Machine Tool # 1

MACHINE TOOL

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Metal Cutting

Metal cutting process consists is removing a layer of metal from blank to obtain a machine part of the required shape and with a specified quality of surface finish. In order to cut the material from blank the cutting tool should be harder than material to be cut, the tool should penetrate the blank and the tool should be strong enough to with stand the forces developed in cutting. Metal cutting process or of two types:. 1) Chip removal process 2) Non Chip removal process Chip removal process: process: In chip removal process the desired shape and dimension are obtained by separating a layer from the parent work piece in from of chip. The various chip forming process are as follows. Turning, Boring, Milling, Grinding, Drilling, Shaping. During the process of metal cutting there is a relative motion between the work piece and cutting tool. This relative motion ------------ depend upon the types. In any metal cutting process the following aims should be achieved. 1) Work piece surface finish should be of desired quality. 2) Metal removal rate should be high. 3) Power consume during the process should be low. 4) Cutting tool life should be more. Chip less Process: Process: In chip less process the metal is given the desired shape without removing any material from the parents work piece. Some of the chip less process are as follows. a) Rolling b) Spinning c) Forging Metal Cutting Principle: Principle: Metal cutting is one of the most important process carry out in an industry. The purpose of any material cutting and

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finish of a component by operation commonly called machining is to produce are desired Shape, Size, removing the excess_____ material in the form of chip from a rough blank a material. The machining process should be carry out at high speeds and feeds and least cutting effort and at lowest cost. Number of interrelated factor effect metal cutting, the must important factor are as follow. a) The properties of work material b) The properties and geometry cutting tool c) The interaction between the tool and the work during metal cutting. During cutting process the following properties work piece material are quite important. a) Hardness b) Toughness c) Abrasive On the other hand the tool material should be hard, strong, tough, and wear resistance. One of the basic requirements for metal cutting operations is that the tool is harder than the work piece. Both are held rigidly and the tool tends to penetrate into the work piece. There are two methods of producing surface by cutting tool. A. Generating B. Forming Generating: Generating:-

In generating a cylinder the work rotates about an axis and the cutting tool moves in a direction parallel to the axis. Forming:Forming:

In forming a cylinder the work is stationary and the cutting tool moves into direction in the axis of the cylinder. BASIC ELEMENTS OF MACHINING: MACHINING: The basic elements of machining are as follows a) Work piece B ) Cutting tool C ) Chip The work piece provide the parents metal from which the unwanted material is removal by cutting action of tool to obtain desired size and shape. For providing the cutting action relating motion between tool and work piece is necessary. Classification of Cutting Tool: Tool:

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Classification of cutting tool depending upon the number of cutting edge the cutting tool used in metal cutting or classified as follows 1. Single point cutting tool. 2. Multi point cutting tool.

Point Single P oint Cutting Tool :This type of tool has an effective cutting edge and removes excess material for the work piece along the cutting edge .Single point cutting tool is of the following types. 1) Ground Type 2) Tipped Type 3) Forged Type 4) Bit Type 1) Ground Type ::-In ground type the cutting edge is formed by grinding the end of tool. 2) Forged Type ::-In this type of cutting tool the cutting edge is formed by rough forging before Harding and grinding. 3) Tipped Type ::-In this type of cutting tool the cutting edge is in the form of small tip make of high grade material which is welded to a shank made up lower grade. 4) Bit Type: Type:- In this type of cutting tool a high grade material of a square, rectangular, some other shape is held mechanically in a tool holder. Single point cutting tool are commonly used in lathe, shapers, planners, boning machines and slotters. single point cutting tool may be left handed or right handed type. A tool is set to be right hand type if the cutting edge is on the right side When viewing tool the point end. A tool is set to be left hand type if the cutting edge on the left side When viewing tool from the point end. Lathes tool, shaper tools, plainer tools, and boring tools are single point tool.

Multipoint Multipo int Cutting Tools: They have more than one effective cutting edge to removes the excess material from the work piece. Milling cutters, drills, reamers broaches and grinding wheels are multipoint cutting tools.

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TOOL GEOMETRY The desirability of getting the maximum use from a tool before it needs regrinding is one of the objectives of tool technology. There are many factors that contribute to cutting tool efficiency. Among the most important of these are the following. 1. the shape of the cutting edge that removes the excess material. 2. the correct selection of the type of cutting tool for the material to the machine. 3. The correct choice of cutting speed and feed. 4. the proper setting of cutting tool relative to work 5. the correct choice and proper application of coolants. The optimum tool geometry depends upon the following factors. 1. work piece material 2. machining variables a) cutting speed b) feeds c) depth of cut 3. material of the tool point 4. type of cutting

TOOL LIFE Tool life is defined as the length of time, a tool will operate before its failure occurs.

Requirements of a cutting tool: For high production machining should possess for following requirements.
It should be strong and rigid.
it should be sharpened with a high class finish.
It should have optimum geometry.
It should be convenient ______ in use. The performance of cutting tool mainly depend on the following:
Cutting tool material properties.
Geometry of cutting tool.
Methods use for clamp, indexing and change of cutting tool. The operational characteristics of a cutting tool are described by machine ability. There are three main aspect_____ of machine ability :

1. Tool life 3.Power required to cut

2.Surface finish

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Types of Single point tool: A single point tool cutting tool is quite commonly used in metal cutting. All single point tools are divided into a number of groups according to: 1. direction of feed such as
Right hands tools
Left hand tools.

2. Shank from such as:





Straight Bent Offset Necked tools

3. Application on a certain type of machine tool such as:





Turning Planning Shaping Boring and tool for special purpose machine tools.

4. Machining processes 5. Shank cross section such as:
Rectangular
Square
Round

6. Tool position relative to work such as:
Radial
Tangential

7. Cutting material such as:
High speed steel tool
Carbide tipped tools
Ceramic tools.

8. Method of manufacturing such as
Solid
Tipped

Types of metal cutting process The metal cutting processes are of two types: 1. Orthogonal cutting process ( two dimensional cutting)

2. Oblique cutting process ( three dimensional cutting) Orthogonal cutting process:

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Orthogonal cutting occurs when the major cutting edge of the tool is presented to the work piece perpendicular to the direction of feed motion. Orthogonal cutting involves only two forces.

Work piece

Tool

Oblique cutting process : This form of cutting occurs when the major edge of cutting tool is presented to the work piece at an angle which is not perpendicular to the feed motion.

Work piece

Tool

Comparison of orthogonal and oblique cutting: cutting:

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Chip Formation: Formation:

S.No. Orthogonal cutting

Oblique cutting

1

The cutting edge of the tool The cutting edge of the tool is remains normal (90) to the inclined at an acute angle to the direction of tool feed. direction of tool feed. 2 The direction of chip flow The direction of chip flow velocity velocity is normal to the is at an angle with the normal to cutting edge of the tool. the cutting edge of the tool. 3 The cutting edge clears the The cutting edge may or may not width of the work piece on clear the width of the work piece. either ends. 4 Only two components of Three mutually perpendicular cutting forces act on the tool. components of cutting forces act at These two components are the cutting edge of the tool. perpendicular to each other. The variable which influence____ the type of chip produced are as follows: 1. properties of material cut especially________ ductility. 2. depth of cut 3. feed rate 4. effective rake angle of tool 5. cutting speed 6. type and quantity of cutting fluid. Three different type of chips formed during metal cutting are as follows: 1. continuous chip 2. Discontinuous chip 3. Continuous chip with a built-up-edge.

chip:-Continuous chip: A continuous chip is obtained when cutting ductile material, such as low carbon steel, aluminum, and copper. This chip is severely deformed and either comes off in the form of a long string, or curls into a tight roll. some very soft and ductile material with a low strength tend to tear away from the parent metal of the work piece rather than shear clearly.

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chip

Work piece

The factors responsible for the formation of continuous chips are given below: ▪ High cutting speed • Ductile material • Large rake angles ▪ Sharp cutting edge • Efficient cutting fluids ▪ Low friction The tool life increases and the power consumption reduces, in case of continuous chips.

Discontinuous chip: The compressed material ahead of tool fail in a brittle manner along the shear zone producing small fragments. such chips are called discontinuous chips. as shown under chip

work piece

Discontinuous chips are associated with brittle materials such as cast iron and free cutting brass. low cutting speeds and lack of rake can also cause discontinuous chips to be formed when cutting ductile materials such as mild steel. Continuous chip with a builtbuilt-upup-edge: When during cutting, temperature and pressure is quite high it causes the chip material to weld itself to the tool face near the nose. this is called “Built up edge”.

As shown under chip

Work piece

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This process gives poor finish on the machine surface. various factors responsible for the production of continuous chips with built-up edges are as follows: • Ductile work piece material • small rake angles • Low cutting speed • Dull cutting edge • Insufficient cutting fluid • high friction at the tool-chip interface • Coarser feed. The extra metal, welded to the nose or point of the tool is called ‘Built-upedge’ (BUE).

*Chip Thickness Ratio or Cutting Ratio:The outward flow of the metal causes the chip to be thicker after separation the parent metal. Metal prior to being cut is much longer than the chip which is removed. as shown in fig,

Chip thickness before cutting = t 1 Chip thickness after cutting = t 2 Chip thickness ratio, r = t1/ t 2 This chip thickness ratio or cutting ratio is always less than unity. If the ratio “ r ” is large, the cutting action is good. A ratio of 1: 2 yield good results. k = 1/ r k= chip reduction coefficient

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when metal is cut there is no change in the volume of the metal cut. t 1. b 1. l 1 = t 2. b 2. l 2 Chip thickness before cutting = t 1 Width of chip before cutting = b 1 Length of chip before cutting = l 1 Chip thickness after cutting = t 2 Width of chip after cutting = b 2 Length of chip after cutting = l 2 It is observed that b 1 = b 2 .’.

t 1. l 1 = t 2 .l 2

t1 = l 2 t2 l2 chip thickness ratio can be easily obtained by measuring l 1 and l 2 From the right angle triangle ABC , we have

AB =

BC = sin β AB BC / sin β = t 1 / sin β

From right angle triangle ABD BD _ sin ( 90 – β + α ) = cos (β – α ) l 2 /AB = cos (β – α ) l2 AB = cos (β – α )

From (1) and ( 2 ) , we get t1 sin β =

t2 sin β

_

cos (β – α )

t1

=

l2

=

r

=

sin β cos (β – α )

_

_

………(1)

………. ( 2)

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cos β cos α + sin β sin α 1 r

r

=

=

sin β _ cos β cos α + sin β sin α

sin β _ cos β cos α + sin β sin α

r cos β cos α sin β r cos α

+ r sin β sin α = 1 sin β

+ r sin α = 1

sin β cos β

r cos α tan β tan β = 1- r sin α

Where r = shear angle α = rake angle

=

1 - r sin α

r cos α

_

………… (formula)

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Cutting Tool Materials: Materials:-The purpose of cutting tool material to remove metal under controlled condition. therefore the tool must be harder than the material which it is to cut. the cutting tools made up of different materials. the cutting tool material should possess the following requirements;1. It should be strong enough to withstand the forces being applied due the cutting tool i.e. bending, shear, compression etc. 2. It should be tough 3. It should be harder than the material. 4. It should be able to resist high temperature. 5. It should be capable of withstanding the sudden cooling effect of coolant used during cutting. 6. The coefficient of friction between the chip and the tool should be as low as possible in the operating range of speed and feed. 7. It should be easily formed to the required cutting shape. The various cutting tool for material can be g as follows. • plain carbon steel • Medium alloy steel • High speed steel (H.S.S) • Non-ferrous alloy steel • Cemented carbides • Ceramics • Diamonds • Cubic boron nitride • UCON • Salon • Carbonate Tool Steel :Plain carbon steels, medium alloy steel and high speed steels are known as tool steel.

Chip Breaker;Chip breaker are provided to control the continuous ribbon-like chip that are formed at high cutting speeds. Continuous chips are dangerous to the operator of the machine. These chips are hard, sharp and hot. The chip

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breaker deflects the chip at the sharp angle and cause to break into small pieces. So that they are easily removed by the coolant or simply allowed to fall into the chip pan of the machine. There are basically two types of the chip breakers: • Groove type • Obstruction type

CuttingFluid Cutting coolants and oils are used during drilling to carry away ____ the heat from the drill point preventing it from overheating. This permits higher cutting speeds and longer drill life. The action of the chips coming out of the hole tends to restrict _____ the entry of the fluid. A continuous supply of cutting fluid should be maintained in order to obtain proper cooling. For severe conditions drills containing oil holes have a considerable advantage. Practically all metals require the use of coolant when being drilled except cast iron which may be machined dry. ===================================