.W. Taylor. On the basis of experimental work, Taylor showed the tool life 'T' and cutting speed 'V' is related to each other as follows. . . . . . . (eqn. 2.5.1) V. Tn = C Where the constant 'n' is called the tool life exponent and the parameter 'C' is known as Taylor’s constant. Making T = 1 in the above equation, we find that C = cutting speed for 1 min tool life. The constants n & C depends upon the tool and work materials, feed and depth of cut, type of coolant and tool geometry etc. Equation 2.5.1 can be written as LogV + n Log T = log C or log T = (1/n) log C - (1/n) log V

. . . . . (eqn 2.5.2)

Cutting speed - Tool life curves can be graphically expressed as shown in fig.2.7. From the graph it can be seen that 'n' is the negative inverse slope of the curve and C is the intercept velocity at T = 1. The following values may be taken for 'n'. n = 0.1 to 0.15 for HSS tools = 0.2 to 0.4 for carbide tools = 0.5 to 0.6 for ceramic tools.

T

V

The equation 2.5.1 can be generalized or modified to include the effects of feed and depth of cut one such relationship is of the form VTnfn1dn2 = C1

. . .. (Eqn .2.5.3)

Where the exponent n, n1, n2 and constant C1 depend upon tool and work materials, tool geometry and type of coolants etc. Following comments can be made from the tool life equation. (1) Smaller the values of exponent, n, (as in HSS) the steeper is the slope of log V - log T line and more is the sensitivity of tool life to changes in cutting speed. Thus the ideal tool material is one, which can be used at any cutting speed without affecting tool life. i.e. when n=1. From this point of view ceramics are superior to carbides and HSS. (2) The larger the value of 'C' the greater is the tool life and superior is the tool material. (3) Even under constant cutting conditions values of ‘n’ and 'C' are found to vary widely. (4) In some cases, the tool life criterion changes with change in the predominant wear mechanism at different cutting speeds. For example in cutting steel with carbide tools, the predominant mechanism changes from adhesion wear at lower cutting speeds to diffusion wear at higher cutting speeds. Consequently the tool life criterion has to be changed from limiting width of flank wear land to limiting depth of crater wear. Since the values of exponent are different for different wear mechanisms the log T - log V graph has two straight-line segments as shown in fig 2.7. (C). 2.6

VARIABLES (FACTORS) AFFECTING TOOL LIFE: The various variables, which affect the tool life, are as under -

1. Tool material 2. Work material 3. Process variables - speed, feed, depth of cut 4. Tool geometry 5. Cutting fluid 6. Vibration behavior of machine tool work system

life) at all cutting speeds. Thus for a given cutting speed cemented carbide will have better tool life than HSS. Hence tool life is dependant on type of tool material. 2. Work material: The properties of the work material that tend to increase the tool life are as follows, (a) softness (or lack of hardness) to reduce cutting forces, cutting temperature & abrasive wear, (b) absence of abrasive component such as slag inclusions, surface scale & sand, (c) presence of desirable additives like lead to act as boundary lubricants and sulphur to reduce cutting forces & temperatures by acting as stress raiser, and (d) lack of work hardening tendency that tend to reduce cutting forces and temperatures and also abrasive wear and (e) occurrence of favorable microstructure, e.g. presence of spheroidized pearlite instead of lamellar pearlite in high carbon steel improves tool life. Similarly in cast irons, a structure that contains large amount of free graphite & ferrite leads to greater tool life than one, which contains free iron carbide. 3.

Process variables: (Speed, feed, depth of cut):

The cumulative effect of speed, feed & depth of cut can be seen from the modified Taylor’s tool life equation. Increase in any one of the above reduces the tool life, but cutting speed has more impact on tool life followed by feed & depth of cut. Tool life is a direct function of temperature. At higher feed, the cutting force per unit area of chip tool contact on rake face & work tool contact on flank face is increased there by increasing the temperature and hence wear rate. Similarly, at higher depth of cut, the area of chip tool contact is increased roughly in proportion to change in depth of cut (such is not the case with feed change where the chip tool contact area changes by larger proportion than change in depth of cut), increasing the temperature & consequently the wear rate. 4.

Tool geometry:

Rake angles, cutting edge angles, and relief angles & nose radius affect the tool life by varying degree. I) The cutting forces, tool temperatures & tool wear decrease with increase in rake angle (fig.2.8 a) consequently tool life improves when rake angles are increased. However larger rake angles make the cutting edge sharper reducing the mechanical strength & making the tool liable to chipping. Therefore there is an optimum rake angle associated with every tool work pair.

Tool li

optimum rake

Negative rake

Positive rake

Effective Rake Angle (a)Effective rake angle versus tool life ii) Large relief angle increases volume of wear required to reach a particular width of flank wear land as seen from fig. 2.8 (b) and also reduces the tendency of rubbing between flank & work piece surface, there by increasing the tool life. However, on the other hand, larger the relief angle smaller is the mechanical strength of cutting edge & more liable the tool is to chipping fracture. Thus there is maximum tool life for optimum relief angle as seen for fig.2.8 (c). f

hf

f

hf

tool Relief angle

tool

Relief Angle

larger relief angle

smaller relief angle EFFECT OF RELIEF ANGLE ON TOOL LIFE

(C) T

The cutting fluid cools the tool & work piece, acts as lubricant and reduces friction at chip tool interface. Therefore the cutting temperatures are decreased & the use of cutting fluid in the tool materials with low value of hot hardness (e.g.) shows appreciable increase in tool life. However in carbides & oxides, which have high value of hot hardness, the cutting fluid has negligible effect on tool forces or tool life. 6.

Vibration Behavior of Machine tool Work System: -

If the machine is not properly designed, if the work piece is long and thin or if the tool overhang is excessive, chatter may occur during cutting. It is known that chatter may cause fatigue failure or calas tropic failure of tool due to mechanical shock. 2.7. Machinability: Machinability is the property of material to be machined, which governs the case or the difficulty with which it can be machined under a given set of conditions. In spite of the efforts made by the number of investigators, so far, there has been no exact quantitative definition of Machinability. It is due to large number factors involved & their complexity in metal cutting process viz. forces & power, tool life, surface finish etc. These are dependent upon number of variable such as work material, cutting conditions, M/C tool rigidity tool geometry. Due to this, it is impossible to combine these factors so as to give a suitable definition for Machinability. It is of a considerable economic importance for production engineer to know in advance the Machinability of work material so that he can its processing in an efficient manner. 2.7.1

Criteria of Machinability: -

The case of machining different materials can be compared in terms of various criteria based on tool life values, cutting forces surface finish. Some other criteria, eg. ease of chip disposal & operator safety can also be employed. 1. Tool Life Criterion: Tool life is usually the most important of the three main criteria used for assessing machinability. This is due to the fact that the tool life can be conveniently expressed in terms of cutting speed (which being the direct function of cutting speed when all other variables are kept constantly. Thus the cutting speed for producing a predetermined value of tool life, termed as specific cutting speed, could be assessed on basis of comparison of machinability of materials. For example if "Vs" is the cutting

Al. alloys Mg. alloys

-

300-1500 600-2000

2. Cutting Force Criteria: This criterion is important, where it is necessary to limit values of cutting force in keeping with rigidity of machine tool & to avoid vibration in machining. If the cutting force is high consequently the power consumption is also high, a larger machine tool may be required, thus increasing the overhead cost and unit production cost. The specific cutting energy of a given material, defined as cutting power required for removing a unit volume of material in unit time, is often considered as index for machinability of a given work material. The larger the specific cutting energy, i.e. higher the cutting forces induced under a set of cutting conditions during the machining of a material, the lower is its machinability index. 3. Surface Finish Criteria: According to this criterion, the work material will have better machinability if under a given cutting conditions, it takes on a better surface finish (Polish or is less rough) than the other. This criterion is used in situations where surface finish is the cause of rejection of machined parts. In such case the first two criteria may not be helpful, because in spite of particular work material permitting the use of higher cutting speeds without an excessive number of tool changes or power consumption, satisfactory finish may not be achieved. 4. Other Criteria: Several other criteria have been put forward to assess machinability of different work materials. Prominent among these are (a) Temperature developed at chip tool interface, (b) depth of hole cut out in a given time by a standard drill that rotates at a standard speed and specified downward thrust (penetration test), and (c) depth of cut produced by a power hacksaw on a standard bar of given material under standard sawing conditions of speed and downward pressure (sawing test). Even physical properties like hardness, tensile strength, shearing strain per unit shear stress in plastic range, and different combination physical, properties ( eg. hardness, specific weight, ductility etc.) have been correlated with tool life in order to use them as machinability measures.

In spite of availability of several machinability criteria, a wholly satisfactory unit of machinability has still not been found. For example, if different tool materials, different cutting conditions or different operations are used to assess the relative machinability for the same set of work materials, different machinability ratings are obtained. Therefore the tables of machinability ratings are used only for general guidance during process planning. 2.7.3. Variables affecting machinability: Machinability is influenced by variables pertaining to machine tool, cutting tool, cutting conditions or work material. These variables are listed as under. A) Machine Variables: 1. Capacity of machine: (Power, torque - accuracy of machine) 2. Rigidity of machine & work holding devices. B) Tool Variables: 1. Tool material (HSS, Carbide, Ceramics etc) 2. Tool geometry (Tool angles, radii, type etc) 3. Nature of tool engagement with work. (Continuous of intermittent, entrance & exit conditions) C) Cutting Conditions: 1. Cutting speed, 2. Feed, 3. Depth of cut D) Work material variables: 1. Hardness, 2. Tensile Shength, 3. Chemical Composition, 4. Microstructure, 5. Degree of cold work, 6. Shape & dimensions of work, 7. Rigidity of work piece, 8. Strain hardenability. 2.8. SURFACE FINISH: (ROUGHNESS) The quality a machined surface is characterized by accuracy of manufacture in aspect to the dimensions specified by the designer. Every machining process leaves its evidence on the surface that has been machined. This evidence is in the form of finely spaced micro-irregularities left by the cutting tool. Each kind of tool leaves its own pattern that can be identified. This pattern in called surface finish or surface roughness. Whenever two machined surface come in contact with each other, the quality of the mating surfaces plays an important role in performance & wear of the mating parts. Some degree of roughness, which may be extremely small, is always present on any surface.

rise in temperature suppresses or reduces formation of built up edge and at V3 it almost disappears. This reduces the height of micro irregularities. Further increase in cutting speed reduces surface roughness. The absence of built up edge formation in curve 'B' for machining of high alloy steels, non-famous metals & cast iron shows decrease in surface roughness from beginning (i.e. no rise in roughness as in curve 'A' is observed) dry

100

refined kerosene

0

0.2

v1 v2 v3 v m/min (a)Effect of cutting speed

activated kerosene

0.14

B

0.075 0.10

A

emulsion

H(RMS) (MICRONS)

(MICRONS)

H

f

(b) Effect of feed b) Feed: Increase in feed deteriorates surface, finish. It is observed that a rate of feed (f) in the range 0.12 mm/rev. to 0.15 mm/rev. has a negligible effect on the height of irregularities but further increase in feed rate increases surface roughness Fig.2.10 (b) shows effect of feed for different coolants on height of surface irregularity. c) Depth of Cut: Increase in depth of cut deteriorates the surface finish. 2) Tool geometry: - The effect tool geometry on surface finish is as follows: a) Nose radius: Increase in nose radius improves surface finish. There is appreciable improvement in surface finish up to 0.3 mm. nose radius. Higher value of nose radius produces less improvement in surface finish & on the other hand it causes chatter during machining. b) Rake angle: Increase in rake angle of tool improves surface finish. c) Relief angle: Increase in relief angle adversely affects surface finish but the effect, as a whole is small. d) Side cutting edge angle: An increase in side cutting edge angle improves surface finish largely because of the reduced chip thickness but the degree of improvement depends upon the rigidity of the work & it’s mounting.

in machining. 2.8.2. Surface finish Terminology: a) Roughness: This includes surface irregularities resulting due to the various manufacturing processes. These irregularities combine to form surface texture. Other definition Roughness (Primary texture): relatively fined-spaced surface irregularities. On surfaces produced by machining and abrasive operations, the irregularities produced by cutting action of tool edges and abrasive grains and by the feed of the machine tool are roughness. Roughness may be considered as being superposed on a wavy surface. b) Roughness Height: It is the height of the irregularities with reference to an average line: The value of roughness height can be expressed in two ways i) Arithmetic average value (Ra) ii) Root mean Square (rms) value. The arithmetic of average height value is given by . . . . .. eqn 2.8.1 Ra = hta -∑ yi/n Where Y = Vertical distance of the profile from centerline. n = Total no. of vertical measurements. hrms =

∑ yi 2 n

c) Roughness Width: It is the distance parallel to the nominal surface between successive peaks or ridges, which constitute the predominant pattern of the roughness. d) Roughness width cut off: It is the greatest spacing of the repetitive surface irregularities to be included in the measurement of average roughness height. It is always greater than the roughness width in order to obtain the total roughness height rating. e) Lay: It is the direction of predominant surface pattern produced & reflects the machining method. Lay: The direction of predominant surface pattern. A typical surface is shown in Fig. 14.9. The identification of the macrogeometrical of microgeometrical errors is based on (l/h) ratio as shown in Fig.14.10. When (l1/h1) > 1000, the deviation is macrogeometrical denoting out-of-roundness, taper or barrerl form. When 500>(l2/h2)≥150 the deviations denote waviness. When (l3/h3) ≤ 50, the microgeometrical deviations are characteristic of surface roughness.

Waviness (secondary texture other definition): - The surface irregularities, which are of greater spacing than the roughness. On machined surfaces such irregularities may result from machine and work deflections, vibrations, etc. h) Flaws: Cracks, scratches & ridges are called flaws. They are not regularly recurring & are imperfections outside the regular pattern of surface texture. other definition FLAWS: Irregularities, which occur at one place or at relatively infrequent intervals in the surface, e.g., a scratch, ridge, hole, crack, etc.

PEAKS ROUGHNESS SPACING

CENTRE LINE

VALLEYS

2.8.2

Ideal Surface Roughness & natural surface Roughness:

The surface finish is dependent upon tool geometry, feed rate & other irregularities or contributing factors in the machining process. Such roughness is called natural surface roughness. If the other contributing factors are entirely eliminated them the surface roughness produced will depend entirely on tool geometry such surface roughness is called ideal surface roughness.

2

4

Where "h" is the height of the geometry of the surface roughness. Now from geometry: f h = ----------------------------------. . . . . . . Eqn 2.8.4 tan "SCEA" + Cot "ECEA" from Eqn 2.8.4 & 2.8.3. we have Ra = f/4.(tan "SCEA" + Cot "ECEA") or Ra = f/4.(tan "Cs" + Cot "Ce") . . . . . . . Eqn. 2.8.5. For rounded corner tool, the ideal value of surface roughness is hCLA = f2/8R and Ra = f2/18 3 R . . . . . . . . . Eqn 2.8.5. Where "R" is nose radius. Thus, a change in the rate of feed is more important than a change in nose radius and depth out has no effect on the surface geometry.

EC

H

f

d ECEA

TOOL

(a)

SCEA

f h/2 I

II

(b)

1S0 have recommended a series of roughness values & corresponding roughness symbols as shown in the following table. Table: Surface roughness values. Sr. No.

Type of operation

1. 2. 3. 4.

Rough machining Fine machining Grinding Lapping

Representation as per IS 3073 - 1967 Symbol Relative values in microns ∇ 8 - 25 ∇∇ 1.6 - 8 ∇∇∇ 0.025 - 1.6 ∇∇∇∇ < 0.025

2.8.5. Cost of surface finish The cost of machining of any component is greatly dependent on the quality of surface roughness required on the part. The cost increased, if the specified roughness on the part decreases. In other words smoother the required surface, the greater is the cost involved. The various reasons for this increase are given below. 1. High feed & depth of cut cannot be used. 2. Frequent tool changes are required as the tool failure criteria permits lower width of wear land. 3. The set up requires more of effective clamping methods, special tools or inserts and more frequent closer inspections.

E

Machining cost

D grinding linear variation

A finish turning

1/h CLA

C

hononing

REAMING ELECTRON BEAM LASER ELECTROCHEMICAL BORING, TURNING ELECTRONIC GRINDING GRINDING HONING ELECTROPOLISH POLISHING LAPPING SUPER FINISHING

Fig 2.15 surface finish produced by machining operations In the figure the inverse of surface roughness (i.e. surface finish) is plotted against machining cost. The increase in cost with reduction in roughness is gradual in rough turning (up to points). But for finish turning it increases exponentially. The surface roughness produced by various machining operations is given in 2.14 (a) 2.8 2.9.1

Cutting Fluids : Functions of cutting Fluid: -

During the metal cutting heat is generated. The heat is generated due to the plastic deformation of the chips, friction between the rake face & chips & friction between clearance face & work piece. The heat generated increases the temperature of both cutting tool & work piece. The increase in temperature of tool decreases hardness & hence wear resistance & life. The effect is more pronounced in H.S.S. tools than in carbide tools. Fig.2.16 (a) shows zones of heat generation. In shear zone maximum heat in generated (i.e. 60 %). In chip - tool zone, 30% heat is generated, whereas in tool-work zone minimum heat (i.e. 10%) is generated. In fig. (2.1% (b) it can be seen that at higher cutting speeds the distribution of heat in chips, tools & work piece is nearly in the ratio 80:10:10

Moreover, apart from heat generation the chips also create problems in metal cutting such as bad surface finish due to abrasion of chips to the tool or finished work piece. On these aspects the functions of cutting fluid can be summarized as follows. 1) Lubrication at chip tool interfaces & work tool interface & hence preventing welding between contact surfaces. 2) Cooling action or heat dissipation to avoid loss of hardness & wear resistance & hence improve tool life. 3) Flushing of chip to protect surface finish & improve tool life. Cutting fluids, besides fulfilling the above functions should have following properties. 1) It should be stable & non-foaming.

Fig 2.17

Effects of cutting fluid in metal cutting

2.9.2

Types of Cutting Fluids: The detailed classification of cutting fluids is shown in 2.16. They are broadly classified into: (i) Neat oil, (ii) Soluble oil, (iii) Synthetic coolant, & (iv) Gaseous fluid. Besides this a true solution i.e. water mixed with corrosion inhibitors, is also used but has poor lubricating action.

Fig.2.13: Family of cutting fluids a) Neat Oil: - is mineral oil, vegetable oil or blends of these oil straight mineral oil i.e. without additives is not suitable for high loading & speed & hence used only in light machining of non ferrous metals like Aluminum & magnesium. Compounded oils (i.e. mineral oil blended with fatty acids like lard oil, oleic acid, sperm oil etc) have high

b) Soluble Oil or Water emulsion: Soluble oil is blend of mineral oil, emulsifying agents & coupling agents. Emulsion is formed by mixing soluble oil with water in the ratio 1:10 to 1:40 for general machining & up to 1:80 for grinding. Conventional emulsions are milky in appearance. Translucent emulsions are made by reducing oil droplet size of emulsion with a high ratio of emulsifier to oil. They have higher film strength & better anticorrosive properties than opaque emulsions. They also provide improved tool lubrication due to fine dispersion of oil globules. Heavy-duty soluble oils i.e. soluble oils blended with fatty acids & EP additives, can withstand heavy cutting pressure & temperature. Heavy-duty soluble oils are used in rich concentration with ratios ranging from 1:5 to 1:15. c) Synthetic Coolants: - They are usually non petroleum products, though sometimes a small percentage of mineral oil is added, many chemical agents blended in water form synthetic coolants. Basically they are coolants, though some are also lubricants. They are used in grinding than in other operations & they are mixed in the ratio 1:50 to 1:250 parts of water. The main problem associated with use of synthetic coolant is its compatibility with the other lubricants, seals metal parts. Because they are chemically active, they react easily with paints, other metal parts, etc. Before deciding on the use of these coolants, this aspect should be thoroughly looked into comparative performance of these three fluids is given in the Table 2.1. d) Gaseous Fluid: The high cost restricts use of gaseous fluid. Mist is commonly used gaseous fluid. Mostly compressed air is used to atomize the coolant & carry it to the point of cutting in the form of mist. Gases like carbon dioxide, Freon, and helium are used for special applications. Table:

Comparative Performance of Various Types of Cutting Fluids

Sr. No. 1.

Soluble oil

Neat oil

Synthetic coolant

Promotes better coolinghence effective in transporting the heat

2.

Prone to bacteria contamination and obnoxious odour-so

Promotes better lubrication-hence effective in reducing heat generation. Good service life, no unpleasant odour

Poor lubricating property like soluble oil, but better cooling property than soluble oil Life better than soluble oil

7. 8. 9.

Susceptible to risk of oil dermatitis Tramp oil like hydraulic and other lubricating oil will reduce the life Low initial cost

Susceptible to risk of skin cancer Little effect with tramp oil

Some chemicals may be toxic Little effect with tramp oil

High initial cost

Low initial cost

2.9.3. Selection of cutting fluids: The major factors which govern the selection of cutting fluids are (i) machining process, (ii) the cutting tool materials, & (iii) the work piece materials. Besides these factors, compatibility with the machine, performance requirements, human interaction & economy must also he look into? Specifying a particular fluid as most suitable for a specific application is almost impossible. When more than one operation is performed on the machine this problem becomes more difficult. Neat oil may be better choice than soluble oil when the cutting fluid is likely to see into the drive or control system. The factors influencing selection of cutting fluid are shown in fig.2.17.

Factors influencing the selection of cutting fluids

grinding 5.

Reaming

3.4

4.3

4

4

6

6

6.

Drilling-deep-hole

5

5

5

5

9

9

7.

Honing

1.2

1.2

2

2

6

6

8.

Automats

1.2

1.2

2.1

24

6.1

6.1

9.

Drilling, Boring, turning and Milling

7

7

9.7

9.7

7

8

10.

Sawing

7

7

7

7

7

7

11.

Grinding

7.8

7.8

8.7

8.7

7

7

(1) Mild sulphurized fatty oils. (2) Mild sulphc chlorinated oils. (3) Medium sulphurized fatty oils. (4) High sulphur fatty chlorinated oils. (5) High chlorinated mild sulphurized fatty oils. (6) Fatty mineral oils. (7) Soluble oil (8) Translucent soluble oil (9) Heavy-duty soluble oil.

SOLVED PROBLEMS: Example 1: Tool life test has been carried at two different cutting speeds. The width of flank wear land has been measured after every 10 minutes at each cutting speed. The width of flank wears land at V = 150 m/min were noted as 0.10, 0.15, 0.22, 0.25, 0.30, and 0.35, min after 10,20,30,40,50,60,70 minutes of cutting respectively. Similarly at the cutting speed "V" = 200 m/min the values of width of flank wear land were noted as 0.20, 0.25, 0.30, 0.40, 0.50, 0.85 mm for cutting time of 10,20,30,40,50,60,70 minutes respectively. If the criterion of tool failure is selected, as 0.30 mm, width of flank wear land, calculate values of "n" & "c" of the Taylor's tool life equation. Solution: As the tool failure criterion is 0.3 mm, the value of cutting time corresponding to this value will be the tool life at that cutting speed. From the above data we have, Tool life T1 = 60 min. for V1 = 150 m/min., and T2 = 30 min. For V2 = 200 m/min. Therefore putting these values in the Taylor's T.L. equation we have. (V1) (T1) n = C i.e., (150) (60) n = C . . . . . . . . . . (i) And (V2) (T2) 2 = C i.e., (200) (30) n = C . . . . . . . . . . (ii) Dividing equation (i) by equation (ii), we have. (150/200) x (60/30) n = 1, or 2n = 1.33 n log 2 = log 1.33 i.e.

Solution: Let m = No. of components produced between consecutive tool changes, V = Cutting speed, (m/min) T = Tool life (min.) f = Feed rate, (mm/rev) L = Length of cut, (mm) D = Work piece diameter (mm) = time to produce one work piece = L/f.N. Tm We express the tool life equation as VTn fn1 = C . . . . . . . . . . (i) Where n, n1 & C are constants. V = π.D.N./1000 . . . . . . . . . (ii) m = T/Tm = ( T.f.N.)/L Therefore, T = (m.L)/f.N. . . . . . . . . (iii) Substituting (ii) & (iii) in (i), we get ( π. D.N./1000) x ( m.L./f.N.)n x fn1 = C N1-n, mn , fn1-n = (1000 C.L. -n/π. D.) or N1-n, mn. fn1-n = C1 . . . . . . . . . . (iv) Substituting values of N,f, & m in (iv) we get. (250)1-n x (311)n x (0.100)n1-n = Cl . . . . . . . (v) (250)1-n x (249)n x (0.125)n1-n = Cl . . . . . . . (vi) and (300)1-n x (144)n x (0.125)n1-n = Cl .. . . . . . (vii) Dividing (vii) by (vi) we get, (250/300) 1-n x (249/144)n = l or (1-n) log (250/300) + n log (249/144) = 0 n = 0.25 Similarly by dividing (v) by (vi) we get n1 = 0.500 Substituting values of n & n1 in (v) we get C1 = 148.4 Putting N = 350, f = 0.150, n = 0.25 & n1 = 0.500 in (iv) we get m = 75 Example 3 : In a laboratory test on turning operation, the following data have been recorded, S.No. V, m/min f mm/rev. d, mm T, min. 1 100 0.10 2.0 120 2. 130 0.10 2.0 50 3. 100 0.12 3.0 70 4. 100 0.12 3.0 65

Putting values of n in (iii) & (iv) & dividing (iv) by (iii) get. (65/70) x (3/2) n2 = 1 or n2 log [3/2] = log 1.66, Therefore n2 = 0.38 Putting values of n in (ii) & (iii) & dividing (iii) by (ii). We get (100/130) x (70/50) n x (0.12/0.10) n1 = 1 n1 log (0.12/0.10) = log (1.18), Therefore n1 = 0.885 Putting these values in eqn (i) we get C = 72.13. Now at V = 120m/min f=0.20 mm/rev, & d=2mm, we get T = 8.77 min. Example 4: The tool life equation for a turning operation is given as 36.5 = V. T0.13, f0.60.60 d0.3 .A 60 min. tool life was obtained using the following cutting conditions; V = 40 m/min, f = 0.25 mm/rev. d=2.0 mm. Calculate the effect on tool life if speed, feed, and depth of cut are together increased by 25% and also if they are increased individually by 25% Solution: Putting V = m/min, f = 0.25 mm & d = 2 mm we get, 40 x (60) 0.13 x (0.25) 0.60 36.50 Now of values of V, f, & d are individually increased by 25% i.e. by Selecting. V = 40 x 1.25 = 50 m/min. f = 0.25 x 1.25 = 0.3125 mm/rev. and d = 2.0 x 1.25 = 2.5 mm Tool life T = [36.50/50 x (0.25) 0.3 x (2.0) 0.3] 1/0.13 = 10.78 min. Similarly, for, V = 40m/min, f=0.3125 mm/rev & d = 2.0 mm, we get T = 21.42 min and for V = 40 m/min, f = 0.25 mm/rev & d = 2.0 mm, we get T = 35.85 min. If all these values are increased together i.e. for V = 50m/min, f=0.3125 mm/rev. and d = 2.5 mm, we get T = 1.26 min. It can be seen that the impact of increase in cutting speed on tool life is maximum. Example 5. The tool life for H.S.S. tool is expressed by the relation V.T.0.14 = C1 and for tungsten carbide is VT0.2 = C2. It at a speed of 24m/min tool life for both the tools is 128 minutes; compare the life of tools at a speed of 30 m/min. Solution For HSS tool, V x T0.14 = C1 . . . . . . . . . . . . (i) i.e. 24 x (128)0.2 = C2

Thus, at higher cutting speed of 30 m/min. WC Tools gives better tool life than HSS tools or the tool life of HSS tool T1 = (29/38) x T2 or 0.7 x T2 at V = 30m/min. Example 6: The cutting speed and tool life relationship for a tool is given by V.T.0.2 = C. During machining, 18 mm bar on a lathe at a cutting speed of 110 m/min. the tool life is found to be 60 minutes. Calculate spindle speed to give a tool life of 5 hours. If length of cut per component is 50 mm, what is the cutting time per piece and how many pieces can be produced between tool changes at a feed of 0.15 mm/rev? Solution: From tool life relationships. C = (110) x (60) 0.2 = 249.47 Therefore for T = 60 x 5 minutes, we have V = 249.47/(300) 0.2 = 79.72 m/min. Hence spindle speed N = 1000 C/π x D = 1410 rpm. Cutting time/piece = L/f. N. = 50 x 60/(0.15 x 1410) = 0.24 min. Therefore, number of components produced in 5 hours tool life 50 x 60 x 6/0.24 = 1249 QUESTIONS 1.What are the various causes or mechanisms of tool wear? Explain. 2.Explain what is meant by the terms - (Explain with neat sketches) 1.Attrition wears 2.Diffusion Wear 3.Abrasive Wear 4.Fatigue Wear 5.Plastic deformation 6.Electrochemical wear & chemical wear. 3.Differentiate between "flank wear" and 'Crater wear' 4.What is the difference between 'Abrasion wear' & 'Attrition wear’? 5.Why tool wear is important in metal cutting? 6.What is meant by 'tool life’? What is its significance to an engineer who is interested in productivity? 7.What are the various tool life or tool failure criteria? 8.State the general from of Taylor's tool life equation and also give the modified Taylor's tool life equation.

18. How will you select the machinability criterion for a given set of conditions? 20. What are the sources of best generation in metal cutting? How is the heat distributed in chip, tool & work piece? 21. What are the basic functions of cutting fluids? 22. What are the effects of cutting fluid in metal cutting? 23. How are the cutting fluids classified? 24. Compare the performance of neat oil, soluble oil and synthetic coolant. 25. What are the various factors influencing selection of cutting fluids? 26. Which coolants would you suggest for turning of following metals with H.S.S. tool? i) Mild steel, 2) Aluminum, 3) Copper, 27. Recommend a cutting fluid for broaching or i) Low Carbon Steel, ii) Stainless steel, iii) Aluminum alloy, iv) Copper. 28. State the factors responsible for surface roughness? 29. Explain the effect of cutting speed, feed and depth of cut on surface finish. 30. What is the effect of tool geometry on surface finish? 31. What is surface finish? Describe the elements of surface texture. 32. Explain the following terms. i) Roughness, ii) Roughness height, iii) Roughness width, iv) Lay, v) Waviness, vi) Flame, 33. What is ideal surface roughness & natural surface roughness? 34. What is hCLA or R-value? Derive an expression for the theoretical roughness of a surface machined with single point tool. 35. What is the effect of surface roughness on cost of production? 36. In general give a broad range of surface roughness that can be achieved in various machining operations. 37. How is surface finish represented on drawing? 38. The tool life for HSS tool is expressed by relation V.T.1/7 C1 and for W-C tool is expressed as V.T.= C. If at a speed of 24 mpm the tool life is 120 minutes, compare the life of tool at a speed of 30 mpm. 39. For a particular machining set up the tool life equation is given as V.T.0.2 = C. During machining of an 18 mm diameter bar on a lathe at a cutting speed of 110 m/min, the life of tool is found to the 60 minutes. Calculate spindle speed to give tool life of 5 hours. If a length of 50 mm per component is machined what is the cutting time per piece and how many pieces can be produced between tool changes? The feed used is 0.15 mm/revolution 40. During the tool life test for HSS tool used to cut special die steel the following observations were recorded. Use the above values to calculate the constants of tool life equation.

***** CHAPTER-III CUTTING TOOL MATERIALS AND NOMENCLATURE 3.1 Introduction: Before 1990, machining had been carried out with plain carbon steel tools or air hardening alloy steel known as Mushet steel. The machining speeds were low due to failure of cutting tool to maintain hardness at high temperature generated due to high cutting speeds. A big break-through has been achieved when F.W. Taylor developed a tool material (called high speed steel), which can operate at relatively higher cutting speeds. The cast cobalt base alloy tools appeared on the scene in 1915. They have higher hot hardness; wear resistance & fewer tendencies to form BUE. In 1926, Germans introduced a new material called sintered tungsten carbide tools, manufactured by the powder metallurgical technique. They can operate at comparatively very high cutting speeds than HSS tools. Addition of carbides of titanium, tantalum & niobium to basic tungsten carbides enhanced range of application of the carbides. Later on coating of titanium carbides has been applied on cemented tungsten carbide increasing the permissible speed by about 50-80% over the conventional carbides. Ceramic tools (or cemented oxide tools) were produced & developed in 1960's. The permissible speed with cemented oxides is two three times higher than those with cemented carbides. However, the brittleness restricts their use to only on rigid or (more of less vibration free) machine tools. In 1971, Union carbide (USA) developed a new material UCON, consisting of columbium, tungsten & titanium, permitting increase in cutting speed compared to carbides. In 1972, a new tool material called cubic boron nitride, (Borozon) with hardness next to diamond has been developed. Consequently, permissible machining speed with these tools is five to eight times that of carbides & can he used to cut hardest materials. Polycrystalline diamond bonded to carbide base has been used as a tool material.

3.2. Desirable Properties of Cutting tool materials: The three most important desirable properties of tool materials are wear resistance, hot hardness & Toughness. This is because during machining, the tool tip is subject to relatively high temperature, intense normal pressure, frictional stress at chip tool contact & work tool contact, impact & vibrations. Apart from these three important properties some other properties such as thermal conductivity for removal of heat from chip tool interface, low coefficient of thermal expansion, weldability, hardenability, dimensional stability, freedom from distortion after heat treatment etc are also desired. a) Wear Resistance: It is also called as abrasive wear resistance (or AWR). Wear resistance, is necessary to enable the cutting tool to retain its shape & cutting efficiency. Wear may be caused due to various mechanisms such as attritition or adhesion, diffusion, abrasion etc. Wear resistance refers to the ability of tool material to retain its sharpness & shape for sufficiently long time while machining a given material at a relatively low cutting speed. Wear resistance & toughness are two independent properties. The gain of one results in loss of other. b) Hot hardness: It is also called as high temperature stability (or HTS). It is a measure of ability of tool material to retain its hardness at elevated temperatures developed at chip-tool interface. Higher the value of hot hardness higher will he the

d) Other desirable properties. The following other properties are also desired by tool materials. i) High Thermal conductivity for quick removal of heat from chip tool interface. ii) Low coefficient of friction for reduction of heat generated due to friction at contact surface. iii) Low coefficient of thermal expansion for reduction of effect of thermal stresses & thermal shocks on material. iv) Resistance to distortion after heat treatment. v) Hardenability to achieve hardness at slower cooling rates during hardening. vi) Weldability for ease of wedding of tool with shank vii) Grindability for ease of grinding after tool failure. viii) Dimensional stability etc.

The basic desirable properties namely RBF, HTS & AWR do not always have concurrent high attributes in any given tool material. Generally, if a material is made more refractory to gain "HTS", it becomes more brittle with low "RBF". If it becomes more abrasion resistant, it also becomes brittle fig.3.31 (a) shows relative "HTS", "AWR", & "RBF" for two arbitrary materials in a ternary plot. Such a plot may be used

AWR TERNARY PLOT 3.3. Carbon Tool Steels: These are plain carbon tool steels to which no appreciable amounts of alloying elements are added. The carbon percentage varies from 0.6 to 1.5% in these steels. Increase in carbon percentage increase toughness and shock resistance whereas decrease in carbon content increases hardness & abrasion resistance. Very small quantities of Silicon, manganese, chromium or vanadium are added for increasing the hardness and grain refinement. Carbon tool steel is broadly classified into two categories water hardening steel (W type) & oil hardening steel (0 types), depending on the quenching media employed during heat treatment. The W-type should be quenched in water or brine for obtaining full hardness. The O-type does not require such a drastic quenching to reach full hardness due to addition of manganese & other alloying elements. The carbon tool steel has good toughness but relatively very rapidly drops after 2000C & their recovery hardness is also poor. These tools are cheaper compared to other tool materials. They have better machinability & Grindability. They are used for manufacturing various tools such as milling cutters, twist drills, turning tools, reamers etc, for use on easy to machine materials like wood, magnesium, brass & aluminum due to their less wear resistance & high toughness. In these cases the chip tool interface temperatures are low (below 2000C) & hence better tool life can be obtained even with carbon tool steels. Form tools of special shapes for small number of work piece are often made of carbon tools steel for reasons of economy. High carbon tool steel are also used for manufacturing tool operating at low cutting, speeds & at lighter cuts viz. hand taps, threading dies, razor blades engraving tools, files, reamers, hand & hack saws. Carbon tool steel with relatively low percentage of carbon are used in hand chisels, hammers, swages etc., where a combination of hardness & shock resistance is required. 3.4. High Speed Steel: High-speed steel is available in large varieties. But is basically high carbon steel to which alloying elements like tungsten, molybdenum, chromium, vanadium & cobalt have been added to improve their hardness, toughness & wear resistance properties. These steels can retain cutting edge hardness at temperature up to 6000C, thus permitting much higher cutting speeds than those possible with carbon tool steel.

M4, M7, M36, M41, M42, M43, M44, M45, M46 M-Type HSS contains 0.8% to 1.25% carbon with other alloying elements such as W (1.5 to 6%), Mo (3.75-9.6%), Cr (3.75 to 4.25%), V(1.15 to 4%), Co (0 to 1.2%), M-Type HSS is usually cheaper than T-type and have greater toughness at the same level of hardness as compared to T-type. The only drawback is decarburizatin during hardening in high molybdenum steel, but this can be avoided by using a salt bath or controlled atmosphere-hardening furnace. The functions of the various alloying elements used in HSS are given below. i) Carbon (C): - If combines with iron to form carbide which makes it respond to hardening, thus increasing hardness, strength & wear resistance. ii) Tungsten (W) & molybdenum (Mo) - These are strong carbide formers and produces fine structure adding to both toughness & hardness. But to produce the desired effect "W" is added in larger quantity compared to "MO". It also improves hot hardness. iii) Chromium (Cr): It improves hardenability & forms various carbides of Chromium, which are very hard. Grain refinement due to addition of chromium improves both toughness & hardness as in case of "MO" or "W". The alloys of “Cr” improve abrasive wear resistance. iv) Vanadium (V): It is strong carbide former & hence used in small amounts. It increases the hot hardness & abrasive wear resistance. v) Cobalt (Co): It is usually added to increase hot hardness to permit use of higher cutting speeds. 3.4.1. Manufacturing of High Speed Steel: H.S.S. is made by conventional method of alloy steel manufacture by powder metallurgical techniques, and the electro slag refining process. The steels produced by last two methods have uniform composition, finer grain structure free from inclusions & segregations. Properties of HSS tools are very significantly affected by heat treatment, which, therefore, must be carefully carried out as recommended by the manufacturer. The surface of HSS tool can be further hardened by work hardening (e.g. ball rolling) A recent development is that of coating HSS tools (by chemical vapour deposition technique) with thin layers (less than 10mim thick) of a refractory metal carbide or nitride e.g. titanium carbide, titanium nitride, hafnium nitride or Albumin. In power metallurgical technique fine powders of desired alloy are produced by atomization, compacted under high pressure, in desired shapes sintered at 1500C to get the HSS tool or tip. Whereas the electro slags refining of process consists of casting a

- Grades M1, M2, M7, of M10 are used for drills & reamer, for steel up to 325 BHN. - Grades M1 is used for Tape. - Grades M2 or M7 is used for gear cutting tools such as hob, gear shaper, shaver etc. - Grade M2 or T2 are used for form tools. 2) Conventional HSS of grades M33, M36, T4, T5, T6, with cobalt are used for heavy duty. They have better hardness & wear resistance but are less tough compared to M2. - Grades T4, T5, T6, are used as tool bits for planning & heavy-duty turning. - Grades M33 & M36 is used for interrupted cuts in tough & scaly forging, for drilling & milling hard alloy steels, titanium, stainless steels & heat resistant materials. 3) High vanadium high speed selects (represented by grades M3, M4, T15). They have higher hot hardness but are less tough than M2 grade & are more difficult to grind. They are used for single point lathe tools, screw machine tools, flat & circular form tools broaches drills etc. for machining of conventional alloys super alloys & refractory metals. 4) High hardness Cobalt steel (represented by grades M41 to M46) have exceptional secondary hardening properties and have very good hot hardness their hardness is much higher than conventional HSS. They are used for machining heat-treated steels, titanium alloys, and high hardness aerospace materials like cobalt & nickel base alloys.

Table 3.1: High Speed Steels: AISI Designat ion M1 M2 M3 M4 M7 M10 M33

e

W

MO

Cr.

V

Co.

0.80 0.85 1.05 1.30 1.00 0.90 0.88

1.75 6.00 6.00 6.00 1.75 1.75

8.50 5.00 5.00 4.50 8.75 8.00 9.50

3.75 4.00 4.00 4.00 4.00 4.00 3.75

1.15 2.00 2.40 4.00 2.00 2.00 1.15

8.25

M36 M41

0.85 1.10

6.00 6.75

5.00 3.75

4.00 4.25

2.00 2.00

8.25 5.00

M42

1.10

1.50

9.50

3.75

1.15

8.25

Conventional HSS. High Vanadium HSS. Conventional HSS with Co High hardness cobalt steel.

(25 to 35%), tungsten (10-25%) & carbon (1.0-3%). Cobalt acts as a solvent or matrix with chromium (which forms carbide) as a major alloying element. Tungsten contributes overall hardness of tool. Carbon increases hardness & wears resistance of the tool. Some times other alloying elements such as Vanadium (to further increase hardness & wear resistance by forming vanadium carbide), Molybdenum, tantalum, columbium (which may replace part of chromium and Tungsten) and Nickel (which increases toughness at the expense of hardness) may he added to them for enhancing specific properties. Cast alloys, have a high wear resistance surface with a soft core produced by the casting process. The structure exhibits long needles of chromium carbide oriented perpendicular to the surface at the periphery of the tool & at center it is oriented at random, resulting in soft core. The alloy carbides have properties intermediate between HSS & cemented carbides. They as less tough & more resistant than high-speed steel. They are used at cutting speeds ranging between that for HSS & carbides. Its important characteristics are high red hardness (up to 7600 C) low cost of friction, excellent resistance to corrosion & high resistance to shock & impact. 3.5.1 Applications: i) They are used for machining cast & malleable iron, stainless steels, non-ferrous metals, nitroloy, bronze, monel, graphites plastics. ii) Due to high shock resistance there performance is better than, carbides an interrupted cuts. iii) Due to castability used for making form tools. iv) Cast alloys can be used at cutting speed. 25-30% higher than for HSS for roughing cuts (Group A & B) & for speeds 30 to 50% higher (than for HSS for finishing cuts (Group C) v) They are used for machining plastics (which contain corrosive curing agents) because of their anticorrosive property (due to high chromium content). vi. They are suitable for operations where wide range of cutting speeds are encountered in a single cut e.g. parting, vii. They can be used in multiple tooling on automats where operations require surface speeds between HSS & carbides. In multiple tooling carbides are used on larger diameters where on the cast alloys are used on smaller diameters cast alloy tool are also used to small diameters jobs where a wide range of speeds are encountered in a single cut as in case of parting.

materials like cast iron. Later on additions of carbides of titanium; tantalum, niobium etc. extended their range of applications. 3.6 Manufacture: The basic manufacturing process of cemented carbides is shown in fig.3.4 and consists of following. i. Refining the ore & reducing in the hydrogen to get powder of W, Ti, Ta, Nb, etc. & Cobalt.

Fig.3.4 manufacturing process for sintered carbides.

i) P group identified with blue colour for (machining of long chipping materials like free cutting steel) available in various grade P01, P05, P10, P15, P20, P15, P30, P35, P40, P45, P50. ii) 'K' group identified with red colour for machining short chipping materials like cast irons & non-ferrous metals, available in K01, K05, K10, K15, K30, K40, iii) 'M' group identified with Yellow colour for general purpose applications, available in grades M05, M10, M15, M29, M30, M40 Hardness & wear resistance increases from K40-K30. . . . . .MK40. . . . . . M05. . . . . .to P50. . . . P01. . . . . i.e. from K to P whereas toughness increases for P to K. 'K' - grade carbides are essentially straight tungsten carbide with cobalt as binder. 'P' grade carbides are combined carbides, tool materials (Wc, Tic, Tac, Nbc) with Cobalt as binder. 3.6.3 Applications & properties: Carbides are used for applications employing higher cutting speeds on machine tools having sufficient power. However the rigidity of the machine tool, tooling & work piece is to be ensured. Carbides have very high hardness & wear resistance, transverse rupture strength (100-300 Kg/mm2) & low coefficient of thermal expansion but have less toughness. Corrosion resistance of carbide is better than that of binder thus strength of skeleton decides strength of cemented carbides & hence it is not used for machining stainless steels. i. Carbides are used for applications employing higher cutting speeds due to high hot hardness on machine tools having sufficient power. ii. Due to good hardness & wear resistance used for work rest blades, lathe centers, seal rings, nozzles, measuring gauges, scribers etc. iii. Due to low coefficient of thermal expansion. Used for making slip gauges & gap gauges. 3.6.4

Coated Carbides & Laminated Carbides: Resistance of straight tungsten carbides to crater wear cab be improved by addition of Tic. Hot hardness is also increased but compressive strength, impact strength & elastic modulus are all reduced by the addition of titanium carbide. The combined effect of good hardness, wear resistance & toughness can be obtained by applying coatings of Tic over Wc, Tip. In coated carbides the coating is applied on all faces of the tip whereas in laminated carbides coating is applied on rake face.

Layer of pure ultra-fine grain TiC (b) COATED Like Tic various other coatings are recommended Titanium Nitride multi-coating. (i.e. Titanium nitride over the Tic coatings), Hafnium nitride, Aluminum Oxide coating. Due to coating or lamination the life of cutting edge is increased wear resistance is increased & about 30-50% higher cutting speeds are permitted compared with carbides. 3.6

Ceramic (Cemented oxides) It appears that the word ‘Ceramics’ is derived from the fact that the materials are made by sintering at extremely high temperature approaching to that of pottery ceramics. Amongst the numerous ceramic tool materials available (viz, Aluminum oxide) silicon carbide, Boron carbide, Titanium Carbide, Titanium boride), the best results have been obtained with alumina (or aluminum oxide) or aluminum oxide 90% combined with other oxides (viz Cr2O3, MgO & Nio). Ceramics are very hard & have a high degree of compressive strength (30000-35000 kg/cm2 even at elevated temperatures. They have good abrasive wear resistance, low coefficient of friction & can retain cutting edge hardness up to about 7600 (& exhibit uniform strength up to 6000C) permitting higher cutting speed (800 m/min). They have low transverse rupture strength (one –half to one – third) of that of carbides) restricting their applications in interrupted cuts only. But through better raw materials, composition modifications & closer control over process variables can be used in interrupted cuts of light to medium severity on cast iron & steel work pieces. Ceramics have poor weldability & hence permit their use on high abrasive & reactive materials at higher cutting speeds. But due to poor weldability brazing of tip with steel shank is not possible. This problem can be solved by epoxy resin cementing of ceramic tools to steel shanks. Low thermal conductivity produces some problem of thermal shock due to rise of thermal gradient but is not of that severity. Cermets are acid resistant, non-magnetic, non-conducting & noncorrosive. They have more flexibility of machining i.e. wide range of materials can be machined at various speeds ranging from 200 rpm to 800 rpm. There are two main

grinding of blanks can be carried out with diamond-impregnated wheels. To get strong hard & highly wear resistant tools, it is necessary to control grain size, density & percentage of impurities. 3.7.2 Machine tool & tool requirements: Due to brittleness of ceramics, a very important consideration in use of ceramic tool is the rigidity of machine tool. Vibration & chatter should be eliminated wherever possible. The tool holder should be rigid & there should be complete contact between the ceramic inserts and their seating surfaces. Not only high speed ranges & horsepower but also rigidity & accurate balancing of machine tool parts & work holding devices are important for proper exploitation of potentialities of ceramic tools. Minimum spindle run out is essential. The tool design for ceramic tools is also very important. Use of negative narrow land ground on cutting edge of tool substantially decreases the tendency of tool tip to fracture. The lands are called K lands & their width varies form 0.2 to 0.3 mm. 3.7.3 Applications: 1) Ceramics provide good surface finish and quality, eliminating finishing operations like grinding, cast iron, known for its abrasive characteristics, can be machined to smooth bright finish using ceramics. 2) Heat treated steels up to 65 Rc can be finished up to 5 µm surface finish, often eliminating the grinding operations, with a considerably improved tool life vis-à-vis carbide. 3) High temperature alloys such as hastelloy, stellite & Monel can also be machined by ceramic tools. 4) Ceramics are used for roll turning, long tube boring, cylinder liner boring, etc. 3.8. Diamond Tools: Diamond because of its high modulus of elasticity, chemical inertness & exceptionally high hardness is ideal for obtaining fine surface finish & accuracy. Though initial cost of the tools in high, the cost per piece is less due to very high tool life. It can be used as cutting tool material, in the form of either a single crystal or a polycrystalline compact. The single crystal diamond may be natural or synthetic. It has both hard & soft axes. If the diamond is used in the direction of soft axis (or parallel to cleavage plane) it wears out quickly. Polycrystalline diamond is produced by sintering very fine powder of diamond at high temperature or pressure. Some times polycrystalline diamond compacts are brazed to each corner of carbide insert. Because of random orientation of diamond particles in diamond compact, any direction can be used for cutting (as there are no hard or soft axes)

0.1 mm/rev & depth of cut should also be limited to 0.5 mm. The cutting speed for single point diamond tools for machining of carbides is 15-25 m/min, for ceramic is 30100 r/min for pure aluminum it is 500-600 m/min. 3.8.1 Applications: i) The diamonds of various forms are used in industrial application such as in grinding wheels, dressing tools, dressing dies, hones, lapping compounds, core drills etc. ii) As a cutting tools single crystal diamonds is used for machining non ferrous metals like aluminum, brass, copper & bronze etc. especially where high silicon content is involved. iii) It is also used for non-metallic materials like plastics, epoxy resins, hard rubbers, glass & also precious metals like gold silver & platinum. iv) Polycrystalline diamond is widely used for machining glass, reinforced plastics, eutectic & hyper – eutectic alloys etc.

Fig. 3.16 Hardness chart for tool materials

toughness. It also exhibits excellent resistance to diffusion & adhesion wear. It is aimed to give 3-5 times more edge life than conventional carbides. It can operate at a cutting speed of 250-500 m/min on steels up to 200 BHN. 3.9.1 Appications: 1) It is recommended for roughing semi finishing & finishing cuts in turning, facing & boring. 2) It is not generally applied to milling, parting off, or for operations, using form tools, 3) It Is basically steel machining grade & not recommended for machining of Iron, stainless steel & super alloys having nickel, cobalt & titanium base.

chemically inert, but an atmosphere of steam it breaks down chemically to boric acid and ammonia 9000 C. (But it has no reaction with oxygen like diamonds at 8000 c) 3.10. Application: (1) CBN is successfully used as a grinding wheel, on HSS tool providing good surface finish, precision high output also on titanium, nitronic, stainless steel stellities. 2. It is used in grinding of hardened steel in the form of lead screws bores, splines, thereales, ball & roller bearing part .CBN cut cool & grinding affects such as burrs of thermal shocks are net produced. 3. They are also used for grinding the sideways of cast iron beds of housing type components. TOOL MATERIALS Q.1 Q.2. Q.3.

Give a brief account of history of development various tool materials. What are the three basic requirements of cutting tool materials ? What is hot hardness and recovery hardness and compare them for various tool materials. Q.4. Define AWR, RBF & HTS. Compare them for Tic, Wc and HSS Q.5. What are the various properties requirement by tool materials ? Q.6. List of various tool materials used in metal cutting. Enlist their advantages and disadvantages. Q.7. Give the composition properties and applications of following tool materials. 1. Carbon tool steels. 2. High speed steels. 3. Cast cobalt alloys. 4. Ceramics 5. Diamond. 6. UCON 7. CBN Q.8. What are the merits of HSS tools manufactured by PMT ? Q.9. Compare the ceramic and carbides as a cutting tool material. Q.10. How are the cemented carbide tools produced. Q.11. What are the coated and laminated carbide tools ? Q.12. What are the various grades of carbides ? What are the application of various grades of carbides.

production cost ( or cost/piece ). At very slow cutting speeds. The time required for production of one component will be high thereby increasing machining, labor & overhead costs. But at very high cutting speeds, the wear of tool will be faster, frequent tool changing, regrinding & setting, leading to higher cost. The effect of cutting speed in unit production cost can be seen from Fig.4.1. The unit cost is obtained by summing up the individual costs.

4.2. Optimization criteria As seen earlier the unit production cost varies with cutting speed. So it was felt necessary to find out some criteria for optimum cutting speed. W.W. Gilbert has evaluated. Two cutting speed (and hence tool lives) I. Cutting speed (or tool life) for maximum cost II. Cutting speed (or tool life) for maximum production.

a) Machining Cost: This is the cost of labor rate plus overhead rate required for the time required for cutting per piece. b) Handling (Idle) Cost: This is the cost of labor rate plus overhead rate required for the time lost in loading and unloading of job. c) Tool Changing cost: This is the cost direct labor rate plus overhead rate required for the time lost in changing of tool. In order to estimate this cost on per piece basis this cost is multiplied by the factor tool failure per piece. d) Tool Cost: This is the product of cost of tool lost per job turned with tool failure per piece. The cost of tool per grinding (in case of HSS tools) or cost of tool/per edge (in case of indexible carbide tips) can be estimated as under: For HSS Ce = Tool cost No. Of regrinds + 1). And Ce = (Cost of Tool bit/No of cutting edge) + tool holder depreciation per cutting edge. The total unit production cost can be estimated for a simple turning operation by summing the above individual cost components for a simple turning process as under. Let L = Length of machining, mm, D = Diameter of Machined part, mm. V= Cutting speed, mm/min, f = Feed, mm/revolution. T = Tool life, mm, T = Machining time, min/piece. Th= Handling time, min/piece, T = Tool changing time; min. Ti = Idle time, min/piece, C = cost per cutting edge, Rs/edge. Cu = Direct labour rate + overhead rate, Rs/min. Now, Machining time, Tm = L/f.N., min = π DL/1000.f. V, min. = K/V, min. Where K = π . D.L./1000f Tool life, T = (C/V) 1/n, min Therefore, Tool failure per piece, = Tm/T = K.V.1/n-1/C1/n Thus, Cp = Machining cost + Handling cost + tool changing cost + Tool cost. Cp = CuTm + Cu.Th + Cu .Tc (Tm/T) + Ce (Tm/T) Cp = Cu ( K/V + Th + K.V.1/n-1. Tc/C1/n) + Ce.K.V.1/n-1/C1/n ….. Let the total production time per piece is given by To = Tm + Th + Tm.Tc/T

4.1

We have, CP = Cu (K/V + Ti + Th + K.V.1/n-1. Tc/C1/n) + Ce.K.V.1/n-1/C1/n dC p = 0 , Gives ∴ dv Cu (-K/V2 + 0 + (1/n-1).V1/n-2)K.Tc/C1/n+ (1/n-1) V1/n-2.K.Ce/C1/n = 0 -V-2 + (1/n-1) V (1/n-2). Tc/C1/n + (1/n-1) V1/n-2.Ce/Cu.C1/n = 0 (1/n-1).V1/n-2/C1/n.(Tc + Ce/Cu) = V-2 V(1/n-2+2) = C1/n/(1/n-1) (Tc +Ce/Cu) V1/n = C1/n/(1/n-1) (Tc + Ce/Cu) Or Vopt =C/[(1/n-1) (Tc + Ce/Cu)]n . . . . . . . . . . 4.3 Putting value of V = Vopt in Taylor’s tool life equation, we get . . . . . . . . . . . . . . . . 4.4.3. Topt = (1/n-1) (Tc+Ce/Cu) This cutting speed correspondence to lowest unit production cost (Fig.4.1) 4.5 Cutting speed for maximum production rate: The cutting speed for maximum production rate can be estimated by neglecting Tool cost (Ce) i.e. in the eq. 4.3 if we put Ce = 0 we get, the cutting speed for maximum production rate i.e.

V’opt = Vopt at Ce= 0 or V’opt =[C/(1/n-1) Tc]n

. . . . . . . . . . . . . . 4.5

Similarly, the tool life for this cutting speed ′ = (1 / n − 1).Tc Topt

. . . . . . . . . . . . . 4.6

The cutting speed for maximum production rate can also be found by differentiation of rate of production w.r.t. Cutting speed & equating to zero as under from eqn.4.2 we have Rp = 1/C (Tm + Th + Tm.Tc/T) = 1/(K/V + Th + (K/V).Ta/C/V)1/n For maximum production rate

C'opt

C'opt>Copt R'opt>Ropt

Copt R'p Rp

production rate

V'opt The high efficiency range may be wideVopt or narrow depending on job conditions. If tool cost is relatively low for a given job the Vopt may be 5% to 10% higher. Where tool cost is high end points of this range can have 30% to 40% differential. It is always preferable to operate at a cutting speed greater than Vopt instead of a slightly, smaller value because at least with slightly higher production cost than Cp corresponding V ′ , we get higher production rate. The increase in production rate and hence increase in revenue may offset the increased unit costs. To investigate this effect, an alternative model based on maximum profit criterion has been developed.

4.7.

Optimum Cutting speed for maximum profit rate:

In the earlier sections we have seen the two criteria viz. minimum cost criterion & maximum production rate criterion. At the cutting speed for minimum cost production rate may be too low to maximize profit rate. While at cutting speed for maximum production rate the cost of production may be too high and hence profit margin too low. Therefore, the cutting speed for maximum profit rate would be different from that for minimum cost speed and maximum production rate speed. 38.3 ECONOMIC CUTTING SPEED (additional material) An increase of cutting speed has two main effects upon the economics of cutting; the metal removal rate is increased, the too] life is decreased. An increase in the metal removal rate will lower the direct cost of metal removal; a reduction in the tool life will increase the costs of servicing and replacing worn-out tools. The two separate effects, and their combined influence upon the total cost of machining, are best illustrated graphically as shown in Fig. 38.1. The following deductions can be made from Fig. 38.1 (a) As V increases, the time required to remove the metal (and hence the cost of its removal) will fall. The cost of cutting ∝ 1/V.

38.4 ECONOMICS OF METAL REMOVAL When a manufacturing process consists of removing metal with a single point tool, the type of tool used or cutting speed chosen can have an effect upon the total cost of the product. It is worth considering this because the removal of metal in this manner is still a major process in the engineering industry. In a roughing operation the object is to remove a certain volume of material at minimum cost or minimum time, or maximum profit, and the type of tool and cutting speed should be chosen accordingly. In a finishing operation the object is to improve a certain area of material until it is of the desired quality of finish. In the following discussion the chosen criterion is the removal of certain volume of material at minimum cost. Again it should be emphasized that the analysis used to obtain the optimum conditions is worthless unless the cost information used is relatively accurate. F. W. Taylor introduced the well-known relationship between the cutting speed used in a metal removing operation and the life of the tool, viz., VT" = C ...(1) where V = cutting speed T= tool life (Although in basic SI units meters and seconds should be used, meters and minutes are the practical units) n = an index closely related to the cutting tool material, and the following values may be used : 0.1 to 0.15 for high speed steel tools 0.2 to 0.4 for tungsten carbide tools 0.4 to 0.6 for ceramic tools C = a constant

The slope of the straight line will give the value of n, and hence a value for C can be obtained. —It can be seen that if cutting speed V is increased, then tool life T will decrease. Hence, metal is removed faster and therefore more cheaply. But tool life is shorter and therefore tools replacement and servicing are more costly. This cost situation is shown in Fig. 38.4. VT = Optimum cutting speed where the total cost of machining a batch of components y is at a minimum. In order to find an expression for VT the tooling cost and metal removal cost (or machining cost) must be added to give the total cost. Then by calculus the turning point of the curve and hence VE can be found. Let H = machining cost/minute i.e., labour cost/minute + over- heads/minute. Let J = tooling cost i.e., cost of changing tool + cost of regrinding + tool depreciation. Let y1 = cost of machining metal/unit volume of metal cut. Let y2 = cost of servicing tools/unit volume of metal cut. Let y = total cost/unit volume of metal cut = y1+y2. =

1 dfV

Where d = depth of cut

f = feed in length/rev V = cutting speed Therefore y1 = cost of machining metal/unit volume of metal cut. = (time to machine a unit volume of metal in minutes) x (machining cost per minute) or

y1 =

1 ×H dfV

Since d and f are constants therefore y1 =

HK V

1 K = where K is a constant. dfV V

The number of tool changes in

K K minutes = where T is tool life in minutes at V TV

cutting speed V. ∴ ∴

∴

Differentiating

JK y2 = TV y2 =

But JK

C   V

C T=  V =

1/ n

×V

JK (V)

1/ n

1− n n

C1 / n 1- n n

HK JK(V) + y = y1 + y 2 = V C1/n

…..(2)

machining costs

∴

1 1− n   V  = .   R  n  C

∴

V   C

1/ n

=

H

1/ n

n R (1 − n )

∴

 n  V = C   R (1 − n ) 

n

…(4)

=VT at the minimum This expression will enable VT to be calculated so that the optimum cutting speed can be found to give minimum cost YT for the batch. It should be noticed that n from Taylor's equation is important in this equation, hence the need to obtain its value accurately. In this analysis we have not included the costs of handling the tool. From eq. (1) V   C

1/ n

=

1 T

1/ n

n V =  C R ( 1 − n)   1 n = T R (1 − n ) R (1 − n ) T= n T (1 − n ) = R n

and from eq.(4)  Hence Or

Since R= ∴

J (eqn .3) H T  T.H   1 − n  = .   R  J   n 

38.5 MINIMUM COST/COMPONENT - The following costs are involved in metal cutting: (i) Machining costs (ii) Tool costs (iii) Tool changing costs

…..(5)

Machining cost = C0 fm Tool cost

= CT .

...(a)

tm T

…(b)

Tool changing cost = C0 t c .

tm T

…(c)

Handling cost = C0 th Where C0 = operating cost (Rs/min) tm = time required to machine the work piece, (min) Ct = tool cost per cutting edge, (Rs) T= average tool life, (min) tc = tool changing time (min/operation) th = handling time (min) (for loading and unloading of work pieces). Average unit cost (Cu) = (a) + (b) + (c) + (d) per piece C u = C 0 .t m + C t . C u = C 0 .t m +

t tm + C 0 .t c m + C 0 .t h T T

tm + (C t + C 0 .t c ) + C 0 .t h T

…(6)

There are two cost factors in this basic model (C0,Ct) and three time factors (tm,tc,th) in addition to the tool life factor. Each of these factors will be discussed briefly. Operating Cost (C0). The operating cost equals the sum of the machine operator's rate plus appropriate overhead. Tool Cost (Ct). The tool cost is the Cost of the insert price and a prorated cost per cutting edge of the complete tool holder and any other parts such as chip breakers, shim

remove a tool that has failed and replace it or index it, reset for size, and be ready for the next cut. Handling Time (th). The handling time is the time in minutes required to load and unload the work piece from the machine. It includes the idle time and time necessary to advance and retract the tool. Tool Life Factor. Tool life is taken from Taylor's equation, VTn = C. The average tool life (T ) in minutes per cutting edge is : C T=  V

1/ n

…(8)

38.6 DETERMINATION OF CUTTING SPEED FOR MINIMUM COST (Vmin) The total cost for an operation is made up of the four individual costs: machining cost, tool costs; tool changing costs, and the handling costs. The interaction of these factors was shown in Fig. 38.5. The cutting speed for minimum cost of a given operation is derived by equating the total cost to the sum of the four individual costs, differentiating the costs with respect to the cutting speed, and setting the result equal to zero. Vmin =

C  1  C 0 t c + C t  − 1 C0  n 

   

…(9)

n

38.7 TOOL LIFE FOR MINIMUM COST (Tm) The minimum-cost cutting speed, Vmin in, is a function of the operating time costs, tool costs, and tool changing time, and is a function of the n and C parameters in Taylor's equation. Since the constant C in Taylor's equation and in equation (38.9) are the same, and if V corresponds to Vmin, then the tool life that corresponds to the cutting speed for minimum cost is: 1 n

 C 0 t c + C t C0 

Tmin =  − 1

   

…(10)

38.8 CUTTING SPEED FOR MAXIMUM PRODUCTION There are times when it becomes necessary to speed production beyond the point of the recommended minimum cost. In this case, it is necessary to know what the

1 n

 

Tmin =  − 1 t c

…(12)

The tool life at maximum production rate is a function only of n, the slope of the curve in Taylor's equation, and the tool changing time. Thus for an HSS tool (n = 0.1) with a tool changing time of 1 min, Tmax =9 min; that is, the tool should last only 9 min. A carbide tool, where n= 0.25 and I min is needed for tool changing, should only last 3 min. 38.10 MAXIMUM PRODUCTION RATE The unit time required to produce a work piece, t t p = t m + t c  m  T

  + t h 

…(13)

Where fm is the machining time tc is the tool changing time th is handling time (idle time) T is tool life. The minimum unit time will result in maximum production rate as Q = Production rate =

1 tp

…(14)

The maximum production rate will correspond to the minimum production time per piece as obtained by differentiating Eq. (38.13) w.r.t. Cutting speed after substituting time-cutting speed relationship (refer back). ∴ Cutting speed for maximum production rate is given by Eq. (38.11) and Tool life for maximum production rate is given by Eq. (38.12) These optimum values are dependent on index, n, and tool changing time, tc .

38.11 MAXIMUM PROFIT RATE Profit rate (PR) = Profit per piece ( P R × Q ) where g is production rate per unit time. Now

Q= =

1 tp

(refer eqs. 38.13 and 38.14) 1

t t h + t m + t c  m  T

  

The total cost (CT) per piece is t C T = K 1 t h + K 1 t m + K 1 t c  m  T

…(15) Where K1 is direct labour cost (Rs) K2 is tool-grinding cost (Rs) th is handling (idle) time per piece (minutes) tm is machining time per piece (min) tc is tool changing time (min) Taking income per piece as I PR =

∴

PR = 1− CT Q

PR = P R .Q = (1 − C T ).Q =

1− CT tp

Substituting the value of CT from eq. (15) above

 t  + K 2  m   T

  

 T  −K 1  tm  tm + tc   + th T  λ πDL = 0 tm = 1000.f .V V πDL λ0 = 1000.f

PR =

Nothing Where

VTn = C0 (Taylor’s equation) Where D is diameter of machined part, mm L is length of machining, mm f is feed, mm/revolution V is cutting speed, m/minute C0 is a constant The equation for profit (16) reduces to I− PR =

K 2λ0 C10/ n

t h + λ0V

−1

+

.V (1 / n ) −1 t cλ0 C10/ n

V

(1 / n ) −1

− K1

…(17) To maximize the profit rate, let dPR =0 dV

from which the following condition is derived (1 − n )[K 2 t h + I − t c ].V 1 / n + λ 0 K 2 V 1 / n −1 − nC10/ n .I = 0

…(18) For a known value of Taylorian exponent, n, the Vopt for maximized profit can be numerically obtained. SOLVED PROBLEMS: Problem 1: Brass components 75 mm long x 50 mm diameter is to be machined on an automat, using a depth of cut of 1.25 mm. Select speed that minimizes machining cost and calculate the corresponding tool life. Also estimate the cutting speed for minimum time of production. Assume that Labour plus overhead rate = Rs. 5/hr. Reconditioning cost of tool edge = Rs.0.25/edge

opt

Topt

[(1 / n − 1) (Te + C e / C u )]n

[(1 / 0.25 − 1) (5 + 0.25 / 0.8)]0.25 And

= 135.6m / min = (G / Vopt )1 / n = 24.0 min.

Similarly,

′ = Vopt at C e = 0 = 152.4m / min and Topt ′ = 50 min . Vopt Problem 2: Mild steel work piece 150 mm long x 100-mm. diameters are to be turned on the lathe using a feed of 0.15 mm/rev. And depth of cut of 2.5 mm., using brazed carbide tipped tool. Find out production cost per piece, cutting speed & tool life for minimum cost of production and maximum production rate, minimum cost of production and minimum production time. The following relevant data is available. 1) 2) 3) 4) 5) 6) 7)

Purchase cost of tool = Rs 110/No. Of regrinds = 10 Labour + Machine + 0.1 rate = Rs.30 per hour. Tool grinding cost = Rs. 2.50/edge. Tool changing time = 5 min. Idle time = 3 min. Tool life equation is V. T.0.25 = 150

Solution: From above data we have Cu = 30/60, Rs/m = 0.50, Rs/min. Ce = 110/10+1 = 10.0, Rs/edge. Cg = grinding cost = 2.50, Rs/edge. Tc = 5 min, . Ti = 3 min. Tm = machining time per piece = π D. L./1000 f.V = π.100.150/1000 x 0.15 x V. = 100.π, V-if-min. 4 -4 T = (C/V)1/n = 1501/0.25/V1/0.25 .V Tool failure per piece = Tm/T-100 π V-1/(1504.V-4) = 100 π V3/1504, Now the cost of production per piece Cp is given by Cp = Idle cost + cutting cost + Tool changing cost + Tool cost + Tool grinding cost. 1) Idle cost = Ti x Cu = 3 x 0.50, = 1.5 Rs./piece 2) Cutting cost = Tm. Cu = 100. π. V-1 x 0.5 = 50 π V-1 Rs/piece 3) Tool changing cost = Tc. (Tm/T) Cu = 5 x 100 π V3/1504 x 0.5

Cp =

1.5 + 50 x Π (48.7)-1 + Π (48.7)3/1504 x (250 + 1000 + 250, 1.5 + 3.22 + 0.0007163 (1500) 5.795, Rs/piece.

Cutting speed for maximum production rate is given by ′ = C / 1 / n − 1, Tc , n = 150 /(3 x 5) .25 = 76.21 m / mm. Vopt ′ = (1 / n − 1) Tc = 1.5 min . Topt Minimum production time = Idle time + Cutting time + tool changing time, = 3 + 100.Π/76.21 + 5 x (100 Π V 76.21)3/1504 = 3 + 4.12 + 1.373 = 8.49 min. Problem 3 : In machining mild steel workpieces stated in earlier problem if the indexible inserts are used the following data is available. Compare the unit cost of machining attainable from the two types of tool and the minimum production time. Tool tip cost = Rs.15/Number of cutting edges =6 Tool holder cost = Rs.350/Number of edges/holder = 700/Solution : From data we have, Cu = 0.50, Rs/min, Ce = (15/6 + 350/700) = (3.0), Rs/edge. Tc = 1, minute Tm = Π D.L./1000 f.V = 100 Π V-1, T = (C/V)1/n = 1504. V-4 Tool failure per piece = Tm/T = 100 Π V3/1504 Cp = 3 x 0.50 + (100ΠV-1) x 0.5 + 1 x (100Π V3)/1504) x 0.5 + 3.0 (100ΠV3/1504) DCp/dV = 0 gives, 150 Π V-2 = Π.150 V2/1504 + Π.900 V2/1504 1504 = ( 3 + 18 ) V4 i.e. V4 = 1504/21 Vopt = 70.07 m/min. Topt = (1/n – 1) Tc = ( 4 – 1 ).1 = 3 min. Cp minimum = 1.5 + 2.24 + 0.107 + 0.638 = 4.48, Rs/piece. ′ = (150 /(1 / n − 1).Tc ) 0.25 = 113.98 = 114 m / min . Vopt ′ = (1 / n − 1), Tc = 3, min . TOPT Minimum cutting time, T = Idle time + Machining time + Indexing time 3 + 100 Π (70)-1 + 100 Π (70)3/1504 x 1 = 7.69 min.

5. 6.

Derive the relation for cutting speed for maximum production rate & from that find out tool life for maximum production rate, A 600 mm long cut is to be made on a 150 mm diameter AISI – 4140 steel bar in a lathe with a depth of cut of 1.5 mm and feed of 0.25 mm/revolution. The Taylors tool life equation is given by

VT0.22 = 475 For above machining operation, two types of tools may be used : (1) brazed tool and (ii) threaway carbide inserts. The following cost data have been collected. a) Machine cost : Machining cost = Rs. 1.00/hour. Machine overhead = 100% of labour. Grinding cost (labour) = Rs. 1.50/hour. Grinding machine overhead = 200 % of labour Idle time = 5 min. b) Tool cost : For brazed tool Initial tool cost . . . Rs. 8.00 Grinding time . . . . . 5 minutes/edge Tool changing time . . . 2 minutes No. of possible regrinds . . . . . 9 i) ii)

For throwaway inserts. Original cost Rs.3.00 Tool changing time = 1.5 min. Total Cutting edges = 8

Plot the cost per piece as a function of cutting speed and hence find the cutting speed for minimum cost for each case. Computes and compare the tool life for minimum cost per piece and for maximum production rate for both type of tools.

7) In machining a component on Automat following data is available estimate the minimum cost of production and the high efficiency range of cutting speed. Length of component, L = mm., Diameter of component, D = mm Depth of cut, d = 1.25 mm, Feed rate, f = 0.15 mm/revolution Idle time, T1 = 1 min/piece, Tool changing time, Tc = 5 min. Labor Plus overhead rate, Cu = 0.1 Rs./min. Tool cost per cutting edge C = 0.25, Rs/min. Tool life relationship, V.T0.25 = 300

selection of tool angles & cutting variables etc, have been described. Classification of cutting tools: Depending upon the number of cutting edges, the cutting tools used in metal cutting are classified as follows. 1. Single point tools - having only one cutting edge and. 2. Multiple points - having more than one cutting edge e.g. milling, reamers, drills, broaches, grinding wheels etc. The single point tools can be classified into various types, depending on various criteria as under. a) According to construction - Solid Brazed tip and Throway Tip. b) According to type of operation - Turning, facing. Boring, Knurling, Threading, parting, forming. c) According to Shape - Cranked, straight, circular, square, d) According to usage on machine tools - Lathe tools, Shaper tools, Planner tools, boring tools. e) According to direction of cut - Left hand cut tool. Right hand cut tools. Single point tool - various parts : A conventional single point tool has sharpened cutting part called its point. The point of the tool is never sharp but is given small radius (called nose radius). The point of the tool is bonded by rake face (along which chips flows on topside, principal flank (or major flank or side flank) on one of the sides-of the tool and auxiliary flank or end flank on end of the tool. The edge formed by Intersection of face and side flank is called side cutting edge (or principal cutting edge or major cutting edge, where as edge formed by Intersection of face and end flank is called end cutting edge (or auxiliary cutting edge or minor cutting edge). The point where the end and side flanks meet is called nose. The bottom portion, of the tool is called base. The position behind the point portion is the holding or mounting portion of tool called shank. The shank may be square rectangular or circular in cross section depending on the type of tool.

reference system. for defining various angles three mutually perpendicular planes are used. (Like conventional drawing practice). These planes are 1) Machine longitudinal plane – (or tool transverse plane) 2) Machine transverse plane – (or tool longitudinal plane) 3) Basal plane perpendicular (1 & 2) (i.e. along the base) In this system as the rake angles are specified in co-ordinate system it is easier to calculated the setting angle for grinding fixture in terms of back and side rake angles. But it has a drawback that the angles are not related to actual position of cutting edge.

The various angles specified in this system are defined below. i) Back rake angle Angle between face & base of tool measured in plane perpendicular to base & parallel to axis of tool (m/c transfers plane) ii) Side rake angle Angle between face & base measured in plane perpendicular to base & longitudinal axis of tool (m/s longitudinal plane) iii) End relief angle Angle between end flank immediately below end c.e. & a line perpendicular to base of tool measured in m/c transverse plane. iv) Side relief angle Angle between side flank immediately below side c.e. & a line perpendicular to base of tool measured in m/c transverse longitudinal plane. v) End cutting edge angle Angle between end cutting edge & a line normal to tool

American Standards Association (ASA) System The American Standards Association (ASA) system specified the tool face by defining its slope in two orthogonal planes : one parallel to, the other perpendicular to, the axis of the cutting tool, both planes being perpendicular to the base of the tool. The two angles thus specified are known as the tool back rake and tool side rake . — In this system, like the British maximum-rake system, the angles were specified independently of the position of the cutting edge and, therefore, gave no indication of the behavior of the tool in practice. The advantage of the system was always considered to be the simplicity of its use in the grinding of single-point cutting tools, and yet, a tool cannot be ground accurately to the back rake and side rake without using equations or curves.

Orthogonal Rake System (ORS or DIN) In this system the various angles (rake) clearance etc. are measured in different planes than that used in ASA system. In DIN system, the back rake angle is measured in a plane which is normal to the base plane but parallel to the trace of side cutting edge in base plane. Like wise the side rake angle is defined as the angle between the rake face & the base plane measured in a plane normal to the trace of side cutting edge in the base plane. The system satisfies the desirable conditions that the two rake angles are specified in relation to respective cutting edges. The drawback of this system is that it needs some calculations to obtain setting edge angles on tool grinding fixture. German System The German (DIN) system also specified two rake angles, called back rake and side rake (Fig), but in the German system the angles were related to the position of the cutting edge. The German back rake was the slope of the cutting edge measured in a plane containing this edge and perpendicular to the tool base; the German side rake was the slope of the tool face measured in a plane that was both perpendicular to the plane in which back rake was measured and perpendicular to the tool base. This system had some physical meaning in relation to the cutting process because both angles were specified in relation to the edge of the tool that performs the cutting operation. —A difficulty arose, however, when the system was used for grinding a cutting tool. The problem is similar to that occurring in the American system.

ISO System (NRS) : In this system, the side rake angle is defined as the angle between the base plane of the tool and the rake face of the tool measured in a plane normal to the side cutting edge. The back rake angle is the angle between the base plane & the rake face measured in a plane normal to the end cutting edge. Stabler suggested backs rake system because he found a good correlation between the cutting forces & the normal rake angle (i.e. side rake angle of NRS) in oblique cutting. He also showed that the tool rake face could be ground by directly setting the angles specified in this system on the tool-grinding fixture. International System (ISO Normal Rake System) — The new ISO recommendation for the nomenclature of cutting tools first establishes systems of planes that can be used to define the various angles of the faces and flanks of the tool. It should first be understood that two systems of angles and planes are required. The first system is the tool-in-hand system of planes and angles and refers to a cutting tool that is held in the hand and is used for the purposes of grinding and sharpening the tool. The second system is the tool-in-use system of planes and angles and refers to the cutting tool being used in a machining operation. The reason two systems are required is twofold : First, in a simple turning operation as the feed is increased, the effective rake angle increases, and the effective clearance decreases; second, it is possible that a cutting tool (particularly a single-point cutting tool) can be held in a machine tool in various orientations, thereby altering the effective angles of the tool.

Thus the tool-in-hand system is defined in relation to the tool base (or, for rotating tools, to the tool axis). The tool-in-use system, on the other hand, is defined in relation to the resultant cutting direction and the direction of feed motion. As explained above, there are two systems (1) Tool-in-hand system (2) Tool-in-use system. For identification, planes and angles in the tool-in-hand system are prefixed by the word tool and those in the tool-in-use system are prefixed by the word working. Figs. show the various tool angles and working angles for a single point cutting tool. British System (Maximum rake system or MRS ORBS) : In this system the rake angle is specified as the steepest slope of the rake face. It is equal to the angle between the rake face & a plane parallel to base measured in plane perpendicular to base of tool. It has been suggested that the system could have been based on the supposition that the chip flows in the direction of the steepest slope (direction MM). However, this assumption has been proved to be wrong. The main advantage of this system is that the specified rake angle is easy to set on tool grinding fixture for grinding the rake face. The angle of chip flow & therefore effective rake angle is however difficult to estimate. Also the rake angle has no relationship with the actual position of cutting edge.

The British system had the advantage that the specified angles could be set on a grinding vise and the face ground to the specified angle. The difficulty with this system was that the angles specified were quite independent of the position of the cutting edge, and, therefore, complicated expressions or a set of curves had to be used to estimate the direction of chip flow. It has been suggested that the system developed from the idea that the chip flows in the direction of maximum slope of the tool face. Since this idea is not even approximately true, the system had no physical significance in relation lo the cutting process. (Face & flank orientations for different nomenclature systems (comparison) Item

1) Location of cutting edges. 2) Orientation of face. 3) Orientation of principal Flank. 4) Orientation of Auxiliary Flank. 5) Nose radius.

Tool Reference system: Orthogonal φ , φe γo , λ

Tool Reference System Normal φ , φe γ n,λ

Machine Reference System American φs ,φe γx , γy

British System φ , φe γ max , φ γ

α0 α'0 r

αn α'n R

αx, αy r

αx, αy R

Tool Signature: (for various systems): The various angles are specified in a particular order viz. rake angle (Back, side), Relief angle (end, side), cutting edge (end, side) radius. For various systems the tool angles may be specified in the following sequence called as tool signature. The shape of a tool may be specified in a special sequence as given 1. American system γy - γx - αy -αx - φe- φs – r (ASA) 2. Orthogonal rake system λ - γ0 - α0 -α'0 - φe - φ – r (ORS) 3. Normal rake system λ - γn - αn -α'n - φe - φ – r (NRS)

However these relationships embodied in eqn 2.32 through eqn.2.35 relating γ0 and λ with γx and γx are dependent on the signs of the angles γ and λ. Significance of Various Tool Angles :

1. Rake Angle (back & side) : The rake angles are given for easy removal of metal chips and are different for different materials. The cutting angle (δ) and shear angle are affected by rake angles. Larger the rake angle, smaller the cutting angle (& larger the shear angle) & the lower the cutting force & power. However the strength of tool & heat dissipation capacity reduces when rake angle is increased. Thus the practical values of rake angles are selected after compromise between larger values for easier. Cutting & small values for strength. In general rake angle is small for cutting hard materials & large for cutting soft ductile materials. An exception is brass, which is machined with small rake angle for preventing digging of tool in work. When we use positive rake angle, the force on tool is directed towards the cutting edge, tending to break it. Carbide being brittle lack shock resistance & will fail if positive rake angles are used on it. Using negative rake angles, directs the force back into the body of tool away from the cutting edge. The use of negative rake angle increases the cutting force. For higher cutting speeds at which carbide cutting tools can be used, this increase in force in less than at normal cutting speeds. High cutting speeds are therefore always used with negative rake, which requires ample power of machine tool (Fig.). The use of indexible inserts has also promoted the use of negative rake angles. An insert with negative rake angle has twice as many cutting edges as an equivalent positive rake angle insert.

the cutting force and heat produced over larger cutting edge. This angle varies from 0 to 900 as SCEA is increased effective cutting edge length for same depth of cut increases (increasing tool life), thickness of chip, reduces, width of chip increase. On the other hand, the larger is the value of SCEA, the greater is the component of force tending to separate work & tool. This tends to promise chatter. For general machining SCEA 150 to 300 is recommended. The shape of work piece also decides SCEA. To produce 900 shoulders, zero degree SCEA is needed. No SCEA IS required for machining castings or forgings with hard scaly skins, because the least amount of tool edge should be exposed to destructive action of the skin. 4) End Cutting edge angle (ECEA) (80 – 150): It provides a clearance or relief to the trailing end of the cutting edge to prevent rubbing or drag between machined surface & trailing end only a small angle is sufficient for this purpose. Too large ECEA takes away the material that supports the point & conducts away the heat. An angle of 80 to 150 has been found satisfactory. Some times small flat (1.6 to 8 mm long) is ground in the front portion next to nose radius to level the irregular surface produced by a roughing tool. End cutting tools like cut off necking tool have no ECEA. Design consideration in single point cutting tool: In the design of single point cutting tool following points are considered. 1. Selection of type of tool material. 2. Selection of tool geometry. 3. Assigning cutting variables (depth of cut, feed, cutting g speed) 4. Calculation of shank dimensions from strength and rigidity considerations. 1. Selection of tool material: A review of history of tool material shows that a new tool material seldom fully replaces an old one. No single tool material has desired properties to withstand the wide range of stresses, abrasion, impact and thermal shock to which a cutting tool is subjected during metal cutting. Each cutting tool has a unique combination of properties that are important for its performance. Plane carbon tool steels are still in use, having survived competition from HSS, carbides & ceramic. Traditional tool materials like HSS continue to undergo substantial improvements in their properties through suitable modifications in their composition by optimizing processing techniques. The applications of various tool materials can be obtained from the standard books, journal & catalogues of manufacturers of tools & machine tools. 2) Selection of tool geometry:

Free machining HSS steels Brazed carbides Throway carbides Cast iron HSS Chard Brazed Carbides Throway carbides Aluminum HSS alloys Copper Alloys

HSS Brazed Carbides Throway carbide

8 -10 5-0 5- 0 5 -10 -5 - 0 -5 15-20 0-5 0 5-10 0- 5 0

8 -12 6 -5 5 -5 -5 15 15 5 10 8 5

5 5 5 5 5 5 12 5 5 8 5 5

5 5 5 5 5 5 10 5 5 8 5 5

15 15 15 15 15 15 5 15 15 5 15 15

For cemented oxide tools the recommended tool geometry is BR (100 to 250), SR (10 to 250), ERA (50 to 100), SRA (50 – 150) ECEA (50 – 150) & SCEA (200 – 600). Usual values of these angles are 20, 15, 15, 5, 10, 45) for BR, SR, ERA, SRA, ECEA & SCEA respectively. 3) Assigning cutting variables to the tool i.e., depth of cut, feed & speed. i) Depth of cut: The value of depth of cut is determined primarily from the magnitude of the machining allowance. If for example a shaft is to be turned to a diameter of 100 mm from a bar of 104 mm diameter, the machining allowance will be (104-100)/2 i.e., 2 mm. The nearer the blank is to the finished part in shape & size i.e., smaller the machining allowance the lesser the amount of metal required to be removed the shorter the time required for machining, the higher the productivity in manufacturing the given part. It is advantageous to remove the whole machining allowance in a single pass, or cut, as is commonly done in rough machining (i.e. depth of cut equal to machining allowance). If the machining allowance is large then it is divided into more than one cut. In semi-finish turning a depth of cut of 0.5 to 2.0 mm. is assigned for finish turning the depth of cut is in the range from 0.1 to 0.4 mm. If the allowance is larger than these values, then these depths of cut refer to the second (final) pass. 2) Feed: In order to reduce the machining time i.e. to increase the productivity, it preferable to apply maximum possible rate of feed, taking into account all the factors, which may influence this rate such as surface, finish, cutting forces available strength & rigidity of work piece & mechanism. In practice feed is usually selected from tables of cutting conditions contained in various handbooks. Thus according to commonly applied tables for machining of free 0

tool (the nearest lower speed or maximum 5% higher speed. This will be actual rotational speed (N) at which the machining operation is to be carried out. It is used to calculate actual cutting speed (Va). The values of the recommended cutting speeds for particular selected tool material, depth of cut & feed can also be selected from standard data available in handbooks or catalogues viz. for rough machining of free machining steels with HSS at a depth of cut of 2.5 mm & feed of 0.4 mm/rev. as can be seen from the table 5.2) & for the same parameters the value of cutting speed for carbide tools is 140 to 170 m/min (table 5.3). These values however will be required to be modified according to available speed (rpm) on the machine tool.

4) Calculation of shank dimensions from strength and rigidity considerations (Tool Shank Design): The shank of a cutting tool is designed for strength and rigidity. The shank of a single point tool may be rectangular square or round in cross section. The rectangular cross section is most often used because the reduction in strength of the shank, at section I-I Fig.33.1, is less than for a square shank when a seat is cu for a tip. Rectangular cross sections with various H-to-B ratios are used. In most cases, H/B = 1.25 or 1.6 for B = 10 to 40 mm. It is advisable to use H/B = 1.6 for semi finishing and finishing operations & H/B = 1.25 for roughing. Square shanks tools are used for boring, turret lathe or screw machine tools, as well as in other cases when the distance from the base of the tool to the line of centers of the machine tool is insufficient to accommodate a rectangular shank. Round shank tools are used for boring and thread cutting. They can be turned in holder to make adjustments. The cross sections of the rectangular tool shank are: B x H = 10 x 16, 12 x 16, 12 x 20, 16 x 20, 16 x 25, 20 x 25, 20 x 32, 25 x 32, 25 x 40, 32 x 40, 32 x 50, 40 x 50 mm.

To determine the maximum permissible size of the shank cross section on a strength basis, it is necessary to equate the actual, bending moment to the maximum moment permitted by the cross section of the shank i.e. Mb = M’b …(33.1) …(33.2) In turn Mb = F2 l kgf. mm and M’b = σb Z kgf.mm …(33.3) Where1 = tool overhang (see Fig.33.1) σb = permissible bending stress of the shank material , kgf per mm2, for Unhardened structural steel with σt = 60 to 70 kgf per mm2 , σb = 20 kgf per 2 mm , for shanks of carbon steel, but heat-treated by the procedure for high-speed steel, the permissible bending stress can be approximately doubled. Z = section modulus of tool shank, mm3. The section modulus of a rectangular cross section is BH 2 Z= mm 3 6 Where B and H are the width and height of the shank at the critical section, mm Hence we can write from Eqs. (33.1), (33.2) and (33.3) above BH 2 Fz l = σ b Z = σb 6 from which 6F l BH 2 = z …(33.4)

σb

In rectangular shanks of a height H = 1.6B B (1.6 B)2 =

6 Fz l

σb

Therefore 6 Fz l mm 2.56σ b Since in square shanks the width is equal to the height, then from (33.4) B=3

BB 2 =

6 Fz l

σb

…(33.5)

πσ b The calculations given above for the plane bending of tool shanks are simple but not entirely exact. Only the force Fz is taken into consideration and only the bending deformation it causes. But three forces – Fz or Ft or Fc – the cutting force, Ff cutting force, Ff - the feed force and Fr - the radial force (Fig. 30.10a and b) - act on the tool in cutting and their action leads to additional stresses so that the shank is subject to combined stresses. Combined stresses are higher (in comparison to stresses in plane bending due to force Fz) by about 100 per cent, and they are influenced by the plan approach angle and the construction of the tool point. Table 33.1 lists permissible stresses when calculations are based on plane bending, but the values take combined stresses into account. TABLE 33.1 Permissible stress values σb for tool shanks of structural carbon steel subject to plane bending (with combined stresses taken into account.)

Shanks

Permissible stress σb kgf/mm2, in plane bending, depending upon the plan approach angle, deg. and the shape of the tool point.

300

450

600

760

900

450 (Bent shank)

Unhardened

12

10

8

6.5

5.5

13

Hardened

24

20

16

13

11

26

- Sometimes it is necessary to carry out checking calculations in respect to the rigidity of the tool shank. The maximum load permitted by the rigidity of the tool can be determined by the formula Fz r =

3 fEI l3

….(33.8)

Where f = permissible deflection of the tool, mm (f ≈ 0.1 mm for rough turning and 0.05 mm for finishing) E = Young's modulus of the tool shank material, kgf/mm2 (for carbon structural steel, E= 20,000 to 22,000 kgf/mm2) I = moment of inertia of the shank cross section (for a rectangular cross section I=

BH 3 and for a round cross section I=0.05 d4, where d is the diameter of the 12

shank, mm).

turret), the number of clamping screws (a tool should be clamped by at least two screws) and the distance between these screws. In choosing the tool length, it is desirable to take into account further utilization of the shank after complete wear of the carbide tip in performing the given machining operation. Design of a parting off tool: The general design procedure followed for single point tool design is also applied for parting tool. Viz. selection of tool material selection of tool geometry, selection of cutting variables & checking the cutting variables & finding tool dimensions & overhang. However the geometry of parting tool varies slightly & some standard shapes are suggested for parting tool as shown in fig. In strength calculations of the critical cross section of a cut off tool is neck. I.e. the place where the tool body terminates in the tool head is considered, values of φ is taken as 10 to 20 relief angle between 60 – 100 , rake angle between 50 – 250 (lower for hard & higher for soft material ). Normally width of parting tool in cutting region bears distinct relationship with the diameter of the work piece to part off. There are very many empirical relationships with diameter of work piece to part off. Width of parting tool (thinner section), b = 0.6 D , mm Length of thinner section of tool 1’ = 4b to 6b, mm Height of cutting parther of tool H = 4b to 10b, mm

Problem 1: In a parting off operation in mild steel with HSS tool the cutting force “FZ” is given by FZ = 264.d.f., where “d” is depth of cut in mm & ‘f’ is feed in mm/rev. If feed is 0.8 mm/rev. & depth of cut is not more than 5 mm, design a suitable cross section of the parting tool, assuming the permissible shear stress of tool material as 40 kg/mm2 & = 4, Young’s modulus for tool material = 2 x 104 kg/mm2. If maximum deflection of tool point is limited to 0.05 mm, find the extent by which the tool can be projected out of the tool post. The work piece and the tool post are assumed to have sufficient rigidity. Solution: Cutting force, “FZ3” = 264 x 5 x 0.8 = 1056 kg.

and δt permissible = 0.05 mm. Therefore 0.05 = 1056 x 13/ ( 3 x 2 x 104 x 4436 ) 1 = 24 mm ≅ 25 mm. Thus, maximum permissible length of overhang is 25 mm. Problem: 2 0

A 10 back rake tool is used for machining on a lathe at a speed of 60 m/min. The diameter of work piece is 100 mm. Find the cross section of rectangular tool shank if maximum permissible deflection at tool point is 0.012 mm & maximum allowable stress in the tool shank is 7.5 kg/mm2. Assume a rectangular shank with height to width ratio 1.6 & tool overhang as 1.3 times the height. The recorded value of cutting force under these processing conditions is 250 kg. Assurance the suitable value of young’s modulus.

Solution:

We know that F2.1 = B.H.2σ/6/t Substituting the values, 7.5 x B.(1.6 B)2/6 = 250 x ( 1.3 x 1.6B) or B = 12.7 mm & H = 20.32 mm. The dimensions may be rounded to next higher preferred size i.e. B = 16 mm & H = 25 mm. These values must be checked for maximum permissible deflection. Assuming E = 20 x 103 kg/mm2, we have δt = F2 .13d/(3..E.I.) Putting the values we get δt = 0, 066 mm i.e. < 0.12 mm As the actual deflection is less than permissible deflection the design is safe. Cutting frequency = N = 1000 x V/(Π x D x 60) = 3.18 cycle/sec. Natural frequency of cutting tool having deflection δt is given by Natural frequency = 0.625/(δt)1/2, cycles/sec. = 7.69 cycles/sec. The natural frequency of cutting tool is much more than the cutting frequency hence the designed cross section with required overhang is safe.

Problem 3 :

The cutting force component on a tool point while machining mild steel with 100 back rake angle, HSS tool is given by empirical formula is 103 kg. for feed = 0.6 mm/rev. depth of cut ≅ 2.2, mm, design a suitable cross section of the tool assuming the shear strength of tool material to be 20 kg/mm2 & Eos of 2.5. The young’s modulus of tool material is 20 x 103 kg/mm2. If the maximum permissible deflection of tool point is 0.04 mm., find ‘δt overhang. Give a neat sketch of designed tool.

During high speed machining of ductile materials, long chips are continuously produced which must be broken into small piece for easy disposal and to protect the finished surface from coiling chips. Further the long chips, which may get entangled, can cause machine stoppage besides being unsafe for the operator. The chip breaker may be added to a cutting tool for this purpose.

DESIGN OF FORM TOOLS FORM TOOLS FOR TURNING APPLICATIONS A form tool is defined, as a cutting tool having one or more cutting edges with a defined profile or contour that will be reproduced as the desired form on the work piece surface. Form tools for turning applications are classified according to type of crosssection, such as flat tools or circular tools or end-form tools as shown in Table 20.1. Flat or block tools are further classified according to setting of the tool with respect to work piece like radial-fed or tangential-fed type. Further form tools are also classified with respect to orientation of tool axis in relation to work piece axis.

Classification of Form Tools Form Tools According to cross-section Flat or block

Circular

according to setting of cutting edge

according to orientation of tool axis

radial

tangential (skiving )

parallel

end-form

angular

Fig.20.1 shows flat tool applications while Fig. 20.2 shows a circular form tool and Fig. 20.3 shows an end-form tool. Fig.20.4 shows the case of angular setting of tool axis of a circular form tool with respect to work axis. When the surface to be formed is accessible only with orientated axis, angular setting is used. Usually such settings are avoided.

to the clearance face. The amount of X is less than the actual depth of form AB produced on the work piece because of the clearance angle a. From the geometry of Fig.20.6, X = AB .cos a .. Eqn. 20.1

Therefore, l 2 = ( R 2 − h 2 ) - r cos y = ( R 2 − r 2 sin 2 y - y cos y Now, X = l 2 cos(a + y )

… Eqn. 20.2

Hence, X = { ( R 2 − r 2 sin 2 y ) - r cos y}cos(a + y) … Eqn. 20.3 Introduction of rake angle to facilitate cutting action modifies the profile on the tool. Consider, as an example, the case of a single point V-notch tool shown in Fig.20.8 where ξ is the included angle to the produced. Let ξ 1 be the included angle ground on the form tool. From the geometry of Fig.20.8,

ξ1 ξ S S = and tan = 2 2X 2 2m ξ m ξ Hence, tan 1 =   tan 2 X 2 tan

=

{

ξ 2 2 2 2 (R − r sin y ) − r cos y cos(a + y ) ( R − r ) tan

}

… Eqn.20.4

Hence, the work piece profile angle, ξ, should be modified into ξ1 to be machined or ground in the form tool so that correct profile is reproduced on the work piece.

DESIGN OF FORM TOOLS 1 Introduction - Form tools are intended for producing the desired contour on a work piece by means of a turning operation. - The form tool is used for production work on capstan and automatic lathes in order to ensure (a) High production rate. (b) Uniform cut shapes on all the parts, (c) Accuracy in work piece shape and dimensions. Mostly form tools are made up of high-speed steel, but now, carbide-forming tools are also gaining popularity, owing to productivity being raised by 30-40%.

be tipped with cemented carbide. Dovetail form tool is called so because it is fitted to its holder through a dovetail joint (d)End form tool End form tool is shown in Fig.33.19d. Fig. 33.19e shows a carbide-tipped circular form tool, where ‘I’ is the body of the cutter, ‘2’ is the contoured tip and ‘3’ is the backing member. (f)A Radially fed form tool shown in Fig.33.19f is fed in the direction of feed during cutting operation, till the final shape is imparted on the work piece. (g)A Tangentially fed form tool (Fig.33.19g) travels at right angle to the axis of the rotating work piece. - Fig.33.19h shows clamping of a circular form tool in the holder. The tool is turned in relation to the holder by means of lever ‘I’ having radial serrations on one side which match those on the tool and by adjusting screw ‘2’.

The relief angle (α) depends upon the type of form tool. On circular form tools it is 100 to 120 and on flat form tools it varies from 120 to 150. On form tools intended for relieving form-milling cutters, the relief angle may reach 250 or 300. Design of a Flat Form Tool (Using Graphical Method) -The profile of a form tool can be determined by (1) Graphical method (2) Analytical method The graphical method is straight and simple, however analytical method is more accurate in determining the dimensions. The design procedure of flat form tool using graphical method has been discussed below: (1) The profile of the work piece is drawn in two views, that is, the front view and the top view. (2)The basic points of the work piece profile with dimensions l1 and l2 are projected on the axis I-I drawn perpendicular to the work piece axis. The projected points are 1′, 2′and 3′ . From point 01 as the center draw circles corresponding to work piece radii r1, r2 and r3 (3) Apex of the cutting tool ‘1’ should lie on the work piece axis.

Design of a Flat Form Tool using Analytical Method. - Refer last one fig. of same topic. -It will be sufficient to determine the dimensions denoted by P2 and P3 in this figure. -If the dimensions C2 and C3 are known or they can be calculated by formulating a set of equations, dimensions P2 and P3 can be readily determined since they are sides of the right angled triangles 1A2 and 1B3. -The following equations are used to solve the right angled triangles. ∈1 = α + γ P2 = C2.cos ∈1 P3 = C3.cos ∈1 Dimensions P2 and P3 should be calculated to an accuracy within 0.001 mm.

Design of Circular Form Tool

-Refer above Fig. which shows the graphical method of determining the profile of a circular form tool. (1) The profile of the work piece is, first, drawn.

Point O2, the intersection of line II-II with the arc drawn from point 1, is the center of the circular form tool. (5) Next, draw line aM along the tool face. For this purpose, draw a line from point 1 at an angle γ to line I-I. By connecting points 1, 2 and 3 (points of intersection of the line representing the tool face with the corresponding circles of radii r1, r2 and r3) with center O2 of the form tool, obtain the corresponding radii R1, R2 and R3 of the form tool. (6) Next, to construct the tool profile in a radial cross-section, it is necessary to draw radial line N/N and to lay off dimensions l1 and l 2 to the right on a line perpendicular to line NN (in the case when the axis of the work piece and circular tool are parallel to each other). The dimensions l1 and l are equal to the corresponding axial dimensions of the work piece. (7) From the end point 1′ of the axial dimensions, lay off dimensions P2 and P3 in a direction parallel to line NN. Dimensions P2 and P3 are equal to the differences between the corresponding radii of the form tool ( R1- R2 and R1 – R3, respectively). On the intersections of the lines corresponding to dimensions P2 and P3 with the lines determining dimensions l1 and l 2 , obtain points 2” and 3”. By connection points 1” , 2” and 3” by straight lines, one obtains the profile of the form tool in a radial cross-section. Circular form tool: The circular form tool is circular in shape having depth ‘x’ or projection of distance ‘x’ produced all around the diameter in the form of annular groves. The outside diameter of circular form tool is determined in accordance with the height of profile to be turned. A graphical method is recommended for this purpose.(Fig.5.17)

1) Draw two concentric circles corresponding to maximum & minimum radius of contour to be turned. 2) Through point A ( on minimum diameter circle ) draw one line inclined at an angle γ, (which represents trace of plane ground to produce the tool face) below the line. “OA” Through same point draw another line above ‘OA’ inclined at an angle ∝ (relief angle). The value of ∝ is 100 to 120 for circular form tool. At a distance K ( = 3 to 12 mm) depending upon chip thickness & amount of chips to be cut, draw a line perpendicular to OA to permit minimum amount of space for chip disposal. From the point of intersection ‘C’ ( of vertical line with the tool face line) draw a line bisecting angle. The point of intersection of this line with line drawn at angle of ∝ is the point being sought as centre of circular form tool 02. From O2 draw circle representing outer diameter of tool with radius O2 A = R To determine diameter of the mounting hole, the wall thickness ‘m’ is taken in the range form 6 to 10 mm. On circular form tool for internal (boring) operations, the tool diameter is taken from 0.6 to 0.85 of the hole diameter. If tool is too small to make a mounting hole for the holder, it is made integral with the holder or welded to the shank. Holding arrangement for form tool : The circular form tool for external operations essentially have a mounting hole for the holder. The radial serrations with 900 degree profile (Normally 34 serrations) are made on one and face of circular form tool for setting up and clamping purposes. By very simple arrangements these serrations are used to turn the tool for sharpening ( by grinding the tool face ). For example in one type of holder

(Fig.5.20) The tool is turned in relation to holder by means of lever ‘1’ having radial serrations on one side which match those on the tools, by adjusting screw ‘2’. The tool holder has tension shaped projection (4) to locate in the matching T-slot on the tool post. The holder is then fixed on the tool post by means of T-bolt (3). After the cutting edge is set to desired height the central bolt is tightened to restrict the rotation of the circular form tool. The flat form tool have dovetail form on the back side. The holder for flat form tool thus must posses a similar matching dovetail form with the help of screws the position of cutting edge can be adjusted to the desired height after re-sharpening. Problems 1) How are the single point tools classified? 2) With a neat sketch explain geometry of single point tool in ASA system? 3) What are the various systems of tool nomenclature? 4) What is tool signature? 5) Explain how will you proceed for designing single point cutting tool ? 6) What is the significance (a) Rake angle (b) Clearance angle? (c) and cutting edge angle ? 7) What is chip breaker? What is the significance of volume ratio of chips in metal cutting ? 8) Explain the various methods of chip breaking with neat sketch. 9) What are form tools? How are they classified? 10) How in the distance “X” to be machined perpendicular the flank of the tool calculated for a zero rake flat form tool? 11) What is the effect of rake and clearance angle on the distance “X” to be machined (Perpendicular to flank) on a form tool? Derive the relation for the same. 12) A 300 clearance angle is to be produced on a work piece by end turning tool with 100 clearance angle and 00 rake angle end form tool? Will the value of angle to be produced on the flank be same? If not explain why? 13) Explain a mounting arrangement for circular form tool with heat sketch?

a) Cutting force FZ = 1105 kg. b) Permissible shear stress of tool material = 40 kg/mm2 c) Young’s modulus for HSS tool material = 20,000 kgf/mm2 d) Permissible deflection of tool = 0.06 mm. Also select the various angles and draw neat sketch of the designed tool.

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CHAPTER 6 DESIGN OF BROACH 6.1. Introduction: Broaching is used for machining through holes of any cross sectional shapes, straight and helical slots, external surfaces of various shape, external and internal toothed gears. A Broach is a multiple point tool used in broaching, usually secured to the main slide of the broaching and travels with the slide. Since the teeth gradually increase in size from front to rear end of the broach, each successive tooth removes a layer of material, thereby increasing size of the hole in internal broaching (or removing

production.

6.2

Basic Process: A broach is usually a tapered bar upon which teeth are cut to produce desired contour in a work piece in a single pass of the tool. A typical broach for producing a round hole is shown in fig.6.3 (a). A broach has these cutting zones; roughing teeth, semi-finishing teeth and finishing teeth. The broach tapers from first roughing tooth to the first of the finishing tooth, the outside diameter of each tooth being larger than the tooth that proceeds. Normally the finishing teeth are all of same diameter. A work piece to be broached, must be provided with a starting hole through which broach is pushed or pulled. This hole is just large to permit the front pilot portion of the broach to enter freely. As the broach advances through the part, cutting starts gradually, and as each succeeding tooth engages the work, it removes small amount of metal. 6.3(b) shows the various geometric elements of broach tooth. The chips in broaching are accumulated in the chip space. Unlike the other machining processes where chips are thrown away.

Cutting variables in broaching: The determination of the cutting variables in broaching consists of assigning the cutting speed, since the feed per tooth ‘SZ’ and width b of uncut chip are predetermined by constructional elements of the broach. The cutting speed is decided more by the surface finish requirements rather than the hardness of the work piece material. For high-speed steels the cutting speeds lie in the range of 3 to 30 mpm moreover increase in cutting speed does not substantially reduce the cycle time as the cutting time is quite small compared to the non cutting time in most of the broaching operations. Round holes require lower cutting speeds, compared to internal surfaces with straight faces. External

The cross sectional area of uncut chip per tooth can be calculated as under.

Where,

a = S2.b.n, mm2 … for spleen broach a = S2 . Π . D , mm2 . . . . For round broach b = splines width, D = diameter of round broach. n = number of spleens (n =1 for keyway broach ) The total cross sectional area of the under formed chip is given as –

A = a.Z, mm2 Where m , Z = number of teeth simultaneously in operation. Chip formation and cutting forces in broaching : All the phenomena of the cutting process occur during chip formation in broaching; deformation, heat generation, formation of BUE, friction and wear. The cutting process is often accomplished in broaching with very thin chips, especially in internal broaching where SZ may be as small as 0.015 mm. The chips formed in broaching are accumulated between the chip spaces or gullet. The chip breakers or notches are usually provided in the cutting edge to produce narrower chips that fill easier in the chip spaces. The cutting force ‘FZ’ depends upon physic-mechanical properties of work material, broach tooth geometry, cut per tooth ‘SZ’ and the number and shape of tooth that are in operation simultaneously. The cutting forded in broaching is – F = KS ( total cross sectional area of uncut chip) (Blunt broach factor) F = KS ( Π.D.SZ x Zmax).K . . . for round broach = KS (b.n.S1 . Zmax . K . . . . for Where, Zmax = maximum number of teeth cutting at a time.

The different modes of cutting in the internal broaches are shown in Fig.6.4. In this figure the layers of metal removed by different broaches is also shown. The following are the common modes of cutting. i) Full form cutting : where each tooth removes a thin layer of metal (chip) on the full width of machined surface, for example along full width of spine or keyway, along the whole length of circumference. Such broaches are often called as plain broaches.(Fig.6.4.a) ii) Generation Cutting : Where each tooth removes a thin layer of metal on the part of the full width of machined surface except the last finished teeth which removes the metal over the entire width of contour of the surface (Fig.6.4.b) Such broaches are comparatively easier for manufacturing the complex forms. iii) Group Cut Broaching : Where the teeth are divided into two or more groups of teeth having same diameter but increasing the width within each group. Though such broaches have on an average double life than the form relieved broaching, greater difficulty is encountered in manufacturing of such broaches. Such procedure is often called as progressive broaching. Types of Broaches: Broaches are broadly classified, based on their purpose into internal broaches (for machining holes) and external or surface broaches (for machining ruled surface of open contour). The internal broaches can further be classified as under i) Solid broach : In which the desired profile of breach is produced from a solid piece (Fig.6.5 a) ii) Sectional broach : in which the desired profile of broach is produced from a solid piece (Fig.6.5 a)

iii) Helical cut broach : in which the teeth are positioned along the same helix as in work piece. If the helix angle is less than 15 degrees the axial pull or thrust will be sufficient to rotate the broach provided broach’s pull or push head are made free to revolve on antifriction bearings. If the helix is greater than 15 degrees, the broach head is to be rotated positively with the help of lead screw and gear box (Fig.6.5.c) iv) Burnishing broach : in which non cutting teeth carry out compression, cold work or burnishing a thin layer of metal. These broaches can enlarge the hole by small amount (Fig.6.3.d) v) Spleen broaches : such as straight spleen broaches (Fig.6.5.e) helical spleen broach (Fig.6.5 g), serration spleen broach (Fig.6.5.h), vi) Keyway broach : which is used for producing keyway in a hole. This broach is of rectangular shape and is guided through a bushing (called horn) with a rectangular slot to guide and support the keyway broach to avoid wear of (Fig.6.5.k, bushing called horn) with a rectangular slot to guide & support the keyway broach. To avoid wear a hardened wear strip may be provided.

Large external broaches are designed as separate sections, or inserts damped in a special holder, several methods of clamping the inserts in the holder are illustrated in Fig. 6.6. Some broaches are made up with separate inserted blades. All the fastening elements of built up broaches with either inserts or inserted blades should be checked by calculating their strength. There is no way of expending internal broaches and one or more finishing teeth are converted into cutting teeth in each sharpening. The size of external broach on the contrary can be adjusted by means of gibs or spacers. Such spacers are shown in Fig.6.6. 6.7. Resharpening of broaches : The broaches are sharpened mainly by grinding the tooth faces. Less frequently, the backoff clearance is ground. The grinding of the tooth face in sharpening a round broach is illustrated in fig. 6.7. To avoid wheel edge interference in which the tooth face is ground square with the broach axis, entirely, eliminating the face angle, the radius of the wheel in section N-N should be less than the radius of curvature of tooth in this section. The backoff clearance of a round broach can be ground in a cylindrical grinder.

6.8. DESIGN CONSIDERATIONS IN BROACHING : The following considerations are done in design of internal broaches. 1. Study of geometry & other features of work piece for getting the necessary information for broach design. The following information is normally obtained in designing a circular or internal broach a) Tolerance on the hole to assign the tolerance to the broach. b) Quality of surface finish to check necessity of burnishing teeth. c) Wall thickness of the component to estimate size variations after broaching. d) Quantity of production to study feasibility of broach design 2) Selection of material for broach. The broaches are normally manufactured from high speed steels (normally grade m2 or m7.). The carbide tool inserts are used in surface broaching of cast iron work pieces because of their good flank wear characteristics against abrasive chips. The brazed carbide broaches are not normally used due to problems accompanied with them at all stages. The built up condition of surface broaches has promoted the use of brazed or index able inserts to a great extent. 3. Calculation of broaching allowance (A) : The allowance for broaching is defined as the total thickness of metal to be removed by broaching ( or it may be called as stock left for broaching. The nominal allowance for round holes, machined by drilling or coredrilling previous to broaching is given machined by drilling or coredrilling previous to broaching is given as

Where

A = 0.005 D + (0.1 to 0.2) L L = Basic diameter of the hole, mm L = Length of the hole to be broached, mm.

5.Selection of broach tooth and chip space The cross sectional area of gullet (chip space) Ag is found from the longitudinal cross sectional area of chip “Ac” Ag = K Ac The volumetric factor K should be taken between 2 to 5 (smaller values for brittle material when a discontinuous chip is obtained) Ac = L.SZ, where L = length of surface broached, mm corresponding to the value of Ag, the other parameters (like pitch ‘t’ width land ‘b’, tooth depth ‘h’ radius ‘r’ and ‘R’) can be selected for a particular type of profile from table 6.3. The pitch can also be calculated by empirical formulae depending on the broaching length. t = (1.25 + 1.5) L . . . . . . . For plain broach. t = (1.45 to 1.9)

L . . . . . . for progressive cutting broaches.

A factor to be kept in mind while selecting broach pitch is that commonly at least three teeth should be in contact with work piece at one time. It is permissible to have two teeth in contact for short parts. Very small parts are broached in stacks of several pieces and the pitch of the broach is determined for the total length of stack. The pitch of sizing teeth ‘t’ (semi-finishing teeth) is taken as ts = 0.6 to 0.8 for round broaches and for other type of broaches ts = t. The pitch of cutting teeth is made variable from t + (0.2 to 1mm) to t – (0.2 to 1 mm). The other parameters of the tooth profile can also be estimated by empirical formulae given below. h = 0.4 t, r = 0.5 h, b = 0.3 t, r = 0.7 t, The value of pitch is required to be reduced if the maximum number of teeth in contact with work piece (i.e. 2 max) at a time is less than three. i.e. Z max = L/t + 1 , should be minimum three.

6. Selection of geometry of cutting and sizing teeth : The back off or clearance angle ( ) can be selected in accordance with the kind of broach and teeth from table 6.4. The sizing or finishing teeth have narrow straight (cylindrical) wear land of a width ‘f’ = 0.05 + 0.02 mm ( fig.6.3 c) The backoff (or the hook or rake or face ) angle are made as small as possible so as to minimize the loss of size in a cross section of the broach when broach is resharpened after grinding the tooth faces. The values of backoff angle for finishing

roughing broaches of all types (2-4)

9. Determination of dimensions of cutting teeth (or roughing teeth) and semifinishing teeth. The diameter of the first tooth is taken equal to the front pilot diameter Dt = D – A. The diameter of each subsequent tooth is incremented by 2 SZ. The cut per tooth for last three finishing teeth preceeding, the sizing or finishing teeth is gradually decreased as suggested in step (2)

10. The dimensions & tolerance of the sizing teeth (or finishing teeth) The diameter of the sizing teeth DS = DmaxJB , where Dmax is the maximum diameter of the broached hols, δ is the change in the hole diameter after broaching (when the diameter is oversized, take sign as ‘-‘and’ +’ when the diameter is undersized. The tolerance on cutting tooth = ± 1/5 SZ but maximum 0.02 mm, the tolerance on finishing tooth = - 1/3 hole tolerance but maximum IT 7

11. Selection of pull end and rear pilot for round breaches : The pull end of the broach serves to engage the broach to the machine through a puller head. The three types of pull ends are given in fig.67.8. The details of each pull end can be obtained from standard IS 7773. These values can be selected from tables 6.7, 6.8 and 6.9.

12. Calculation of length of broach : The total length of broach is given as L = Length of toothed portion + length of pull end + length of rear pilot. Length of torthed portion, L = tc Zc + tS . Zs Where, tc , ts represents pitch for cutting of sizing teeth and Zc & ZS represents number of cutting and sizing teeth. 13. Determination of force and calculation of strength of broach – The force required for broaching & hole of desired dimension in a work piece is calculated from the specific cutting force for the work piece material (Table 6.10) For round broach, F = KS Π D.S.Z Zmax K The broach strength for pull end breaches is checked for tensile failure is the permissible pulling force ‘F’ given by product of area of critical cross section and safe tensils strength of broaching material should be at least equal to the broaching force F. or F. The area of critical cross section is the area of gullet cross section for first teeth. Critical cross section area = Π ( Dt – 2 h )2/4 Where ‘Dt’ is the diameter of pilot or first tooth and “h” is the depth of gullet. The values of permissible stresses for material of pull and push broaches can be obtained from table 6.9.

Solved Example Example 1 : Design a circular broach for machining a cylindrical hole, diameter D = 25H7 (+ 0.021) and length 10 = 562 ± 0.95 in a toothed wheel blank of free cutting steel (σ t = 70 kgf/mm2) Solution : 1) The broach material selected for this job is HSS (m2) 2) Broaching allowance ‘A’ and diameter of premachined hole ‘DO A = 0.005 D + 0.12 L = 0.9735 say 1.0 mm D0 = D – A = 25 – 1.0 = 24 mm. 3) Cut per tooth SZ From standard table (62) S2 is selected as 0.03 mm for steel. Assuming the number of semi finishing teeth as B the SZ is distributed as ½ SZ – 0.15 mm, 1/3 SZ = 0.01/mm, and 1/6 S = 0.04 mm. 4. Selection of broach tooth & chip space dimensions : Longitudinal cross sectional area of chip = L.SZ = 50 x 0.03 = 1.5 mm2 Ag = cross sectional area of gullet = K x 3.15 = 4.5 mm2 From table (6.3) for re-utilize profile nearest value of Ag is 5.8 mm and of pitch is 7 mm. and h = 2.3, b = 3.0 r = 1.25, OR These dimensions can also be estimated by the empirical formulae as under. T = 1.25 L = 8.8 9.0 mm H = 0.4 t = 3.6 mm R = 0.5 h = 1.8 mm B = 0.3 t = 2.7 mm Let us assume the values selected from standard table i.e. t =7.0, h = 2.3 mm, b = 3.0, r = 1.25 mm. The pitch for finishing or sizing teeth = ts = 0.8 t = 5.6 mm. Now checking for Zmax i.e. maximum number of teeth in contact Zmax = maximum number of teeth in contact = L/t + 1 = > 3 hence the condition that Zmax should be ≥ 3 is satisfied and the selected of pitch is safe. 5.Selection of geometry : From table 604 & 6.5 we get ∝ roughing = 3 + 30’ , , roughing = 150 ∝ Semifinishing 20 + 1 Semifinishing = 150 ∝ Sizing = 10 + 151 √ sizing = 50 6. Selection of number of chip breakers From table 6.6 for D = 25 mm number of chip breakers is equal to 12 and width m = 1.0 mm r = 0.3 mm

dimension is kept constant i.e. 25.016 mm. 9. Dimensions and tolerance of sizing teeth DS = Dmax ± = 25.021 – 0.005 = 25.016 mm Assuming the diameter will be oversized by 0.005, δ is ‘-‘ Tolerance on cutting teeth = ± 1/5 SZ = ± 0.006 Tolerance on finishing teeth = -1/3 rd tolerance on hole = - .0007 10. Selection of pull end & rear pilot dimensions – From table 6.7 d1 = 22 mm, d2 = 17 mm. d4 = 22 mm, C = 0.5 mm L1 = 140 mm, L2 = 25 mm, L3 = 25 mm, L4 = 16 mm, r1 = 0.3 and r2 = 1.0 mm ∝ = 300 for pull end Dt = 24.0 -.04 -.073 From table 6.8 Drp = minimum diameter of broached hole = 25 –0.021 1rp = 25 -0.041 t = 1.5 11. Length of broach – (L1) Length of portion = Tc Zc + ts .Zs = 7.20 + 5.6 x 6 = 173.6 Length of front pilot = L1 + 65 (for tapered portion) = 75 (for cylindrical front pilot) = 140 + 65 + 75 = 280 mm Length of rear pilot = 25 L = 173.6 + 280 + 25 = 478.6 mm 12. Force & strength calculations F = Ks Π D SZ Zmax K. Ks = 425 Kgf/mm2, D = 25 mm, SZ = 0.03 mm, Zmax = 8, K=1.25 F = 10008.75 Kgf. Cross sectional Area of critical section = Π (Dt-2h)2/4 = 295 mm2. Permissible putting stress for HSS = 35 kgf/mm2 Permissible pulling force F’ = 35 x 295.44 = 1034 kg/mm2 F1 > F hence the design is safe. PROBLEMS 1. What is broaching ? What are its advantages and disadvantages over boring ? 2. What are the geometric elements of broach teeth ? 3. Explain the constructional features of broach teeth with a neat sketch.

For variants of the problem see following table.

1) Lips: These are main cutting edges of the drill and are formed by the intersection of the flank and flute surfaces. For efficient cutting, the lips should be straight, equal in length and symmetrical with the axis of the drill. 2) Chisel edge: This is the point end of the web and it is formed by the intersection of flank surfaces. 3) Helix angle: Helix angle practically determines the rake angle at the cutting edge of the drill. As the H. A. decreases, the rake angle also decreases and makes the cutting edge stronger. With the lower H. A. the chip election through the flutes is not efficient. However, low helix drills are recommended for hard materials like marble, slate, carbon and hard rubber. With increase in helix angle, the rake angle increases and the cutting edge becomes weaker. High helix drills, which are also known as fast helix drills, are recommended for soft material like copper. All alloys, Zinc alloys, molded plastics, etc. 4) Point angle (2ϕ) : The most commonly used point angle is 1180. Reducing the point angle leads to an increase in the width of cut, and it is generally adopted for brittle

between the tangent to the flanks and the tangent to the surface of revolution at that point. The actual value of the relief angle during drilling also depends on the feed. A higher feed results in reduced working clearance. This is explained by the fact that the drill not only rotates but also travels axially during cutting. (Fig.7.2.a)

7) Rake angle : (γ) Rake angle is the angle, the flute surface makes with the normal to the surface of revolution described by the lip. The rake angle acquires its maximum value at the periphery of the drill, where in a plane parallel to the drill axis (plane A-A), it is equal to the helix angle w of the helical flutes. (Fig. 7.2.b) The minimum value of the rake angle is at the apex of the point. The rake angle at the chisel edge is negative so that the cutting angle exceeds90 degree and the cutting conditions are unfavorable. A larger rake angle, however, reduces the lip angle, leading to more rapid heating of this part of the drill and consequently, to maximum wear. In deep-hole drilling with a large diameter drill, a wide chip is formed that is difficult to dispose of through the flutes. Such a chip also increases friction and impedes cutting fluid delivery to the drill lips. The width of the chip can be reduced by providing special chip-breaker grooves or notches either on the face (Fig.8.3.b) or on

FORCES ACTING ON DRILL The feed s (mm per revolution) is the amount the drill advances axially in one revolution (or in one revolution of the work if it rotates and the drill only advances). A drill has two main cutting edges (lips) and the feed per lip is s s z = mm per rev 2 As in turning operations, the feed can also be measured in millimeters per minute : sm = sn mm per min The thickness a of the undeformed (uncut) chip (Fig.182a) is measured in a direction perpendicular to the drill lip s a = sz sin ϕ = 2 sin ϕ mm The width b of the undeformed chip is measured along the lip and is equal to the length of the lip : D mm b = 2 sin ϕ The cross-sectional area of the unreformed chip per lip is

ns ns l = hole length or depth, mm ∆ = over travel ( l or 2 mm), mm y = length of travel required before the drill cuts the full diameter, mm. 7.4. CHIP FORMATION IN DRILLING : A drill is more complex than a single point tool. The cutting process in drilling also proceeds under more complex conditions due to the reasons explained below. Chip disposal from and cutting fluid delivery to the drill lips present difficulties, there is considerable friction between the chips and the flute surfaces and between drill and machined surface, a sharp drop in cutting speed (from Υ max to zero) occurs along the drill lips so that at various points of the lips the layer being out is deformed and out at different speeds. Non-uniform deformation is also due to the variable angle along the lip of the twist drill, i.e. the chip deformation (contraction) decreases as the point on the lip approaches the drill periphery (owing to increase in V and γ ). These factors create more severe conditions for chip formation in drilling than in turning. 7.5 : Force acting on a drill : The resultant forces of resistance of cutting can be resolved into three components at each point of the lip viz., (Fig.7.5) 1) Force ‘F’ : acting upward impede penetration of drill in work. k 2) Force Fh : acting horizontally ( and are supposed to be counter balanced ). 3) Force ‘Fz’ : acting horizontally & responsible for setting up moment of resistance (Mrc = Fz .x) 4) Force F1 : acting vertically on the chisel edge. 5) Force Ff : due to the flow of chip (Friction force) Thus total axial thrust force “F” can be written as (2 Fv + F1 + F f ) F= ∑ Out of total resistance, Fm is 40%, F1 is 57% & Ff is 3%. The value of ‘F’ applied to drill must be less than or equal to maximum force permitted by the feed mechanism. The total moment of forces of resistance to cutting (M) is made up of the moment of forces ‘Fz’( = 80% M), moment of the forces due to scraping and friction of the chisel edge, Mce , ( = 12% of M), moment of the friction forces on the margin Mm , and the moment of the forces of friction of the chip on the drill and the machined surface, Mc, (Mm + Mc ) = 8%M) M = Mrc + Mce + Mm + Mc

formed chip thickness. Thus, upon increasing angle 2ϕ axial thrust is increased (due to increase in Fz) and torque is reduced. c) Cutting fluid efficiency : Efficient application of cutting, fluid reduces axial thrust & torque. d) Drilling depth : Increase in depth deterio-rate cutting conditions & hence adversely affect the thrust & torque. e) Cutting speed: Axial thrust & torque first decreases with an increase in cutting speed. 7.6 REAMING:

High feeds tend to reduce the accuracy of the hole and the quality of the finish. Reaming is one of the important operation used in the manufacturing of interchangeable parts of mass production and provides the most economical means of achieving precision fits and interchangeability. The maximum diameter of reamer must be equal to the maximum diameter of the hole minus (0.15 x Hole tolerance). The minimum diameter of reamer must be equal to the maximum diameter of the hole minus (0.35 x Hole tolerance). The reamers usually have even number of teeth to facilitate diameter measurement . Use is made of non-uniform angular pitch “W” which helps is improving surface finish. The value of “W” for a particular number of teeth can be selected from standard table. The reamers may be straight flutes or helical fluted (with helix grooves directed against direction of rotation. QUESTIONS i) What are the various types of drills? Give their uses. ii) Explain the elements of twist drill ? With a neat sketch. iii) Draw a drill point (standard) showing rake angle and clearance angle. iv) Draw a neat sketch showing the various forces acting on a twist drill. v) What is the difference between a drill and a reamer ? vi) What is the various types of reamer and drill shanks ? vii) Draw a neat sketch showing geometric elements of straight fluted reamer. viii) Explain the difference between drilling & boring ? ix) What are the various types of drill points ? x) Explain the difference between drilling & boring ? xi) Explain clearly the need for reamers. Why cannot drills be made to finish holes to a close tolerance ?

Milling is the machining process in which metal is removed by advancing a work piece against a rotating multipoint cutting tool called milling cutter. As the cutter rotates, each tooth removes a small amount of material from the advancing work for each spindle revolution. What distinguishes milling from other machining processes is interrupted cutting, relatively small size of chips and variation of chip thickness within a chip itself. The milling process is generally divided into two basic forms, referred to as peripheral milling and face milling. In peripheral milling, the finished surface is parallel to the axis of the milling cutter and is generated by teeth located on the periphery of the cutter. Types of milling cutters :

8.3 Cutting elements of milling cutters : The geometry of plain milling cutter is shown in Fig.8.2.a. The rake angle when measured in plain normal to cutting edge (section c.c.) is called normal rake angle. And when rake angle is specified in a plane perpendicular to cutter axis.(Section D.D.). The following formula can be used to find the normal rake angle “γ” when the radial rake angle “γ” when the radial rake angle “γ” is given. Tan Υ = tan Y’. Sin. ϕ + tan w. cos ϕ Where ϕ is the entrance angle in case of face milling cutter and is defined as the angle made by main cutting edge with the peripheral cutting edge. For a plain milling cutter with helical flutes, this angle coincides with helix angle ‘w’. The radial relief angle is measured in a plane perpendicular to cutter axis (Section D-D). It is the angle between the tangent to the tooth flank at the point being considered on the main cutting edge and tangent to the circle described by this point. The value of normal relief angle can be obtained as

Tan α = tan αn.cos w tan α n Tan α = Sinϕ

. . . . . for plain milling cutter.

. . . . . for peripheral cutter. The recommended ranges of various angles are α = 120 to 300, αn = 60 to 150, for face milling cutters and αh = 100 to 250 for side milling cutters.

8.4. MILLING PROCESS AND CUTTING VARIABLES: The tooth of plain milling cutter removes chip of varying thickness whose section is confined within two arcs of curate trochaics. The chip formation in milling is accompanied by the same phenomenon as in single point cutter tool, however, there are certain inherent features. 1. The tooth comes in contact with work for relatively short period of time during one revolution, hence during rest of the time it cools down giving favorable effect to tool life. 2. The tooth is subjected to impact loads, shortening the tool life & may lead to catastrophic failure. 3. The chip thickness is not constant but varies along the entire length e.g. In conventional or up milling is minimum at start or entrance of tooth & maximum at end or exit, whereas, vice versa for down or climb milling. Let, Number of teeth on milling cutter = Z, width of cutter = Bc , diameter of cutter = D, depth of cut = t, width of cut = B, chip thickness measured in a radial section = ‘a’. The angle for which tooth is in contact with work piece i.e. the tooth contact angle = δ, feed per revolution = S, feed per minute = Sm, feed per tooth = Sz , rotational speed of cutter = N, number of teeth in contact, = m

Elements of cutting process in straight flute

, amax = Sz ‘sin; δ The cross sectional area of uncut chip removed by one tooth of a straight flute cutter is (denoted by f) F = a.b = B.Sψ. sin To find total cross sectional area of all uncut chips it is necessary to know the number of teeth that are simultaneously in operation and instantaneous tooth constant angle for each tooth. The number of teeth that are simultaneously in operation on a straight flute cutter is m = δ.Z/(Angle between adjacent teeth) = δ.Z/360. or t / D − t / D 2 ) / 360 m=Z. Sin-1( 2 If 1 < m < 2, then maximum two teeth are simultaneously in operation, if 2 < m < 3, then maximum three teeth are in operation simultaneously. Since, 2 2 Sin δ = 1 – cos2δ = 2 t / D − t / D Thus, the value of number of teeth simultaneously in operation depends upon (1) ratio t/D, (ii) D, (iii) Z. The larger t & Z, and the smaller D is, the greater ‘m’ will be for any cutter (Specified with D and Z), m will depend only on the depth of cut. The number of teeth in operation for a helical flute cutter can be determined by graphical method or by the formula given below. m = δ.Z/360 + B.Z/(Π.D.cot w) The larger t, z, B and w are, and the smaller ‘D’ is, the greater the number of teeth in operation, simultaneously. The machining time in peripheral milling is Tm = ( 1 + y + ∆ )/ Sz . Z.N.) min. Where, ‘1’ is length of milled surface, ‘y’ is cutter approach and ‘∆’ is over travel.

8.5. Conventional (UP) Vs Climb (Down) milling : Milling can be accomplished with cutter rotating in the direction opposite to feed of the work piece (Fig. 8.5.a), or in the same direction (Fig. 8.5.b). The first method is called conventional, or up milling the second is called climb or down milling. The comparison of these milling processes is given in the following table.

POINTS 1. Uncut chip thickness

CONVENTIONAL MILLING Zero at entrance max. at exit

CLIMB MILLING Max. at entrance & zero at exit. 2. Load Gradually increases Gradually reduces 3. Machining of work Easy, as cuts from under the Cuts through the scale & piece with Sandy skin foundry skin, breaking it from hence the cutter life or scale in casting or under with & the hence cutter reduces. forging life is good. 4. Effect of cutting Tends to lift the w/p from table W/p is forced against the forces of vibration & or fixture, or lift work table guide ways, eliminating surface finish. increasing the clearance excessive. Clearance – in between the table & bed or joining surfaces. Thus saddle ways which leads to reducing vibrations and vibrations & bad S.F. improving surface finish. 5. Accumulation of In front of surface to be Chips accumulation is chips machined pick-up by cutter & behind the cutter, does not impairs surface finish. harm surface finish. 6. Power consumption More as more power is Less compared to consumed in feed conventional milling. traverse nut is The force is such that the 7. Excessive back lash The continuously in contact with backlash is not eliminated. in table screw & nut. lead screw on same side. So backlash is eliminated.

8.6. Cutting forces in Milling: In milling, the rotating cutter has a no., of cutting edges, which engage with the work piece only in a part of its rotary path, the remaining being through air. This results in pulsation of cutting forces. The chips thickness varies along the cut and for calculation of cutting force, an average value of chip thickness, should be taken into account.

The tangential (peripheral) force ‘F’ sets up the moment of resistance of cutting (M) and tends to bend the arbor. M = Fz.D/2 , Kgf-mm. 1) This moment of resistance should be overcome by the torque developed by the electric motor of the milling machine. Thus main drive mechanism is designed and power required in milling is calculated on basis of force Fz. 2) The radial force Fy exerts pressure on the basis of force Fz and also tends to bend the cutter arbor. Thus cutter arbor is subjected to bending due to resultant of two forces Fz & Fy (i.e. R) and tortional force due to moment of resistance to cutting. 3) The horizontal or feed force “Fh” is used in designing the feed mechanism of milling machine, in calculating the required damping force for the work piece and designing various components of milling future. 4) The vertical force Fy in climb milling tends to keep the cutter pressed against the work piece, while in conventional milling tends of lift the work piece from table. The value of aggregate tangential force “R”, can be calculated from the equation. R = Ks . m. as . B Kgf Where, Ks = Specific cutting force corresponding to given material as = Average value of thickness of chip. m = number of teeth simultaneously in contact

Depending upon the hand of the flute spiral, force ‘Fa’ either tends to slide the cutter off the arbor or holds it against the shoulder on the spindle nose, the axial force can be compensated by using interlocking cutters with helical flutes of different hands. The power required for milling can be calculated from the tangential force Fz E = Fz .V, Kgm/min. The following relationships can be used to determine the forces Fh and Fv. Fh = ( 1 to 1.2 ), Fz and Fv = (0.2 to 0.3) Fz for conventional milling And Fh = (0.8 to 0.9) Fz & Fv = (0.75 to 0.8 ) Fz for climb milling. QUESTIONS 1) What is milling ? How it differs from the turning process ? 2) What is the difference between up milling and down milling ? 3) With a neat sketch explain the geometrical elements of a helical flute plain milling cutter ? 4) What are the various types of milling cutter ? Explain geometry of a face milling cutter ? 5) How are the maximum number of teeth in operation decided in milling ? What are the factors affecting this ? 6) What is tooth contact angle ? Derive the relation for the same. 7) What are the components of tangential force “R” in milling be resolved ? 8) What are the effects of various components of cutting forces in milling ? 9) Why are the inter locking cutters used ? 10) He is the time is milling a flat surface calculated ?

Shaping is carried out in gear shaping machines with a multiple edge tool called a gear shaper cutter. Broaching is done with a special cutting tool and is seldom employed as a gear cutting method. Shaving is a gear-finishing process using a cutting tool in the form of a gear ( or rack in some cases ) with teeth on the flanks of which narrow grooves separated by narrow lands are provided. These narrow grooves are called serrations. Grinding is employed as a gear-finishing process. Finemodule gear are sometimes ground from the solid. Gear teeth are cut by two general processes called : (1) Form cutting in which the shape of the cutting edge of the tool is identical with the shape of the tooth space of the gear (disk and end-mill type gear milling cutters and gear shaping cutter – heads operate on this principle ), and (2) generating, in which the tooth flanks are obtained as a result of machining with a tool whose cutting edges reproduce the profile of the conjugate rack or the profile of a tooth of a conjugate gear. During the machining process, the tool and gear blank form a conjugate (properly meshing) toothed pair, or gearing.

together without slipping. In addition to the rolling (generating) motion, the cutter reciprocates along with the axis of the gear blank. This is the primary cutting motion. Chips are cut in the down stroke of the cutter. In the return stroke the blank is withdrawn slightly from the outer (to prevent the flanks of the cutter teeth from rubbing against the machined surfaces of the gear teeth). At the beginning of the working stroke, the blank is advanced to the cutter again.(Fig.9-2 & Fig.9.4)

In addition to the reciprocating and rotary motions, the shaping cutter is also in fed to the depth of the teeth of the gear being cut. This process has following advantages over the hobbing process. 1) Gears with adjacent shoulders can easily be produced. 2) The gears produced by the method are of very high accuracy. 3) Both internal and external gears can be cut by this process. 4) Cluster gears can be manufactured. However with this process worm & worm wheels cannot be produced.

9.2.2. Geometric elements of gear shaper cutter : The gear shaper cutter are of different types. These cutter have different geometry depending upon the type of gears produced viz, spur gears or helical gears. A rotary gear shaper is basically a gear in which teeth are relieved to provide cutting edges and clearance. According to construction the gear shaping cutters are classified

Gear hobbling process : Hobbling is machining process similar to milling, but in hobbling the work piece is not held stationary but is caused to rotate with a definite ratio to the velocity of the cutter. The cutter is called a hob, the chief feature of which is a thread or lead developed to produced teeth on a cylindrical work piece. Hobs are used for the production of spur gears, helical gears, worm gears, spine shafts, etc.

Hobbling is process of generating a gear by means of a rotating cutter called hob. It is a continuous indexing process. Gear hobbling is faster than milling, because several teeth are cut at a time and because of the continuous meshing process. In gear hobbling, the cutting tool and work piece rotate in a constant relationship while the hob is being fed into work. A hob resembles a worm, with gashes made parallel to its axis to provide cutting edges. For in volute gears the hob has essentially straight sides at a given pressure angle. The hob, is fed into the gear blank to the proper depth and the two are rotated together as if they are in mesh. Each hob tooth cut its own profile, but the accumulation of these straight cuts produces a curved form of the gear teeth, thus the name generating process. The gear cutting with a hob involves three basic motions all of them occurring at a time. The hob and a blank have a rotating motion and the third one is radial advancement for the hob, thus causing the cutting and indexing simultaneously.

9.4. Thread Cutting Tools : 9.4.1. General Screw threads are produced by three methods : 1. Cutting with a cutting tool made of carbon tool steel, high speed steel or cemented carbide (single-point threading tools and chasers, taps, threading dies, self-opening die heads, thread milling cutters and thread generating cutters). 2. Grinding with fine-grain single-rib and multiple-rib wheels. 3. Rolling (based on plastic deformation) with cylindrical dies, flat dies and heads with narrow thread-rolling dies (with longitudinal travel of the head or the work). 9.4.1 Thread Tapping A tap is used to cut internal threads. A tap is a screw on which longitudinal straight or helical flutes have been milled to form cutting edges. It operates with two simultaneous motions: rotation of the work or tap and tap advance along the thread axis. Taps can be classified into the following main types hand, nut, machine, master, die, adjustable and collapsible. The principal parts and constructional elements of a tap are shown in Fig.9.9. The thread lengthily refers to the part of the tap on which thread is cut. It is made up of the chamber and sizing section. The chamber or cutting section is the front tapered end of the tap and serves for rough cutting of the thread. The sizing section cleans up the thread cut by the chamber. The shank is the portion of the tap by which it is held in a chuck or tap wrench; the square serves for transmitting the torque to drive the tap. Elements determining the construction of a tap include the flutes for accommodating the chips, lands and core (central portion of the tap below the flutes which joins the lands).

Another measure for reducing friction in tapping is to provide & back taper on the tap. This makes the major and minor diameter of the thread smaller near the shank than those at the chamber by the following amount; 0.05 to 0.10 mm per 100 mm for ground taps and taps in which threads are formed by rolling; 0.08 to 0.12 mm per 100 mm for underground taps. Fig.9.10

If a tap is used to cut thread in a hole with longitudinal slots or recesses (the tapping of a threading die with a die tap can serve as an example) the number of flutes should not be a multiple of the number of clearance holes in the threading die for vice verse), since otherwise the tap lands may drop into the clearance holes. To ensure productive operation, the tap flutes should be of a shape that provides sufficient chip space (without appreciably weakening the tap) and enables the tap to be necked out of the hole without damaging the thread with the heel of the lands. Three of the more widely used shapes of flutes are shown in Fig.9.10.c. In Type a (Fig. 9.10.c the flute is milled with a convex half-circle cutter profiled to a single radius. In backing out the tap the heel of the lands may cut a chip and spoil the thread. This shape of flute is used only in exceptional cases and then only for hand master taps.

Most taps have straight flutes. Certain special taps have helical flutes. The direction of chip flow can be changed by changing the hand of the helical flutes on the tap. Taps with helical flutes of different hands are illustrated in Fig. Flutes of the type shown in Fig. Drive the chips forward, ahead of the tap, and can be used for tapping through holes. Chip flow is forward the shank for a tap with flutes of the opposite hand (Fig.9.11.b.). This is applicable for tapping blind holes. A straight-flute tap will also direct the chips forward, ahead of the tap, if a spiral point is ground on the cutting face of each land at the chambered end. It is formed at an angle λ (Fig.9.11.c) with the tap axis.

In hand tapping, the work is usually distributed between two or three taps (a set of taps is used). Only the finishing (No.3) tap has a full thread profile. The roughing and middle (Nos. 1 and 2) taps have reduced major diameters. The chamber length differs on three taps. It is longest on the roughing tap (4S) and shortest on the finishing tap (1.5S to 2S). The most commonly applied stock removal distribution has 50 to 60 per cent removed by the roughing tap, 28 to 30 per cent by the middle tap, and 16 to 10 per cent by the finishing tap. Taps are made of high-speed steel or, more frequently, of carbon tool steel. 9.5. Thread-Cutting Dies A threading die is an internally threaded tool used to cut external screw threads by screwing on the work piece. Threads are usually cut in one pass. Threading dies may be solid or split; they may be round, square or hexagon (Fig.9.13 a, b and c ), spring (Fig.333d) or two-piece adjustable dies for a hand stock. A threading die operates in a manner resembling the operation of a tap, except that it cuts external and not internal, thread. Parts and constructional elements of round threading dies (Fig.9.14). The elements associated with the cutting process are: rake angles γ and γN and angle λ; die lands (land width B and width of gap between lands H1); clearance holes; chamber length l1; chamber angle ϕ; die thickness H; number of lands z; chamber relief K and relief angle a. Elements associated with the dimensions of the formed screw threads are; major, minor and pitch diameters of the thread; angle of thread and thread pitch. Elements which provide for mounting the die in a machine tool or die stock are : outside diameter D1 rim thickness e and e1, adjusting slot, spot holes for clamping screws and spot holes for adjusting screws. Round thread-cutting dies are used to cut threads and to size previously cut threads. Thread cutting is accompanied by the removal of a considerable amount of chips, and the clearance holes must be large enough to avoid being clogged by the chips. Only a very thin layer of metal is removed in sizing screw threads and therefore dies for this purpose do not require large clearance holes. Such dies may also be of lower strength.

PROBLEMS 1. What are various types of gear cutting processes? 2. What are the various gear cutting tools operating by form cutting principle? Give geometric details of an/one. 3. What are the various types of gear cutting tools operation by the Generating principle ? Explain the geometric element any one. 4. What is gear shaping? What are it’s advantages of hobbling? 5. What are the advantages of gear hobbling? 6. What are the various methods of thread production? 7. What are the various types of flutes widely used in taps? 8. How can the direction of chip flow be changed? 9. Draw a neat sketch of a tap showing various elements? 10. Draw a neat sketch of a die, showing various elements? 11. Explain the difference between milling a spur gear manufacturing on hobbling Machine tap ?