1-theory+of+metal+cutting.pdf

Manufacturing Process-II

UNIT – I THEORY OF METAL CUTTING

Subject: 15MEC304 - Manufacturing Process-II Class: B.Tech – V Sem - Mechanical ‘B’ section

15MEC304 Manufacturing Process II 3 credit course - 3 hrs. / week

– Wednesday (3pm-4pm) – Thursday (9.45am-10.45am) – Friday (2.00pm-3.00pm) Evaluation Periodical 1 - 15 Periodical 2 - 15 Internal assessment – Assignments & Class test – 20 End semester exam - 50 Email id : [email protected]

15MEC304 Manufacturing Process II - Syllabus SECTION I Theory of metal cutting: Types of metal cutting processes, Mechanism of chip formation- Forces and temperature in metal cutting, Tool life - Machinability and surface finish: Cutting tool materials and cutting fluids. Cylindrical Surface Machining: Basics of turning process, lathe and its Accessories: operations, process parameters. Turret and capstan lathes. Machining time calculations. SECTION II Drilling Machines: Types, operations, process parameters. Boring machines: types, operations, process parameters. Design considerations for drilling, reaming and tapping. Flat and Profile Machining: Milling operations-Milling machines: types, operations, process parameters. Planing and shaping machines- types, operations. Broaching machines-types, operations. Gear machining processes. SECTION III Finishing Processes: Fundamentals of abrasives – Grinding wheels-The theory of grinding processGrinding operations and machines- grinding fluids- design considerations for grinding: Finishing operations- Super finishing processes-lapping and honing. CNC Machines: NC, CNC and DNC, types, construction, tool and work holding devices, part programming- manual and computer automated programming. Modern machining processes: EDM, ECM, USM, Abrasive and water jet machining, High Energy Beam machining, High speed machining.

Text Books

What is manufacturing process?

Labor

Power

Tooling

Machinery

The process of converting raw materials into finished products.

Product

Raw materials

Manufacturing Process

Profit

Manufacturing Processes

Manufacturing Processes: Casting

Manufacturing Processes

Extrusion

Manufacturing Processes: Forming and Shaping

Manufacturing Processes

Manufacturing Processes: Forming and Shaping

Manufacturing Processes

Manufacturing Processes: Joining

Manufacturing Processes

Manufacturing Processes: Machining

Contents ➢ Introduction to machining ➢ Classification of machining processes ➢ Mechanism of chip formation ➢ Heat generated in metal cutting ➢ Tool wear and Tool life

➢ Cutting tool material ➢ Cutting fluids ➢ Machinability

Introduction to Machining Process ➢ Machining is the process of producing a work piece by removing unwanted material from a block of metal, in the form of chips.

Why Machining is required?

Machined Parts

Micro Machined Parts

Characteristics of Machining Process 1. Improve the dimensional accuracy & tolerance of the components produced 2. Internal & external surface features can easily produced Machined Parts

3. Specified surface characteristics or texture can be achieved on a part 4. May be economical to produce a component

Micro Machined Parts

Classification of Machining Process ➢Machining or Cutting Processes – material removal by a sharp cutting tool, e.g., turning, milling, drilling.

➢Abrasive processes – material removal by hard, abrasive particles, e.g., grinding. ➢Non-traditional Processes or Special Processes - various energy forms other than sharp cutting tool to remove material.

Classification of Machining Process

Examples of Machining Processes

Examples of Machining Processes

Basic Elements of Machining The Basic Elements of Machining Operations are ➢ Work piece

➢ Cutting Tool ➢ Chip

Cutting Parameters

RPM-N

Cutting Speed ➢ Distance travelled by the work surface in a unit time with reference to the cutting edge of the tool ➢ It is usually expressed in m/min.

➢ Cutting speed to be used depends upon work piece material, cutting tool material, machining process,

cutting fluid, machine tool used, surface finish required and tool life.

Feed ➢ Distance advanced by the tool (assuming job remains stationary) for each revolution of the work piece.

➢ It is usually expressed in mm/rev. ➢ Low feed produces fine surface finish.

➢ Feed to be used depends upon size of work piece, tool geometry, tool & work piece material, cutting fluid, cutting speed and depth of cut

Depth of Cut (DOC) ➢ Perpendicular distance between the machined surface and the un machined surface or the previously machined surface. ➢ It is usually expressed in mm.

Assignment 1 - Broad classification of Machine Tools Due: 12 July 2017

• • • • • • • • •

According to direction of major axis According to purpose of use According to degree of automation According to size According to precision According to number of spindles According to blank type According to type of automation According to configuration

Machining-Chip Formation

Figure 21.2 (a) A cross-sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).

Mechanism of Chip formation The form of machined chips depend mainly upon : ➢ Work material ➢ Material and geometry of the cutting tool ➢ Levels of cutting velocity and feed and also to some extent on depth of cut ➢ Machining environment or cutting fluid that affects temperature and friction at the chip-tool and work-tool interfaces.

Knowledge of basic mechanisms of chip formation helps to understand the characteristics of chips and to attain favourable chip forms.

Types of Chip in Machining 1. Continuous chip 2. Discontinuous chip 3. Continuous chip with Built-up Edge (BUE)

Continuous chips • Continuous chips are usually formed with ductile materials at high rake angles and/or high cutting speeds. • A good surface finish is generally produced. ➢ Ductile work materials ➢ High cutting speed ➢ Small chip thickness (fine feed) ➢ Sharp cutting edge ➢ Large rake angle ➢ Less friction between chip tool interface through efficient lubrication

Continuous chips Compression

Shear stress develops Reaches or exceeds the shear strength Yielding or slip takes place resulting shear deformation

Continuous chips

Discontinuous chips Discontinuous chips occur when machining hard brittle materials such as cast iron. Brittle failure takes place along the shear plane before any tangible plastic flow occurs. Discontinuous chips will form in brittle materials at low rake angles (large depths of cut).

Discontinuous Chips Discontinuous chips usually form under the following conditions:

1.Brittle work piece materials 2.Work piece materials that contain hard inclusions and impurities, or have structures such as the graphite flakes in gray cast iron. 3.Very low or very high cutting speeds. 4.Large depths of cut. 5.Low rake angles. 6.Lack of an effective cutting fluid. 7.Low stiffness of the machine tool.

Discontinuous Chips Wedging action of the cutting edge

small crack develops

sharp crack-tip stress concentration

crack quickly propagates, under stressing action, and total separation takes place

Continuous Chips with Built-Up Edge (BUE) BUE, consisting of layers of material from the work piece that are gradually deposited on the tool, may form at the tip of the tool during cutting.

As it becomes larger, BUE becomes unstable and eventually breaks up. Part of BUE material is carried away by the tool side of the chip; the rest is deposited randomly on the work piece surface. The process of BUE formation and destruction is repeated continuously during the cutting operation, unless measures are taken to eliminate it.

Continuous Chips with Built-Up Edge (BUE) The tendency for a BUE to form is reduced by any of the following practices: 1. Increase the cutting speeds 2. Decreasing depth of cut 3. Increasing the rake angle 4. Using a sharp tool 5. Using an effective cutting fluid 6. Use a cutting tool that has lower chemical affinity for the work piece material.

Chip Breakers Continuous chips are not always desirable, particularly in automated machine tools, it tends to get tangled around the tool and operation has to be stopped to clear away the chips.

(a) Schematic illustration of the action of a chip breaker. Note that the chip breaker decreases the radius of curvature of the chip and eventually breaks it. (b) Chip breaker clamped on the rake face of a cutting tool. (c) Grooves in cutting tools acting as chip breakers. Most cutting tools used now are inserts with builtin chip breaker features.

Cutting Tool Classification 1. Single-Point Cutting Tools – One dominant cutting edge – Point is usually rounded to form a nose radius – Eg: Turning uses single point tools

2. Multi-Point Cutting Tools – More than one cutting edge – Motion relative to work achieved by rotating – Eg: Drilling and milling use rotating multiple cutting edge tools

Cutting Tool Classification

Figure: (a) A single-point tool showing rake face, flank, and tool point; (b) A helical milling cutter, representative of tools with multiple cutting edges.

Right-hand Cutting Tool and Insert

Fig: (a) Schematic illustration of right-hand cutting tool. Although these tools have been produced traditionally from solid tool-steel bars, they have been replaced largely with (b) inserts made of carbides and other materials of various shapes and sizes.

Single Point Cutting Tool Geometry Geometry of positive rake single point cutting tool End cutting edge angle (ECEA)

Top View Nose Radius (NR) Side cutting edge angle (SCEA)

Back rake angle (αb) Side rake angle (αs)

Lip angle Front View Side View

Side relief angle (SRA)

End relief angle (ERA)

Single Point Cutting Tool Geometry Geometry of negative rake single point cutting tool End cutting edge angle (ECEA)

Top View Nose Radius (NR) Side cutting edge angle (SCEA) Side rake angle (αs)

Back rake angle (αb)

Lip angle Side View

Side relief angle (SRA)

End relief angle (ERA)

Front View

FV

(Lip angle)

Side view

(side clearance angle)

Top view

Tool Signature It is the system of designating the principal angles of a single point cutting tool. The signature is the sequence of numbers listing the various angles, in degrees, and the size of the nose radius. There are several systems available like American standard system (ASA), Orthogonal rake system (ORS), Normal rake system (NRS), and Maximum rake system (MRS).

Tool Signature For example a tool may designated in the following sequence:

8-14-6-6-6-15-1 1. Back rake angle is 8 2. Side rake angle is 14 3. End relief angle is 6 4. Side relief angle is 6 5. End cutting Edge angle is 6 6. Side cutting Edge angle is 15 7. Nose radius is 1 mm

➢

The system most commonly us (ASA), which is: Tool Signature- ASA

✓ The system most commonly used is American Standards Association (ASA) ✓ Also called as Machine Reference System as the three planes for describing the angles based on configuration and axes of machine tool ✓ The single point tool is designated as:

Bake rake angle, Side rake angle, E cutting Edge angle, Side cutting Ed

αb - αs - θe - θs - Ce - Cs - r Bake rake angle Side rake angle End relief angle

Side relief angle, End cutting Edge Angle Φe

Side cutting Edge angle Φs

Nose radius

Nomenclature of Single Point Tool The most significant terms in the geometry of a cutting tool angles are: – Rake angle » Back Rake angle » Side Rake angle – Relief or clearance angle » End relief » Side relief – Cutting edge angle » End Cutting edge angle » Side Cutting edge angle – Nose Radius

Rake Angle ➢It is the angle formed between the face of the tool and a plane parallel to its base ➢If this inclination is towards the shank, it is known as back rake or top rake, when it is measured towards the side of the tool, it is called side rake. ➢These rake angles guide the chips away from the cutting edge, thereby reducing the chip pressure on the face and increasing the keenness of the tool so that less power is required for cutting ➢An increased rake angle(+ve rake) will reduce the strength of the cutting edge ➢Hence tools used for cutting hard metals are given smaller rake angles whereas those used for softer metals contain larger rakes. 8 – 100 140

Negative rake ➢If the face of the tool is so ground that it slopes upwards from the point it is said to contain negative rake ➢It obviously reduces the keenness of the tool and increases strength of the cutting edge ➢Such a rake is usually employed on carbide tipped tools when they are used for machining extra hard surfaces like hardened steel parts and for taking intermittent cuts ➢A tool with negative rake will have a larger lip angle, resulting in a stronger tool ➢Favourable for tipped tools. Normally varies from 5 to 10 degrees.

Negative Rake Tools ➢ Typical tool materials which utilize negative rakes are: ➢Carbide ➢Diamonds ➢Ceramics(At high speeds)

➢ These materials tend to be much more brittle than HSS but they hold superior hardness at high temperatures. ➢ The negative rake angles transfer the cutting forces to the tool which help to provide added support to the cutting edge.

Relief or Clearance angle – Ground on the end and side faces of a tool to prevent it

from rubbing on the work piece. – To enable only the cutting edge to touch the work piece and cut freely without rubbing against the surface of the job

Side Relief angle: • Angle ground directly below the cutting edge on the flank

of the tool

End Relief angle: • Angle ground from the nose of the tool

Cutting edge angle • Ground on a tool so that it can be mounted in the correct position for various machining operations.

Side Cutting edge angle • Allows flank of the tool to approach the work piece first • Spreads the material over a greater distance on the cutting edge, thereby thinning out the chip. • Approximately 150

End Cutting edge angle • Allows the cutting tool to machine close to the work piece during turning operations • Usually 20 – 300

Nose Radius • Rounded tip on the point of the tool Functions: • Strengthens finishing point of tool • Improves surface finish on work • Should be twice amount of feed per revolution • Too large – chatter; • Too small – weakens point • Values: 0.4 mm to 1.6 mm

Methods of Metal cutting The Basic Methods of metal cutting ➢ Orthogonal Cutting or 2D cutting The cutting edge of the tool is perpendicular to the cutting velocity, the process is called orthogonal cutting ➢ Oblique Cutting or 3D cutting

The cutting edge of the tool is not perpendicular to the cutting velocity but set an angle with the normal to the cutting velocity

Orthogonal and Oblique Cutting Orthogonal cutting

Oblique cutting

Work

Work

Feed Feed Tool Tool

• The cutting edge of the tool remains inclined at an acute angle to the direction of tool feed or • The cutting edge of the tool work feed. remains normal to the direction of tool feed or • The direction of the chip flow velocity is at an work feed. angle with the normal to the cutting edge of the tool. The angle is known as chip flow angle. • The direction of the chip flow velocity is normal to the • Three components of forces (3D cutting) cutting edge of the tool. – Cutting Force – Radial force • Two components of forces – Cutting Force – Thrust Force.

• 2D cutting

– Thrust Force or feed force

• The cutting edge being oblique, the shear force acts on a larger area and thus tool life is increased.

Oblique Cutting – Influence of inclination angle

Forces acting on a cutting tool

The force system in general case of conventional turning process

‘Turning’ Forces For Orthogonal Model Velocity of Tool relative to workpiece V WORKPIECE

F C Tangential 'Cutting' Force (67%) DIRECTION OF ROTATION

Fr Radial Force (6%)

Longitudinal F t 'Thrust' Force (27%)

'A'

'A' CUTTING TOOL

DIRECTION OF FEED

Fc

Ft

Note: For the 2D Orthogonal Mechanistic Model we will ignore the radial component

End view section 'A'-'A'

Forces acting on a cutting tool

61

Orthogonal Cutting Model A simplified 2-D model of machining that describes the mechanics of machining fairly accurately.

Ernst and Merchant Theory Chip thickness ratio

a

t2 B C

t1 D

A

Ernst and Merchant Theory Shear Angle

As density remains the same before and after cutting, l1b1t1 = l2b2t2

In orthogonal cutting, b1=b2 l1t1 = l2t2 t1/t2 = l2/l1

Velocity Relationship



Using sine rule,

v vs v  c  sin(90  (  a )) sin  sin(90  a )

vc vs   cos(  a ) sin  cos a v

vc  v  r v = cutting velocity vc = chip velocity vs = shear velocity

 sin   r  cos ( - a )   

= velocity of tool relative to work = velocity of chip relative to tool = velocity of chip relative to work

vc 

vs 

v sin  cos(  a )

v cosa cos(  a )

64

Chip Thickness Ratio

The ratio of to/tc is known as the cutting ratio, r, expressed as:

Where, r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separation

65

Determining Shear Plane Angle Based on the geometric parameters of the orthogonal model, the shear plane angle  can be determined as:

r cos a tan   1  r sina When shear angle is small The plane of shear will be larger, chip is thicker and higher force required to remove wherethe r =chip. chip ratio, and a = rake angle When shear angle is large The plane of shear will be shorter, chip is thinner and hence lesser force required to remove the chip. The shear angle is determined from chip thickness ratio. 66

Forces in Metal Cutting

F, N, Fs, and Fn cannot be directly measured Forces acting on the tool that can be measured: – Cutting force Fc and Thrust force Ft

67

Fs = Shear force, which acts along the shear plane, is the resistance to shear of the metal in forming the chip Fn = Force acting normal to the shear plane ,is the backing up force on the chip provided by the work piece F = Frictional resistance of the tool acting against the motion of the chip as it moves upward along the tool N = Normal to the chip force, is provided by the tool Fc = Horizontal cutting force exerted by the tool on the work piece Ft = Vertical force which helps in holding the tool in position and acts on the tool nose

1. Merchant circle is useful to determine the relation between the various forces & angles. 2. In the diagram two force triangles have been combined & R & R’ together have been replaced by R 3. The force R can be resolved into two components Fc & Ft

4. Fc & Ft can be determined by force dynamometers 5. The rack angle (α ) can be measured from the tool & forces F & N can then be determined 6. The shear angle (Ф) can be obtained from it’s relation with chip reduction coefficient 7. Now Fs & Fn can also be determine

Merchant’s Circle Diagram Chip

α Tool

Fs

∅

Fc

Clearance Angle

(β - α)

Work

Fn Ft

R

α F

β N

78

The procedure to construct a Merchant’s circle diagram Chip Fs

Fc

∅

Tool Clearance Angle Work

Fn Ft

α

R

α F

β

N

79

Relationship of various forces acting on the chip with the horizontal and vertical cutting force from Merchant circle diagram Frictional Force System D C (90-α) α (90-α)

E

Fc

α

Fc

α

B

α

Tool

(β - α)

Work R

Ft

α F

F β

α Clearance Angle

Fn R

Ft

∅

O

(β - α)

G

Chip Fs

α

β N

N A

F  OA  CB  CG  GB  ED  GB  F  FC sin a  Ft cosa N  AB  OD  CD  OD  GE  N  FC cosa  Ft sin a

The coefficient of friction F   tan  N Where   Friction angle 82

Relationship of various forces acting on the chip with the horizontal and vertical cutting force from Merchant circle diagram Shear Force System B A

(90-∅) C

∅

∅ Fn

Work

R

Ft R

Tool

(β - α) Fn

D

α Clearance Angle

∅

O

(β - α)

(90-∅) Ft

Fc

Fs Fc

Chip Fs

α

α

α F

∅ β N E

FS  OA  OB  AB  OB  CD  FS  FC cos  Ft sin  FS  RCos(   a   

Also:

FN  FS tan(    a )

FN  AE  AD  DE  BC  DE  FN  FC sin   Ft cos

83

Relationship of various forces acting on the chip with the horizontal and vertical cutting force from Merchant circle diagram α

Chip

Tool

Fs Fc

Clearance Angle

∅ (β - α)

Work

Fn R

Ft

α

F

F  FC sin a  Ft cosa N  FC cosa  Ft sin a FS  FC cos  Ft sin  FN  FC sin   Ft cos FN  FS tan(    a )

β N

Ft = R Sin (β-α) Fc = R Cos (β –α) 84

Coefficient of Friction Coefficient of friction between tool and chip:

F   N Friction angle related to coefficient of friction as follows:

  tan  The ratio of F to N is the coefficient of friction, μ, at the tool-chip interface, and the angle β is the friction angle.

F Ft  Fc tana Coefficient of friction,    N Fc  Ft tana The coefficient of friction in metal cutting generally ranges from about 0.5 to 2. 75

Shear Stress Shear stress acting along the shear plane: F   s A s

where As = area of the shear plane,

t1b1 As  sin 

Shear stress = shear strength of work material during cutting 77

Shear Strain in Chip Formation

Figure: Shear strain during chip formation: (a) Chip formation depicted as a series of parallel plates sliding relative to each other, (b) One of the plates isolated to show shear strain, and (c) Shear strain triangle used to derive strain equation.

Shear strain in machining can be computed from the following equation, based on the parallel plate model: Where, ε =Shear strain,  = Shear plane angle and a = Rake angle of cutting tool

Power required in Metal cutting The Power consumed/ work done per sec in cutting: The Power consumed/ work done per sec in shear: The Power consumed/ work done per sec in friction:

PC  Fc  v

Ps  Fs  vs PF  F  vc

The total Power required:

P  Pc  Ps  Pf

90

Sources of Heat in Metal Cutting During Metal cutting, heat is generated in three region as shown in Figure.

1. Around shear plane 2. Tool-chip interface 3. Tool-work interface

Sources of heat and Causes of development of cutting temperature in Machining During machining, heat is generated at the cutting point from three sources, as indicated in Fig. Those sources and causes of development of cutting temperature are: ➢ Primary shear zone (1) where the major part of the energy is converted into heat. ➢ Secondary deformation zone (2) at the chip – tool interface where further heat is generated due to rubbing and / or shear. ➢ At the worn out flanks (3) due to rubbing between the tool and the finished surfaces.

Cutting Temperatures Cutting temperatures are important because high temperatures, 1. Reduce tool life. 2. Produce hot chips that pose safety hazards to the machine operator.

3. Can cause inaccuracies in work part dimensions due to thermal expansion of work piece material.

Distribution of Heat

Temperature distribution in Metal Cutting

Measurement of tool-chip interface temperature

Control of cutting temperature ➢ Proper selection of material and geometry of the cutting tool ➢ Optimum selection of cutting speed, feed and depth of cut

➢ Proper selection of cutting fluid

Cutting Fluids

Lubricants – purpose is to reduce friction… usually oil based Coolants – purpose is to transport heat… usually water based Both lose their effectiveness at higher cutting speeds!

Characteristics of a Good Cutting Fluid ➢ Good cooling capacity ➢ Good lubricating qualities ➢ Stability (long life) ➢ Rust resistance ➢ Nontoxic ➢ Transparent ➢ Nonflammable 89

Economic advantages to using Cutting Fluids • Reduction of tool costs – Reduce tool wear, tools last longer

• Increased speed of production – Reduce heat and friction so higher cutting speeds

• Reduction of labor costs – Tools last longer and require less regrinding, less downtime, reducing cost per part

• Reduction of power costs – Friction reduced so less power required by machining 91

Tool wear and failure During Machining, tool is subjected to three factors ▪ Forces ▪ Temperature ▪ Sliding action Due to this factor the tool life will start giving the following unsatisfactory performance after some time • Loss of Dimensional accuracy • Increased surface Roughness • Increased Power Requirements When the tool wears out, it is either replaced or reconditioned by grinding. This results in loss of production due to machine down time

Tool wear and failure Tool wear Depends on

➢ Hardness ➢ Tool material ➢ Type and condition of work piece ➢ Cutting speed, feed and DOC ➢ Tool Geometry ➢ Tool Temperature ➢ Surface finish of tool ➢ Cutting fluid

Tool wear and failure The usefulness of tool cutting edge is lost through ➢ Wear ➢ Breakage

➢ Chipping ➢ Deformation

Tool failure implies that the tool has reached a point beyond which it will not function satisfactorily until it is re-sharpened.

Tool Wear: Mechanisms ➢ Adhesion ➢ Abrasion

➢ Solid state Diffusion

96

Adhesive wear

Abrasive wear

Diffusion wear

100

Preferred Mode of Tool Failure: Gradual Wear ➢ Fracture and temperature failures are premature failures ➢ Gradual wear is preferred because it leads to the longest possible use of the tool ➢ Gradual wear occurs at two locations on a tool: ➢Crater wear – occurs on top rake face

➢Flank wear – occurs on flank (side of tool)

Tool Wear

Figure: Diagram of worn cutting tool, showing the principal locations and types of wear that occur

Tool Wear

Tool Wear

Tool Wear vs. Time

Figure: Tool wear as a function of cutting time Flank wear (FW) is used here as the measure of tool wear Crater wear follows a similar growth curve

Effect of Cutting Speed

Figure: Effect of cutting speed on tool flank wear (FW) for three cutting speeds, using a tool life criterion of 0.50 mm flank wear

Tool Life Tool life is defined as the time interval for which tool works satisfactorily

between

re-sharpening of the tool.

two

successive

grinding

or

Factors affecting Tool life ➢ Tool material ➢ Hardness ➢ Work material

➢ Surface roughness of work piece ➢ Profile of cutting tool ➢ Type of machining operation ➢ Cutting speed, feed and depth of cut ➢ Cutting temperature

Taylor Tool Life Equation This relationship is credited to F. W. Taylor (~1900)

vT Where,

n

C

v = cutting speed; T = tool life; and

n and C are parameters that depend on feed, depth of cut, work material, tooling material, and the tool life criterion used

n is the slope of the plot C is the intercept on the speed axis

Cutting Tool Materials

Cutting Tool Materials • Selection of cutting tool material is very important. • Performance will determine efficiency of operation.

111

112

Tool materials

Hardness of the cutting tool

Properties of cutting tool materials

Inserts

Insert Strength

Insert Specification

Carbon Steels • Limited tool life. Therefore, not suited to mass production • Can be formed into complex shapes for small production runs • low cost • suited to hand tools, and wood working • Carbon content about 0.9 to 1.35% with a hardness ABOUT 62 Rockwell

High Speed Steel (HSS) ➢ Very highly used alloy steel ➢ Can be hardened to various depths ➢ Good wear resistance ➢ High toughness ➢ Good for positive rake angle tools. ➢ Two basic types of HSS ➢ Molybdenum: ( M Series) ➢ Tungsten: (T Series)

High Speed Steel T Series • 12 – 18 % Tungsten • Chromium, vanadium etc. M Series – – – – – –

10% Molybdenum Chromium, Vanadium, Tungsten, Cobalt Better abrasion resistance Less expensive Less distortion 95% of HSS used is M series

High Speed Steel ➢ In addition 2.5 % to 10%cobalt – used to increase the red hardness/ hot hardness

➢ 0.8% C and rest iron ➢ HSS is used for drills,milling cutters,single point cutting tools,reamers and dies.

High Speed Steel ➢ It looses hardness above 600ºC ➢ Used at cutting speeds 40 to 50 m/min ➢ can cut materials with tensile strengths up to 75 tons/sq.in. ➢ Hardness is in the range of 63-65°C Rockwell

Coated HSS ➢ TiN Coatings- PVD Process – drills & taps ➢ Longer life, high productivity ➢ Reduced power consumption ➢ Coating is approximately three times harder than base tool material ➢ Coating-lubricity-improves chip flow- less heat generation ➢ Higher cutting speed

Stellite • Cobalt 40 to 50% , Chromium 27 to 37% • Tungsten 14 to 29 %, Carbon 2 to 4% • It can not be heat treated and are used in the as cast form • It looses its hardness about 800ºC • Better tool life than HSS • Weak in tension, fails under shock load if not • supported

Cemented Carbides ➢ Introduced in the 1930s. These are the most important tool materials today because of their high hot hardness and wear resistance. ➢ There may be other carbides in the mixture, such as titanium carbide (TiC) and/or tantalum carbide (TaC) in addition to WC. ➢ The main disadvantage of cemented carbides is their low toughness.

Cemented Carbides

Cemented Carbides – General Properties ➢ High compressive strength, but low to moderate tensile strength ➢ High hardness (90 to 95 HRA)

➢ Good hot hardness ➢ Good wear resistance ➢ High thermal conductivity ➢ High elastic modulus - 600 x 103 MPa (90 x 106 lb/in2) ➢ Toughness lower than high speed steel

Cemented Carbides ❖ This hard tool material is produced by a powder metallurgy technique, sintering grains of tungsten carbide (WC) in a cobalt (Co) matrix (as the binder, it provides toughness). ❖ Particles 1-5 μm in size are pressed & sintered to desired shape in a H2 atmosphere furnace at 15500 C. ❖ Amount of cobalt present affects properties of carbide tools. As cobalt content increases – strength, hardness & wear resistance increases.

Coated Tools

Coated Tools

Coated Tools

Ceramics • Magnesium and chromium added to obtain superior quality • High degree of compressive strength • Good abrasion resistance • Low friction co-efficient • Uniform strength up to 1200ºC • Hot hardness-1400 ºC

Diamond • Diamond is the hardest substance ever known of all materials. • Low friction, high wear resistance. • Ability to maintain sharp cutting edge.

• Use is limited because it gets converted into graphite at high temperature (700 °C). Graphite diffuses into iron and make it unsuitable for machining steels. • It is used as a coating material in its polycrystalline form, or as a single- crystal diamond tool for special applications, such as mirror finishing of non-ferrous materials.

Cubic Boron Nitride ➢ Next to diamond, cubic boron nitride (CBN) is hardest material known. Retain hardness up to 1000°C. ➢ By bonding 0.5 mm thick polycrystalline CBN onto a carbide substrate through sintering under pressure.

➢ CBN is used mainly as coating material because it is very brittle. ➢ In spite of diamond, CBN is suitable for cutting ferrous materials. Applications: machining steel and nickel-based alloys.

Cubic Boron Nitride ➢ SPD and CBN tools are expensive. ➢ Made by bonding (0.5-1.0 mm) Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under Pressure. ➢ While carbide provides shock resistance CBN layer provides high resistance and cutting edge strength. ➢ Cubic boron nitride tools are made in small sizes without substrate.

General operating characteristics of cutting tool materials

Machinability ➢ The term machinability refers to the ease with which a metal can be cut (machined) permitting the removal of the material with a satisfactory finish at low cost.

➢ Machinability Index Machinability index (%) = cutting speed of material for 20 min tool life) / (cutting speed of free cutting steel for 20 min tool life) * 100%

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Criteria for Machinability ➢ Rate of Production ➢ Tool life ➢ Surface finish ➢ Power consumption

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Rate of Production ➢ Number of machined parts in unit time

➢ Rate of material removal

Tool life

➢ Period for which tool cuts efficiently and economically between regrinds ➢ Volume of metal removed

➢ Number of pieces machined ➢ Equivalent cutting speed

Life of the tool is over when ✓ Spoiled cutting edge ✓ Poor surface finish ✓ Chatter marks on the job ✓ Dimensional instability ✓ Overheating or fuming ✓ Sudden increase in cutting force and power

➢ Time units 140

Tool life Equation ➢ vTn = c

➢ v fn dg Tn = C1

➢ c = cutting speed for one minute of tool life ➢ n = 0.1 for HSS, 0.125 to 0.25 for carbide tools

➢ Effect of nose radius v T0.0027 = 331 R0.244 ➢ Effect of side cutting edge angle v T0.11 = 78(es + 150)0.064 ➢ In terms of metal removed, ➢ Volume/rev = pdf D mm3

➢ Volume/min = pdf D N mm3/min ➢ Volume/ T min = pdf D N T mm3 ➢ L = pdf D N T mm3 = 1000 v df T mm3 = v df T cm3

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Surface Finish ➢ Visual inspection

➢ Surface roughness blocks ➢ Profilometer ➢ Profilograph To improve finish

To deteriorate finish

➢ Cutting speed

➢ Feed

➢ Nose radius

➢ Depth of cut

➢ True rake angle ➢ Machine tool rigidity ➢ Proper cutting fluid

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Power Consumption ➢ Hardness

-

Increases cutting force and power

➢ Structure

-

Fibrous structure requires more power

➢ Rake angle -

Reduces cutting force

➢ Feed/DOC -

Increases power consumption

Effect of ➢ Cold Working

➢ Grain size ➢ Microstructure ➢ Tooling

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Dynamometers Mechanical type

Piezoelectric type

Strain Gauge type

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