metal cutting mechanical meanufactuing.pptx

metal cutting

1. Chapter : 1 Mechanics of Metal Cutting

2. Introduction -Metal Cutting Metals are shaped in to usable forms through various processes . No-cutting shaping: No chip formation takes place, and the metal is shaped under the action of heat, pressure or both. Ex : Forging, drawing , Spinning, Rolling, Extruding,etc . Cutting shaping: The components are brought to the desired shape and size by removing the unwanted material from the parent metal in the form of chips through machining. Ex : Turning, Boring, Milling, Drilling, Shaping, Planning, Broaching , etc .

3. Introduction -Metal Cutting The ever increasing importance of machining operations is gaining new dimensions in the present industrial age. The growing completion calls for all the efforts to be directed towards the economical manufacture of machined parts. Basic objectives of the economical and efficient machining practice : 1 . Quick Metal Removal (or MRR) 2 . High class surface finish 3 . Economy in tool cost 4 . Less power consumption 5 . Economy in the cost of replacement and sharpening of tools. 6 . Minimum deal time of machine tools .

4. Basic Elements of Machining The basic elements of machining operations are: 1 . Work piece 2 . Tool 3 . Chip

5. For providing cutting action, a relative motion between the tool and work piece is necessary. This relative motion can be provided by: 1 . Either keeping the work piece stationary and moving the tool. Or 2 . By keeping the tool stationary and moving the work. Or 3 . By moving both in relation to one another . Basic Elements of Machining

6. Influence of Parameters on Machining The work piece provides the parent metal, from which the unwanted metal is removed by the cutting action of the tool to obtain the predetermined shape and size of the component . The chemical composition and physical properties of the metal of the workpiece have a significant effect on the machining operation. The tool material and its geometry are equally significant for successful machining. The type and geometry of the chip formed are greatly effected by the metal of the work piece, geometry of the cutting tool and the method of cutting. The chemical composition and the rate of flow of the cutting fluid also provide considerable influence over the machining operation .

7. ORTHOGONAL AND OBLIQUE CUTTING

8. ORTHOGONAL AND OBLIQUE CUTTING Oblique Cutting

9. ORTHOGONAL AND OBLIQUE CUTTING

10. ORTHOGONAL AND OBLIQUE CUTTING ORTHOGONAL OBLIQUE

11. Orthogonal Vs Oblique Orthogonal Cutting The cutting edge of the tool remains normal to the direction of tool feed orwork feed. The direction of chip flow velocity is normal to the cutting edge of the tool. (chip flow angle) The angle of inclination ‘i’ of the cutting edge of the tool with the normal to the velocity Vc is zero . The angle between the direction of chip flow and the normal to the cutting edge of the tool , measured in the plane of the tool face is zero. The cutting edge is longer than the width of the cut. Oblique Cutting The cutting edge of the tool always remains inclined at an acute angle to the direction of tool feed or work feed . The direction of chip flow velocity is at an angle β with the normal to the cutting edge of the tool. (chip flow angle ) The cutting edge of the tool is inclined at an ‘i’ with the normal to the direction of work feed or tool feed Vc . Three mutually perpendicular components of cutting forces act at the cutting edge of the tool. The cutting edge may or may not be is longer than the width of the cut.

12. Classification of Cutting Tools The cutting tools used in metal cutting can be broadly classified as: 1 . Single point tools : Those having only one cutting edge. Ex : Lathe tools, shaper tools, planer tools, boring tools, etc. 2. Multi-point tools: Those having more than one cutting edge. Ex : milling cutters, drills, broaches, grinding wheels, etc. The cutting tools can be classified according to the motion as: 1 . Linear motion tools: Ex : Lathe, boring, broaching, planing , shaping tools, etc. 2 . Rotary Motion tools: Ex : milling cutters, grinding wheels, etc. 3 . Linear and Rotary Motion tools: Ex : drills, honing tools, boring heads, etc.

13. Principal Angles of Single Point Tools

14. Principal Angles of Single Point Tools

15. Tool Signature

16. Tool Signature It indicates the angles that a tool utilizes during the cut . It specifies the active angles of the tool normal to the cutting edge . Some of the common systems are : American System British System Continental System International System

17. Tool Signature American System It defines the principal angles like side rake, back rake, nose, etc. with regarding to the cutting edge and with out any reference to their locations . This system of nomenclature does not give any indication of the tool behavior with regard to the flow of chip during the cutting operation. The three reference planes adopted for designating different tool angles are similar to conventional machine drawing.

18. Tool Signature American System For example a tool may designated in the following sequence : 8-14-6-6-6-15-1 1. Bake 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

19. British System This system defines the Maximum Rake. The variation of tool parameters in this system are indicated in the order of : Bake rake Side rake End Relief angle Side relief angle Side cutting edge angle Nose radius Tool Signature

20. Tool Signature Continental System This system indicates the German System. The various tool parameters are specified with reference to the tool reference planes . International System It is internationally adopted system, developed recently . It incorporates the salient features of tool nomenclature of different systems in it .

21. Reference Planes The following two systems of reference planes are used to describe the geometry and locate the different parameters of single point cutting tool . The Coordinate System The Orthogonal System

22. Reference Planes The Coordinate System: The tool being held in hand against a stationary work piece (Tool in Hand System). Base plane : The horizontal plane which contains the base of the shank of the cutting tool. Longitudinal plane : Vertical plane normal to the base plane and parallel to the direction of feed (f ) . Transverse Plane : Plane perpendicular to the both the above reference planes and is parallel to the depth of cut (d ). This combination of reference planes are known as Coordinate System of Reference Planes.

23. Reference Planes The Orthogonal System: This system assumed as the cutting tool is operating against the work piece. Base Plane : Horizontal Plane contains the base of the cutting tool. Cutting plane : Plane which is perpendicular to the base plane contains the principal cutting edge (c). Orthogonal Plane : third plane which is perpendicular to the both of the above planes. This set of reference planes is known as Orthogonal System of reference planes.

24. Tool Geometry in Coordinate System The Coordinate System: Also called as ASA System of tool signature. Because of the nomenclature of the reference planes X,Y,Z it also called as X-Y-Z Plane System. Various tool angles shown in figure are : αy = Top Rake / Back Rake angle α x = Side Rake angle βy = End Relief / Clearance angle βx = Side Relief / Clearance angle Φe = End Cutting Edge Angle Φs = Side Cutting Edge Angle θ= Nose Angle The order of representation of various parameters as: Back Rake , Side Rake, End Relief, Side Relief, End Cutting Edge, Side Cutting Edge, Nose Radius. The values of Nose Radius θ will depends on the values of Φe and Φs For Example: 8,10,6,6,6,10,2

25. Tool Geometry in Orthogonal System The Orthogonal System: Also called as Orthogonal Rake System (ORS) or International System. Because of the nomenclature of the reference planes L,M,N it also called as L-M-N Plane System. Due to the cutting tool operating on the work piece, many tool parameters are variables in this system. Their actual values are effected by the tool position with regarding to the work piece in actual operation.

26. Tool Geometry in Orthogonal System The Orthogonal System: Various tool angles shown in figure are : Φ0 = Plane Approach angle Φ1 = Auxiliary cutting edge angle λ = Angle of Inclination α = Orthogonal Rake Angle γ = Side Relif Angle β = Wedge Angle δ = Cutting Angle ( = γ + β ) α1 = Side Rake Angle γ1 = End Relief Angle β1 = Side Wedge Angle The order of representation of only main parameters as: Inclination , Orthogonal Rake , Side Relief, End Relief, Auxiliary Cutting, Approach , Nose Radius. For Example: 0,10,5,5,8,90,1

27. Inter-Relationship Between ASA and ORS System

28. Chip Formation The fig. represents the shaping operation, where the work piece remains stationary and the tool advances in to the work piece towards left. Thus the metal gets compressed very severely, causing shear stress. This stress is maximum along the plane is called shear plane. If the material of the workpiece is ductile, the material flows plastically along the shear plane, forming chip, which flows upwards along the face of the tool.

29. Chip Formation The complete plastic deformation of the metal does not take place entirely along the shear plane only. It actually occurs over a definite area PQRS. The metal structure starts getting elongated along the line PQ below the shear plane and continues above the shear plane and continues up to the line RS where its deformation is completed. The complete area PQRS is known as shear zone. The shape of the shear zone is a wedge shape, with its thicker portion near the tool and the thinner one opposite to it. This shape of shear zone is one of the reasons to curl the chip. The produced chip is very hot and its safe disposal is very necessary.

30. Chip Formation The tool will cut or shear off the metal, provided by ( i ) The tool is harder than the work metal, (ii) The tool is properly shaped so that its edge can be effective in cutting the metal, (iii) The tool is strong enough to resist cutting pressures but keen enough to sever the metal, and (iv) Provided there is movement of tool relative to the material or vice versa, so as to make cutting action possible.

31. Types of Chips The chips produced during machining can be broadly classified as three types. 1. Discontinuous or Segmental Chips 2. Continuous Chips 3. Continuous Chip with built-up edge

32. Types of Chips Discontinuous or Segmental Chips This type of chips produced during machining of brittle materials like cast iron and bronze. These chips are produced in the form of small segments. As the tool advances forward, the shear plane angle gradually reduces until the value of compressive stresses acting on the shear plane becomes too low to prevent rupture. At this stage, any further advancement of the tool results in the fracture of the metal ahead of it, thus producing a segment of the chip. With further advancement of the tool, the processes of metal fracture and production of chips segments go on being repeated, and this is how the discontinuous chips are produced.

33. Types of Chips Discontinuous or Segmental Chips These are also produced in machining of ductile materials when low cutting speeds are used adequate lubricant is not provided. This causes excessive friction between the chip and tool face, leading to the fracture of the chip in to small segments. This will also result in excessive wear on the tool and the poor surface finish on the work piece. Other factors responsible : smaller rake angle, too much depth of cut.

34. Types of Chips 2. Continuous Chips The basis of the production of the continuous chip is the continuous plastic deformation of the metal ahead of the tool, the chip moving smoothly up the tool face. This type chip is produced while machining a ductile material, like mild steel, under favorable conditions, such as high cutting speeds and minimum friction between the chip and the tool face. The friction between the chip-tool interface can be minimized by polishing the tool face and adequate use of coolant. Other factors responsible : bigger rake angle, finer feed and keen cutting edge.

35. Types of Chips 3. Continuous Chips with built-up edge While machining ductile material when high friction exists at the chip-tool interface results the continuous chips with built-up edge. The normal reaction of the chip on the tool face is quite high. It is maximum at the cutting edge or nose of the tool. This gives rise to an extensively high temperature and compressed metal adjacent to the tool nose gets welded to it. The chip is also sufficiently hot and gets oxidized as it comes off the tool and turns blue in colour . The extra metal welded to the nose of the tool is called built-up edge.

36. Types of Chips 3. Continuous Chips with built-up edge Metal in built-up edge is highly strain hardened and brittle. During the chip flow up the tool, the builtup edge is broken and carried away with chip, rest of it bonded to the work piece and make it rough. Due to the built-up edge the rake angle also altered and so is the cutting force. Other factors responsible : low cutting speed, excessive feed, small rake angle, lack of lubricant.

37. Types of Chips 3. Continuous Chips with built-up edge Adverse effects of built-up edge formation: • Rough surface finish. • Fluctuating cutting force, causing, vibrations in cutting tool. • Chances of carrying away some material from the tool by the built-up surface, producing crater on the tool face and causing tool wear. Precautions to avoid built-up edge formation: • The coefficient of friction at the chiptool interface should be minimized by means of polishing the tool face. • Adequate supply of coolant. • Large rake angle. • High cutting speeds and low feeds.

38. Chip Thickness Ratio

39. Chip Control and Chip Breakers The chips produced during machining, specially while employing higher speeds in machining of high tensile strength materials, need to be effectively controlled. Higher speeds causing to higher temperatures resulting chip will be continuous, of blue colour and take the shape of coil. Adverse effect of coiled chips on machining: • Effects the tool life by spoiling the cutting edge, creating crater and rising the temperature. • Lead to poor surface finish on the work piece. • If the chip gets curled around the rotating w/p or tool, it may be hazardous the machine operator. • If a large and continuous coil is allowed to be formed, it may engage the entire machine and even the work piece, its quite dangerous. • Very large coils offer a lot of difficulty in their removal. Such difficulties are not encountered while machining materials like brass and cast iron.

40. Chip Control and Chip Breakers The chip breaker break the produced chips into small pieces. The work hardening of the chip makes the work of the chip breakers easy. If the job requirements do not call for very strict chip control the common methods used for chip breaking are : 1. By control of tool geometry : By grinding proper back rake and side rake according to the speeds and feeds. 2. By obstruction method : by interposing a metallic obstruction in the path of the coil.

41. Chip Control and Chip Breakers When a strict chip control is desired, some sort of chip breaker has to be employed. The following types of chip breakers are commonly used. 1. Groove type 2. Step type 3. Secondary Rake type 4. Clamp type

42. Velocity Relationships

43. Force relationship in Orthogonal cutting The relationships among the cutting forces were established by Merchant with the following assumptions. 1. The cutting velocity always remains constant. 2. Cutting edge of the tool remains sharp throughout the cutting. 3. There is no side ways of flow of the chip. 4. Only continuous chip is produced. 5. There is no built-up edge. 6. No consideration is made of the inertia force of the chip. 7. The behavior of the chip is like that of a free body which is in the state of stable equilibrium under the action of two resultant forces which are equal, opposite and collinear.

44. Force relationship in Orthogonal cutting 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 or Tangential force which helps in holding the tool in position and acts on the tool nose

45. Merchant’s circle It is useful to determine the relation between the various forces and angles. In the diagram two force triangles have been combined and R and R’ together have been replaced by R. The force R can be resolved into two components Fc and Ft. Fc and Ft can be determined by force dynamometers. The rake angle (α) can be measured from the tool, and forces F and N can then be determined. The shear angle (f) can be obtained from it’s relation with chip reduction coefficient. Now Fs & Fn can also be determined.

46. The procedure to construct a Merchant’s circle diagram

47. The procedure to construct a Merchant’s circle diagram

48. Relationship of various forces acting on the chip with the horizontal and vertical cutting force diagram

49. Relationship of various forces acting on the chip with the horizontal and vertical cutting force diagram

50. Relationship of various forces acting on the chip with the horizontal and vertical cutting force diagram

51. Forces in a single point tool in Turning It is a case of oblique cutting in which three component forces act simultaneously on the tool point. Ft = The feed force or thrust force acting in horizontal plane parallel to the axis of the work. Fr = The radial force, also acting in the horizontal plane but along a radius of the work piece. (along the axis of the tool) Fc = The cutting force acting in vertical plane and is tangential to the work surface. (also called as tangential force) In case of orthogonal cutting, only two component forces come in to play (Fr = 0)

52. Stress and Strain acting on the chip

53. Stress and Strain acting on the chip

54. Stress and Strain acting on the chip (contd..)

55. Work Done in Cutting

56. Horse Power Calculation

57. Popular Metal CuttingTheories A large number of researchers have been working on various aspects of metal cutting for over 100 years all over the world. Most of these theories, with slight variations in their assumptions and results, have generally been developed encompassing three main parameters: (1) shear angle, (2) rake angle, (3) angle of friction. The most popular of these theories are, 1. Ernst – Merchant theory 2. Lee and Shaffer’s theory

58. Theory of Ernst andMerchant (1944) This theory is based on minimum energy consumption. It implies that during cutting the metal shear should occur in that direction in which the energy requirement for shearing is minimum. The other assumptions made by them include: (a) The behavior of the metal being machined is like that of an ideal plastic. (b) At the shear plane, the shear stress is maximum, is constant and independent of shear angle.

59. Theory of Ernst andMerchant (1944)

60. Theory of Ernst andMerchant (1944)

61. Theory of Ernst andMerchant (1944)

62. Lee and Shaffer’s Theory They analyzed process of orthogonal metal cutting by applying the theory of plasticity for an ideal rigid plastic material. The principle assumptions made by them include : (a) The work piece metal ahead of the cutting tool behaves like an ideal plastic metal. (b) The deformation of the metal occurs on a single shear plane. (c) There is a stress field with in the produced chip which transmits the cutting force from the shear plane to the tool face and ,therefore, the chip does not get hardened. (d) The chip separates from the parent metal at the shear plane.

63. Lee and Shaffer’s Theory

64. Force System in Multipoint Cutting There are several metal cutting operations which involve the use of multipoint cutting tools. Ex: Drilling, Milling, Broaching, Hobbing , etc. Force System in Drilling : During drilling, a lot of axial pressure on it in order to make it penetrate in to the material. Due to this pressure, all the drill elements are subjected to one or other forces.

65. Force System in Drilling Principal Forces in Drilling : FH : An equal and opposite horizontal force acting on both lips of the drill, and thus neutralizing each other. FV : Vertical force acting at the center of the drill in a direction opposite to that of the applied pressure. FV1 : Vertical force acting in the same direction as FV, on the lips of the drill. Ff1 : Frictional force due to rubbing of upward flowing chips against the wall of the hole and flutes of the drill. Ff2 : Frictional force due to rubbing between the drill margin and the hole surface. P : the applied axial pressure or thrust force acting along the axis of drill to press it in to the work piece material.

66. Force System in Drilling

67. Force System in Drilling

68. Force System in Drilling

69. Force System in Milling A milling cutter carries a number of cutting edges. All of them do not perform the cutting action simultaneously. At a particular moment,some of the edges are fully engaged in cutting, some others are engaged partially and many others are not engaged in cutting at all. As a result : A continuous chip is not formed and Tool geometry keeps on changing during cutting. This leads to : vibrations in the machine tool variati0ons in the cutting speed, inferior surface finish, reduction in the tool life. variation in cutting forces.

70. Force System in Milling

71. Force System in Milling

72. Force SysteminMilling

73. CUTTING SPEED, FEED AND DEPTH OF CUT Cutting speed :(m/min) Cutting speed of a cutting tool can be defined as the rate at which its cutting edge passes over the surface of the work piece in unit time. It is normally expressed in terms of surface speed in meters per minute (m/min). It considerably effects the tool life and efficiency of machining. If it is too high, the tool gets overheated and its cutting edge may fail, needing regrinding. If it is too low, too much time is consumed in machining and full cutting capacities of the tool and machine are not utilized, which results in lowering of productivity and increasing the production Cost.

74. CUTTING SPEED, FEED AND DEPTH OF CUT Feed Rate: (mm/rev) Feed of the cutting tool can be defined as the distance it travels along or into the work piece for each pass of its point through a particular position in unit time. For example, in turning operation on a lathe it is equal to the advancement of the tool corresponding to each revolution of the work. However, it is computed and mentioned in different machine tools and different operations. For example, in planning it is the work which is fed and not the tool. Similarly, in milling work involving the use of a multi-point cutter, the feed is basically considered per tooth of the cutter.

75. CUTTING SPEED, FEED AND DEPTH OF CUT The cutting speed and feed of a cutting tool is largely influenced by the following factors. Materials being machined. Material of the cutting tool. Geometry of the cutting tool. Required degree of surface finish. Rigidity of the machine tool being used. Type of coolant being used.

76. CUTTING SPEED, FEED AND DEPTH OF CUT Depth of Cut : (mm) It is indicative of the penetration of the cutting edge of the tool into the work piece material in each pass, measured perpendicular to the machined surface. It determines the thickness of the metal layer removed by the cutting tool in one pass. For example, in turning operation on a lathe it is given by: Depth of cut = (D-d)/2 Where, D=Original diameter of the stock in mm d=Diameter obtained after turning, in mm.

77. SOURCES OF HEAT INMETAL CUTTING During metal cutting, heat is generated in three region as shown in Fig. 1. Around share plane 2. Tool-chip interface 3. Tool-work piece interface

78. SOURCES OF HEAT INMETAL CUTTING Around share plane It is the region in which actual plastic deformation of the metal occurs during machining. Due to this deformation heat is generated. A portion of this heat is carried away by the chip, due to which its temperature is raised. The rest of the heat is retained by the work piece. It is known as Primary Deformation Zone.

79. SOURCES OF HEAT INMETAL CUTTING Tool-chip interface As the chip slides upwards along face of the tool friction occurs between their surfaces, due to which heat is generated. A part of this heat carried by the chip, which further raises the temperature of the chip. And the rest transferred to the tool and the coolant. This area is known as Secondary deformation zone. The amount of heat generated due to friction increases with the increase in cutting speed. It is not appreciably effected with the increase in depth of cut. When the feed rate is increased the amount of frictional heat generated is relatively low. But, in that case, The surface finish obtained is inferior.

80. SOURCES OF HEAT INMETAL CUTTING Tool-work piece interface That portion of tool flank which rubs against the work surface is nother source of heat generation due to friction. This heat is also shared by the tool, work piece and the coolant used. It is more pronounced when the tool is not sufficiently sharp.

81. SOURCES OF HEAT INMETAL CUTTING Fig. shows the distribution of the heat generated during metal cutting, neglecting the heat flowing to the atmosphere. On an average, about 70% of the total heat is carried away by the chip, about 15% is transferred to the tool and the remaining 15% to the work piece. With an increase in the cutting speed a higher amount of heat is absorbed by the chip and lesser amount is transferred to the tool and the work piece. It is an obvious advantage in high speed machining. Some of the heat generated is also shared and carried away by the flowing cutting fluid, when used. The shear angle also effects the heat generation. A larger share angle leads to a smaller heat generation in the primary deformation zone.

82. SOURCES OF HEAT INMETAL CUTTING In metal cutting, the amount of heat generated per unit of time is given by the thermal equivalent of the mechanical work done. Now, the mechanical work done(W.D) is given by : WD = Cutting force ( kgf ) X Cutting velocity (m/min = Fc x VC kgf m/min. Now, if Q be the amount of total heat generated in cutting the metal, then, ( / min) 427

83. TOOL FAILURE A properly designed and ground cutting tool is expected to perform the metal cutting operation in an effective smooth manner. If, however, it is not giving a satisfactory performance it is indicative of the tool failure and the same is reflected by the following adverse effects observed during the operation. 1. Extremely poor surface finish on the work piece. 2. Higher consumption of power. 3. Work dimensions not being produced as specified. 4. Overheating of cutting tool. 5. Appearance of a burnishing band on the work surface. During the operation, a cutting tool may fail due to one or more of the following reasons: 1. Thermal cracking and softening 2. Mechanical chipping 3. Gradual wear

84. TOOL FAILURE Thermal cracking and softening It has already been seen earlier that a lot of heat is generated during the process of metal cutting. Due to this heat the tool tip and the area closer to the cutting edge becomes very hot. Although the cutting tool material is quite hard to withstand this temperature, still every tool material has a certain limit to which it can withstand the elevated temperature without losing its hardness. If that limit crossed, the tool material starts deforming plastically at the tip and adjacent to the cutting edge under the action of the cutting pressure and the high temperature. Thus the tool loses its cutting ability and is said to have failed due to softening. The main factors responsible for creating such conditions of tool failure are cutting speed, high feed rate, excessive depth of cut, smaller nose radius and choice of a wrong tool material.

85. TOOL FAILURE Thermal cracking and softening The temperature ranges within which the common tool materials can successfully operate without losing their hardness are: Carbon tool steels 2000C -2500C High speed steels 5600C-6000C Cemented carbides 8000C -10000C On account of fluctuations in temperatures and severe temperature gradients the tool material is subjected to local expansion and contraction. This gives rise to the setting up of temperature stresses or thermal stress, due to which cracks are developed in the material. These cracks, known as Thermal cracks, emanate from the cutting edge and extend in words. The tool failure due to this aspect is known as failure due to thermal cracking or due to thermal stresses.

86. TOOL FAILURE Mechanical chipping Mechanical chipping of the nose and/or the cutting edge of the tool are commonly observed causes of tool failure. The common reasons for such failure are too high cutting pressure, mechanical impact, Excessive wear, too high vibration and chatter, weak tip and cutting edge, etc. A typical form of mechanical chipping is shown in fig. This type of failure is more pronounced is carbide tipped and diamond tool due to the high brittleness of tool material.

87. TOOL FAILURE Gradual wear When a tool is in use for sometime it is found to have lost some weight or mass, implying that it has lost some material from it, which is due to wear. The following two types of wears are generally found to occur In cutting tools: 1. Crater wear 2. Flank wear

88. TOOL FAILURE Crater wear The principal region where wear takes places in a cutting tool is its face, at a small distance (say ‘a’) from its cutting edge. This type of wear generally takes places while machining ductile materials, like steel and steel alloys, in which continuous chip is produced. The resultant feature of this type of ware of a crater or a depression at the tool chip interface. This type of wear, or the formation of crater on the tool face, is due to the pressure of the hot chip sliding up the face of the tool. The metal from the tool face is supposed to be transferred to the sliding chip by means of the diffusion process.

89. TOOL FAILURE Crater wear The shape of the crater formed corresponding to the shape of the underside of the chip. The principal dimensions of the formed crater are its breadth ‘b’ and depth ‘d’ as shown in fig. A continued growth of crater will result in the cutting edge of the tool becoming weak and may finally lead to the tool failure. At very high speed, and the consequent high temperatures (say 10000C), the H.S.S tool will fail due thermal softening of material, while the tools made from harder materials, like those containing tungsten carbide, cobalt etc., will not wear out so rapidly. Higher feeds and lack of cutting fluids increase the rate of crate wear.

90. TOOL FAILURE Flank wear Another region where an appreciable amount of wear occurs is the flank below the cutting edge. It occurs due to abrasion between the tool flank and the work piece and excessive heat generated as a result of the same. The abrasive action is aided by the hard micro constituents of the cut material provide a lot of abrasive material readily.

91. TOOL FAILURE Flank wear The entire area subjected to flank wear is known as wear land. This type of ware mainly occurs on the tool nose, front and side relief faces. The magnitude of this wear mainly depends on the relative hard nesses of the work piece and tool materials at the time of cutting operation. And also the extent of strain hardening of the chip. When the tool is subjected to this type of ware, the work piece loses its dimensional accuracy, energy consumption is increased and the surface finish is poor.

92. TOOL FAILURE Flank wear The effect of flank wear is expressed in terms of the width (or height) of wear land, which is dependent on time. This height is a linear measure and is symbolically denoted by VB, WL, FW or hf millimeters.

93. TOOL FAILURE Flank wear The total flank wear consists of three main components, drawn between the wear land height (VB) and time (t). The first component (A), which exists for a small duration, represents the period during which initial wear takes places at a rapid rate. The second segment a rapid rate. The second segment (B), which exists for a very long duration, represents the period during which the wear progress uniformly. The last segment (C) represents the region in which wear occurs at a very rapid rate and results in total failure of the tool. That is why this region is also known as the period of destructive wear.

94. TOOL FAILURE Effect of Cutting Speed on Flank wear Effect of cutting speed on tool flank wear (VB) for three cutting speeds, using a tool life criterion of 0.50 mm flank wear.

95. MECHANISAM OF WEAR The wear mechanism of cutting tool is a very complex phenomenon. However, the common mechanisms supposed to be responsible for causing wear are the following: 1. Abrasion 2. Adhesion 3. Diffusion 4. Chemical wear

96. MECHANISAM OF WEAR Abrasion It is a type of mechanical wear. Under this mechanism, hard particles on the underside of the sliding chip, which are harder than the tool material, plough into the relatively softer material of the tool face and remove metal particles by mechanical action. The material of the tool face is softened due to the high temperature. The hard particles present on the underside of the chip may be: a) Fragments of hard tool material. b) Broken pieces of built-up edge, which are strain hardened. c) Extremely hard constituents, like carbides, oxides, scales, etc., present in the work material.

97. MECHANISAM OF WEAR Adhesion It should have been quite clear that due to the excessive pressure a lot of friction occurs between the sliding surface of the chip and the tool face. This gives rise to an extremely high localized temperature, causing metallic bond between the materials of the tool faces and the chip. But, the surfaces of both the chip underside and the tool face, although appear to be smooth apparently, are microscopically rough. Therefore, the contact between these surfaces is not truly a surface contact but a point contact.

98. MECHANISAM OF WEAR Adhesion Due to the excessively high temperature at the chip-tool inter face a metallic bond takes places between the chip material and tool materials at the contact points, in the form of small spot welds, as shown in the diagram. When the chip slides, these small welds are broken. But this separation is not along the line of contact. A small portion of the welded tool contact is also carried away by the sliding chip. Thus, small particles from the tool face continue to be separated through this phenomenon so transferred from the tool face to the chip will depended upon the contact area and relative hardness of chip and the tool materials.

99. MECHANISAM OF WEAR Diffusion Solid state diffusion, which consists of transfer of atoms in a metal crystal lattice, is another cause of wear. This Transfer of atoms takes places at elevated temperature from the area of concentration to that of low concentration. The favorable condition for diffusion is provided by the rise in localized temperature over the actual contract area between the chip underside and the tool faces. In such a condition, the tool material to the chip material at the points of contact. This weakens the surfaces structure of the cutting tool and may ultimately lead to tool failure. The amount of diffusion depends upon: a) Temperature at the contact area between the tool faces and the chip b) The period of contact between the tool face and the chip. c) The bonding affinity between the materials of the tool and the chip

100. MECHANISAM OF WEAR Chemical wear This type ware occurs when such a cutting fluid is used in the process of metal cutting which is chemically active to the material of the tool. This is clearly the result of chemical reaction taking places between the cutting fluid and the tool material, leading to a change in the chemical composition of the surface material of tool.

101. TOOL LIFE Tool life can be defined as the time interval for which the tool works satisfactorily between two successive grindings (sharpening). When the tool wear is increased considerably, the tool loses its ability to cut efficiency and must be reground. The tool life can be effectively used as the basis to evaluate the performance of the tool material, assess machinability of the work piece material and know the cutting conditions. There are three common ways of expressing tool life: a) As time period in minutes between two successive grindings. b) In terms of number of components machined between two successive grindings. This mode is commonly used when the tool operates continuously, as in case of automatic machines. c) In terms volume of material removed between two successive grindings. This mode of expression is commonly used when the tool is primarily used for heavy stock removal.

102. TOOL LIFE The method of assessing the tool life in terms of the volume of material removed per unit of time is a practical one and can be easily applied as follows: Volume of material removed per unit time Where, D= dia. Of work piece in mm t=depth of cut in mm f=feed rate in mm/rev N=No. of revolutions of work per minute If ‘T’ be the time in minutes to tool failure, then: Total volume of metal removed to tool failure:

103. TOOL LIFE We also know that the cutting speed,

104. TOOL LIFE Tool life equation : Taylor’s tool equation Tool life of a cutting tool may be expressed as: Where, V= cutting speed in m/min T=Tool life in min C=A constant (numerically equal to cutting speed that gives the tool life in one min) n=A constant (depends on finish, work piece material and tool material)

105. FACTORS AFFECTINGTOOL LIFE 1. Cutting speed 2. Feed and depth of cut 3. Tool Geometry 4. Tool material 5. Work material 6. Nature of cutting 7. Rigidity of machine tool and work 8. Use of cutting fluids.

106. EFFECT OF CUTTING SPEED

107. FEED AND DEPTH OF CUT Tool life can be defined as the time interval for which the tool works For a give Tool life,

108. TOOL GEOMETRY Many geometrical parameters (tool angles) of a influence its performance and life. For example, the Ranke angle has a mixed effect. If it is increased in a positive direction the cutting force and the amount of heat generated are reduced. Obviously, this should help increase help increase the life of cutting tool. But, if it is very large the cutting edge is weakened and also its capacity to conduct heat is reduced. Thus, a considerable increase of the positive rake results in reduction of mechanical strength of the tool and, hence, lowering of tool life. Since the a above two effects are opposite to each other, for an effectively economical tool life it is necessary to strike a balance between the two, for which the optimum value of rake angle needs to be used. This value varies from -50 to +100. Cemented carbide and ceramic tools are generally provided negative rake.

109. TOOL GEOMETRY Similar contradictory effects are observed with the variations in relief angles or clearance angles . These angles are provided on the cutting tool to prevent rubbing of tool flank against the machined work surface. They, Thus, help in a very lowering the amount of heat generated and, therefore, increasing the tool life. But a very large relief angle results in weakening of tool and, hence, reduction of tool life. Again, therefore, a balance needs to be struck and only an optimum value should be used. These angles normally vary from but in special cases, such as in carbide tipped tools, a higher value up to can be used to prevent rubbing of shank.

110. TOOL GEOMETRY The two cutting edge angles also have their influence on tool performance. The front cutting edge angle, also known as end cutting edge angle, effects the tool wear. Up to a certain optimum value an increase in this angle permits the use of higher speeds without an adverse effect on tool life. But, an increase beyond that value will result in reduction of tool life. It generally varies from The side cutting edge angle or the plane approach angle has a complex effect on tool life If this angle is smaller, higher speeds can be employed. A larger end cutting edge angle increase tool life. Some other geometrical parameters effecting the tool life are: a) Inclination angle . Tool life increase with the increase in this angle up to an optimum value. b) Nose radius . While it increase abrasion, it also helps in improving surface finish and tool strength and, hence, the tool life.

111. WORKMATERIAL The microstructure of the work material plays a significant role because it directly effects the hardness of the material. For example, presence of free graphite and ferrite in cast iron and steel imparts softness to them. Pearlitic structure is harder than this and the martensitic structure is the hardest. Similarly, scale formation and presence of oxide layer on the work surface serve as abrasives and, therefore, have a detrimental effect on tool life. The increase in cutting temperature and power consumption vary directly as the hardness of work piece material. Consequently, higher the hardness of the work material greater will be the tool wear and, therefore, shorter will be the tool life. Adverse effects on tool life are also experienced in machining if pure metal because of their tendency to stick to the tool face, specially at high temperature. This results in more friction and, hence, high amount of wear on tool and, therefore, a shorter tool life.

112. NARTURE OF CUTTING Toll life is also effected by the nature of cutting i.e. Whether it is continuous or intermittent. In the latter case, the tool is subjected to reaped impact loading and may give way much earlier than expected until it is made substantially strong and tough. In continuous cutting, a similar tool will have a relatively longer life. RIGIDITY OFMACHINE TOOL ANDWORK Both the machine tool and the work piece should remain rigid during the machining operation. If not, vibration will take place and then the cutting tool will be subjected to intermittent cutting instead of continuous cutting. This will result in impact loading of tool and, therefore, a shorter life.

113. USE OF CUTTING FLUIDS Cutting fluids are used in machining work for helping the efficient performance of the tool operation. They are used either in liquid or gaseous form. They assist in the operation in many ways, such as by cooling the tool and work, reducing friction, improving surface finish, helping in breaking of chips and washing them away, etc. These factors help in improving tool life, permitting higher metal removal rate and improving the quality of surface finish.

114. TOOLMATERIAL The main characteristics of a good cutting tool material are its hot hardness, wear resistance, impact resistance, abrasion resistance, heat conductivity, strength, etc. What is important to tool life is the likely changes in these characteristics at high temperature because the metal cutting process is always associated with generation of high amount of heat and, hence, high temperatures. The cutting speed has the maximum effect on tool life, followed by feed rate and depth of cut. All these factors contribute to the rise of temperature. That is why it is always said that an ideal tool material is the one which will remove the largest volume of work material at all speeds. The tool material which can withstand maximum cutting temperature without losing its principal mechanical properties (specially hardness) and geometry will ensure maximum tool life, and, hence, will answer the most efficient cutting of metal. We, therefore, conclude that the higher the hot hardness and toughness in the tool material the longer the tool life.

115. CHARACTERRISTICS CUTTING TOOL MATERIALS The material used for the manufacture of cutting tools should posses the following characteristics: 1. Ability to retain its hardness at elevated temperatures, called hot hardness. 2. Ability to resist shock, called toughness. 3. High resistance to wear, to ensure longer tool life. 4. Low coefficient of friction, at the chip-tool interface, so that the surface finish is good and wear is minimum. 5. Should be cheap. 6. Should be able to be fabricated and shaped easily. 7. If it is to be used in the form of brazed tips, its other physical properties like tensile strength, thermal conductivity, coefficient of thermal expansion and modulus of elasticity, etc., should be as close to the shank material as possible to avoid cracking.

116. TYPES OF CUTTINGTOOLMATERIALS The following materials are commonly used for manufacturing the cutting tools. Selection of a particular material will depend on the type of service it is expected to perform. 1. High Carbon Steel 2. High speed Steel 3. Cemented Carbides 4. Stellite 5. Cemented Oxides or Ceramics 6. Diamond

117. High Carbon Steel Plain carbon steels having a carbon percentage as high as 1.5% are in common use as tool materials for general class of work. However, they are not considered suitable for tools used in production work on account of the fact that they are not able to withstand very high temperature. With the result, they cannot be employed at high speeds. Usually the required hardness is lost by them as soon as the temperature rises to bout 2000C – 2500C. They are also not highly wear resistant. They are used mainly for hand tools. They are, however less costly, easily forgeable and easy heat treat. High carbon medium alloy steels are found to be more effective than plain high carbon steels. These steels, are provided better hot hardness, higher impact resistance, higher wear resistance etc., by adding small amounts of tungsten chromium, molybdenum, vanadium, etc., which improves their performance considerably and they are able to successfully operate up to cutting temperatures of 3500C.

118. High Speed Steel (HSS) It is a special alloy-steel which may contain the alloying elements like tungsten, chromium, vanadium, cobalt and molybdenum, etc., up to 25 percent. These alloying elements increase its strength, toughness, wear resistance, cutting ability to retain its hardness at elevated temperatures in the range of On account of these added properties the high speed steel tools are capable of operating safely at 2 to 3 times higher cutting speeds than those of high carbon steel tools. The most commonly used high speed steel is better known by its composition of alloying elements as 18-4-2 i.e., the one that contains 18%W ,4% Cr and 1% V. Another class of H.S.S. contains high proportions of cobalt (2 to 15%) and is known as cobalt H.S.S. It is highly wear resistance and carries high hot hardness. A highly tough variety of H.S.S., known as Vanadium H.S.S., carries 2%V, 6%W, 6% Mo and 4% Cr. It is widely favored for tools which have to bear impact loading and perform intermittent cutting.