Textbook of Production Technology PDF

416 A Textbook of Production Technology cutting. It is expressed in metres per minute (mpm). It is thus the amount of length that will pass the cutting edge of the tool per unit of time. Feed. It may be defined as the relatively sma...

416 A Textbook of Production Technology cutting. It is expressed in metres per minute (mpm). It is thus the amount of length that will pass the cutting edge of the tool per unit of time. Feed. It may be defined as the relatively small movement per cycle of the cutting tool, relative to the work piece in a direction which is usually perpendicular to the cutting speed direction. It is expressed in millimetres per revolution (mm/rev) or millimetres per stroke (mm/str). It is more complex element as compared to cutting speed, since it is expressed differently for various operations. For example, in turning and drilling, the feed is the axial advance of the tool along or through the job during each revolution of the tool or job; for the shaper and planer, it is lateral offset between the tool and work for each stroke and for multitooth milling cutters, feed is the advance of the work or cutter between the cutting action of two successive teeth (expressed basically as mm/per tooth). Depth of Cut : The depth of cut is the thickness of the layer of metal removed in one cut, or pass, measured in a direction perpendicular to the machined surface. The depth of cut is always perpendicular to the direction of feed motion. Selection of Cutting Speed. The cutting speed to be used will depend upon the following factors : (i) Work Material. Hard and strong materials require a lower cutting speed; whereas soft and ductile materials are cut at higher cutting speeds. Depth of cut D N b d Initial position of tool Position of tool after one  revolution of job Cs t f Feed Fig. 6.13. Elements of Machining Process. (ii) Cutting Tool Material. Special cutting tool materials, for example, cemented carbides, ceramics, Stellite and H.S.S. will cut at much higher cutting speeds than alloy or carbon steel tools. (iii) The Depth of Cut and Feed. A light finishing cut with a fine feed may be run at a higher speed than a heavy roughing cut. (iv) Desired cutting tool life. The tool life is a direct function of cutting temperature which increases with increase in cutting speed. Thus as the cutting speed is increased, cutting tool life is decreased. (v) Rigidity and conditions of the machine and tool and the rigidity of the work. An old, loose machine working with a poorly supported tool on a thin bar, will not cut at such a high speed, as a good machine with rigid tool operating on a well supported bar of reasonable dimensions.  Machining Process Table 6.2 Cutting Speeds and Feed Rates Work material Cutting Speed V in mpm Feed rate f H.S.S. tool Carbide tool Stellite tool in mm/rev. Turn Ream and Drill Turn Turn Rough Finish Thread Rough Finish Rough Finish Rough Finish Mild Steel 40 60 7.5 to 15 30 90 180 50 75 0.625 to 2.0 0.125 to 0.75 Cast Steel 15 24 3.5 12 45 100 24 33 0.5 to 1.25 0.125 to 0.175 Grey C.I. 18 27 3.5 13 60 100 33 45 0.4 to 2.5 0.2 to 1.0 Aluminium 90 150 15 72 240 360 120 180 0.1 to 0.5 0.075 to 0.25 Brass 75 100 18 60 180 270 90 150 0.375 to 2.0 0.2 to 1.25 Phosphor Bronze 18 36 4.5 13 120 180 30 50 0.375 to 0.75 0.125 to 0.5 417 418 A Textbook of Production Technology Selection of Feed. Feeds, to be used, will depend upon the following factors : (i) Smoothness of the finish required. A coarse feed will give wider and deeper machining marks and an inferior finish to a fine feed. A blunt nosed tool will give a better finish than a sharp tool for the same feed. (ii) Power available, condition of the machine and its drive. The product of the speed, feed and depth of cut gives the amount of metal being removed and hence the power necessary. A coarse feed on a poor or badly driven machine will be harmful both for the machine and the tool. This will also result in slipping of the drive or belt. (iii) Type of Cut. As a general rule, give coarsest feed possible for a roughing cut because finishing is unimportant. For a finishing cut, the feed should be fine enough to give the class of finish required. (iv) Tool Life. The cutting temperature increases with increase of feed, resulting in decreased tool life. A simple rule of thumb connecting feed and nose radius is, for rough turning, : f (mm/rev.) = 0.5 × Nose Radius 2 The maximum recommended feed rate is of the order of of the nose radius. 3 The higher feeds apply for inserts :- (i) having a strong cutting edge with at least a 60° cutting edge angle (ii) that are single sided. (iii) that are used with a smaller entering angle than 90°. (iv) that are used in materials with good machinability and moderate cutting speeds. Selection of Depth of Cut. The depth of cut to be used will depend upon the following factors :- (i) Type of Cut. Use large depths of cut for roughing operations than for finishing operations. (ii) Tool Life. The cutting temperature increases with increase of depth of cut, resulting in decreased tool life. (iii) Power Required. As discussed above, the cutting speed multiplied by area of cut (feed x depth of cut) gives the metal removal rate, which gives the power requirements. For a given area of cut, a large ratio of depth of cut to feed usually gives the most efficient performance as well as a better surface finish. The three elements of machining process are shown in Fig. 6.13, for a simple turning operation on a centre lathe. The cutting speed is given as : DN V , m / min. 1000 where D = Diameter in mm of work or cutter N = rev. / min. of work or cutter. V = cutting speed of work or cutter The common values of cutting speed and feed are given in Table 6.2. The depth of cut can be taken as equal to 4 to 5 mm in rough turning. 0.5 to 2 mm for semi finish turning and 0.1 to 0.4 mm for finish turning. Machining Process 419 Depth of cut is usually takew 3 to 5 times the feed for rough opeatious. The values for finishing operations are usually small. 6.7. THERMAL ASPECTS OF CHIP FORMATION Work is done during the process of chip formation, which results in the generation of heat. The work also is done in the plastic deformation of the layer being cut and the layers adjoining machined surface and the surface of the cut and in overcoming friction on the tool - face and flank. The heat balance in chip formation can be written as : Total amount of heat generated ⎧ Amount of heat carried away in chips + ⎪⎪ Amount of heat remaining in the cutting tool + =⎨ ⎪ Amount of heat passing into the workpiece + ⎪⎩ Amount of heat radiated into the surrounding air. On an average, for a lathe operation, the above heat dissipation percentages are : 50 to 86%, 10 to 40%, 3 to 9% and 1% respectively Chip of the total amount of heat generated. In finish Tool operations, more heat (in per cent) passes into the work than in rough operations. Heat passing into tool reduces its hardness and makes it less wear - resistant. Heat evolved in the chip formation zone and at the interface between the tool and the chip and at the tool-work interface Work-piece strongly affects the condition of the rubbing surfaces (by changing their co-efficient of Fig. 6.14. Regions of Heat Generation. friction), machining accuracy and the whole cutting process, and the related phenomenon, that is, deformation, tool wear, Built - up edge formation and work hardening etc. Fig. 6.14 shows the regions where the heat is mainly generated. The distribution of heat, both in the chip and the tool, is non- uniform. Hence, they are heated to non-uniform temperatures. The temperature in layers of the chip nearer to the tool-face will be higher than in those farther away. The highest temperature in the work-piece is observed at the point of contact of the tool with the work. It then decreases farther away from the machined surface. The highest temperature in the tool is observed in the boundary layers of the areas of contact with the chip and the job. The temperature farther away will depend upon the heat conduction. The tool-face is heated to a higher temperature than the flank. The tool-face receives heat both from the highly heated chip and from the considerable work done in overcoming the friction of the chip on the face. On the other hand, the flank is in contact with the surface of the cut and the machined surface, which undergo less plastic deformation than the chip. Also, the work done in overcoming friction is less on the tool flank than on the tool - face. The temperature on the tool-face is higher than the average temperature of the chip ; the thicker the chip, the greater the difference in temperature will be. 6.7.1.Factors Affecting Cutting Temperature. Factors affecting cutting temperature are: Work material, Tool material, Cutting variables cutting speed, feed and depth of cut, Tool geometry and the cutting fluid used. 1. Work Material. The cutting temperature is strongly affected by the mechanical properties of the work material, more resistance it will offer in chip formation. So, more work will have to be 420 A Textbook of Production Technology done for metal cutting. This will result in more heat generation and consequently higher cutting temperature. The higher the thermal conductivity of the work material, the lower is the developed temperature. Similarly, tool materials with higher thermal conductivity will result in lower cutting temperatures than the tool materials with lower thermal conductivity. 2. Cutting Variables. Even though, the cutting forces decrease with an increase in cutting speed, but it is substantially smaller than the increase in speed. Therefore, more heat will be generated with an increase in cutting speed, since Heat generated = cutting force × cutting speed hus, the cutting temperature (maximum temperature on the tool surface) increases with the cutting speed. The cutting forces increase with the increase in the rate of feed, resulting in increased heat generation. But the amount of heat generated and hence the rise in cutting temperature, will be slower than the increase in feed. So, the effect of feed on cutting temperature is lesser than of the cutting speed. The effect of depth of cut on the cutting temperature is even less than that of the feed. 3. Tool Geometry. The cutting temperature is affected mainly by the rake angle, plan approach angle and the nose radius. The rake angle has a complex influence on the cutting temperature. As discussed earlier, as the rake angle is reduced (thereby increasing the cutting angle), the work done gets increased, leading to increased heat generation. But as the cutting angle increases, the material behind the cutting point increases. This results in better heat conduction into the tool shank, thereby, lowering the temperature at its contact surfaces. A negative rake angle causes greater deformation than a positive one and leads to more heat generation during metal cutting. The larger the plan approach angle, the higher the cutting temperature will be and more the tool is heated by cutting. The larger the nose radius, the greater the deformation and the cutting force, and more heat will be generated in chip formation. However, the increased nose radius results in increased length of the active part of the cutting edge and the mass of the tool point. This promotes better heat removal both into the tool shank and into the workpiece. This heat removal intensity is more predominant, leading to reduction in the cutting temperature. So, the cutting temperature decreases with an increase in nose radius of the tool. The larger the cross-sectional area of the tool shank will help in increased removal of heat by conduction resulting in lower cutting temperature. 4. Cutting Fluids. The cutting fluids help in reducing the cutting temperature by : reducing friction, facilitating chip formation, absorbing and carrying away a part of the generated heat. The cooling effect of the cutting fluids gets increased with their higher specific heat and thermal conductivity. 6.7.2. Measurement of Temperature in the Cutting Zone The various techniques used for the measurement of temprature in the cutting zone (work- tool interface) are :- 1. Tool-work thermo-conple 2. Thermo-couples embedded in the tool and/or the workprice. 3. Radiation Pyrometers 4. Temperature sensitive paints 5. Indirect calorimetric techniques, etc. Machining Process 421 1. Tool-work Thermo-couple :- This is the most common and simpler technique for measuring temperature in the cutting zone. Here, the e.m.f. generated between the tool-work interface (hot junction) and their cold ends (cold junction) is taken as the mesure of average temperature in the cutting zone. The e.m.f. generated is measured with a millivoltmeter. Fig. 6.15. shows schematically a typical layout of tool-work thermocouple. Both the work and the tool should be insulated from the machine tool. There should be a good contact between the two junctions and the measuring instrument. For this, a copper disk is mounted at the free end of the lathe spindle. The edge of the disk dips in a cup of mercury. The circuit is completed with mercury and the tailend of the tool through a millivoltmeter. The magnitude of e.m.f. generated will depend upon the temperature difference between hot and cold junctions, and the nature of tool and workpire. The measured e.m.f. is converted into temperature with the help of a calibration curve. Insulation Cu-disk Work-piece Spindle Hg Contact Tool insulated from Machine Tool feed Milli-Voltmeter Fig. 6.15. Tool-Work Thermo-couple. The calibration is done with the help E1 Milli of a standerd thermo-couple, Fig. 6.16. A chip voltmeter B B and a piece of tool material are heated in an Chip oven or lead bath at known temperatures. The E2 Tool resulting thermal e.m.f. E1 is measured. A standard thermo-comple (chromel-alumel) E2 Lead Crucible records the bath temperature at which the bath e.m.f. E1, is measured. If the plot of E1 versus  is the same while  increases at it is while Electric heater  decreases (that is, there is no hysteresis), the calibration is satisfactory. Fig. 6.16. Tool-Work Thermo-couple Calibration. 2. Embedded Thermo-couples :- This technique has been successfully used for determining the temperature distribution on the rake face of a cutting tool. But, the method involves considerable effort. 3. Radiation Pyrometers :- Here, the infrared radiation from the cutting zone is monitored with a radiation pyrometer and interpreted in terms of temperature. The method involves taking photographs of the side face of the cutting zone and also of strips of known temperatures. The intensities of radiation at different points in the cutting zone (tool, chip, workpice) are compared 422 A Textbook of Production Technology with strips of known temperatures. This will give temperature distubution on the tool, chip and workpice, Fig. 6.17. The technique has been exetensively used by Boothryod. The drawback is that it indicates only the surface temperatures. Also, the accaracy of the results depends upon the emissivity of the surfaces, which is very difficult to determine accurately. Chip 690 700 720 720 730 680 660 650 Tool 640 t = 0.061 mm 630 620 Temperature °C Workpiece Fig. 6.17. Temperature Distrubution in Cutting Zone. 4. Temperature sensitive Paints :- These paints change colour at rather critical temperatures. The technique is useful for determining overall temperature distribution. 5. Indirect Calorimetric techniques :- Here, the heat distubution between tool, work and chip can be messured for comparison with analysis. 6.8. TOOL WEAR AND TOOL LIFE During any machining process, the tool is subjected to three distinct factors : forces, temperature and sliding action due to relative motion between tool and the workpiece. Due to these factors, the cutting tool will start giving unsatisfactory performance after some time. The unsatisfactory performance may involve : loss of dimensional accuracy, increased surface roughness, and increased power requirements etc. The unsatisfactory performance results from tool wear due to its continued use. When the tool wears out, it is either replaced or reconditioned, usually by grinding. This will result in loss of production due to machine downtime, in addition to the cost of replacing Chip or reconditioning the tool. Thus, the study of tool wear Tool Crater Face is very important from the stand point of performance Wear and economics. Due to a large number of factors over which the tool wear depends (hardness and type of tool material, type and condition of workpiece, dimensions of cut, i.e., feed and depth of cut, tool Work Flank Piece geometry, tool temperature, which, in turn, is a function Flank of cutting speed, surface finish of tool temperature and Wear cutting fluid), the majority of studies in tool wear are Fig. 6.18. Tool Wear. based on experimental observations, since the analytical study will be very difficult. Machining Process 423 Tool wear or tool failure may be classified as follows: (a) Flank wear. (b) Crater wear on tool face (c) Localized wear such as the rounding of the cutting edge, and (d) Chipping off of the cutting edge. Flank wear and crater wear are shown in Fig. 6.18. Flank wear is attributed usually to the following reasons : 1. Abrasion by hard particles and inclusions in the workpiece. 2. Shearing of the micro welds between tool and work-material. 3. Abrasion by fragments of built-up edge blow- ing against the clearance face of the tool. Crater wear usually occurs due to : 1. Severe abrasion between the chip and tool face. 2. High temperatures in the tool-chip interface reach- ing the softening or melting temperature of tool resulting in increased rate of wear. The sharp increase in wear rate after the interface temperature reaches a certain temperature is attributed to ‘diffusion’. diffusion is the movement of atoms between tool and chip materials resulting in loss of material from the face of the tool. It depends upon the workpiece materials, in addition to temperature. So, unless these conditions are favourable, crater wear due to diffusion may be absent. Crater wear is more common in cutting ductile materials, which produce continuous chips. Also, it is more common in HSS (high speed steel) tools than ceramic or carbide tools, which have much higher hot hardness. The reasons for ‘Nose wear’ may be one or more of the reasons discussed above. Chipping of the tool may occur due to the following factors : 1. Tool material is too brittle. 2. As a result of crack that is already in the tool. 3. Excessive static or shock loading of the tool. 4. Weak design of the tool, such as a high positive rake angle. 6.8.1. Tool Life. The total cutting time accumulated before tool failure occurs, is termed as ‘tool life’. There is no exact or simple definition of tool life. However, in general, the tool life can be defined as tool’s useful life which has been expended when it can no longer produce satisfactory parts. The two most commonly used criteria for measuring the tool life are : 1. Total destruction of the tool when it ceases to cut. 2. A fixed size of wear land on tool flank. On carbide and ceramic tools, where crater wear is almost absent, tool life is taken as corresponding to 0.038 or 0.076 mm of wear land on the flank for finishing respectively. As discussed above, tool wear and hence tool life depends on many factors. The greatest variation of tool life is with the cutting speed and tool temperature which is closely related to cutting speed. Tool temperature is seldom measured and much study has been done on the effect of cutting speed on tool life. Tool life decreases with increased V, the decrease being parabolic. To draw these curves, the cutting tools are operated to failure at different cutting speeds. In 1907, Taylor gave the following relationship between cutting speed and tool life, VTn = C... (6.1) where V is the cutting speed ), T is the time (min) for the flank wear to reach a certain dimension, i.e., tool life, C is constant and n is an exponent which depends upon the cutting conditions. If cutting speed-tool life curves are plotted on a log-log graph, straight lines are obtained, (Fig. 6.19), 424 A Textbook of Production Technology n is the negative inverse slope of the curve and C is the intercept velocity at T = 1. The results are valid only for the particular test conditions employed. Thus ‘C’ is the cutting speed for tool life of 1 min. 60 Log Tool Life n Tool Life min 30 d a b c 0 0 60 120 180 240 300 Log Cutting Speed Cutting Speed m p m (a) (b) Fig. 6.19. Cutting Speed—Tool Life Curves. The following values may be taken for ‘n’ : n = 0.1 to 0.15 for HSS tools = 0.2 to 0.4 for carbide tools = 0.4 to 0.6 for ceramic tools The tool life also depends to a great extent on the depth of cut d and feed rate per revolution, f. Assuming a logarithmic variation of C with d, the equation (6.1) can be written as, VTn. dm = C... (6.2) It has been seen that decrease of life with increased speed is twice as great (exponentially) as the decrease of life with increased feed. Considering feed rate also, the general equation can be : VTn. dm. fx = C... (6.3) 6.8.2. Variables Affecting Tool Life. Tool life is primarily affected by a high temperature in thin surface layers subject to wear. The variables affecting cutting temperature will also affect tool life. These variables are: workpiece material, tool material, cutting variables, tool geometry and cutting fluids. The effects of these variables on cutting temperature has already been discussed under Art. 6.7.1. 6.9. MACHINEABILITY In spite of efforts by a number of investigators, so far there has been no exact quantitative definition of machineability. This is because of a large number of variables involved and their complexity. However, the major factors involved in metal cutting are : forces and power absorbed, tool wear and tool life, surface finish, dimensional accuracy and machining cost. These factors depend upon a large number of variables, such as properties of work materials, tool geometry, cutting conditions, machine tool rigidity etc. Due to this, it is impossible to combine these factors, so as to give a suitable definition for machineability. Many authors give a qualitative measure of machineability of a material as : (1) the ease with which it could be machined, (2) the life of tool before tool failure or resharpening, (3) the quality of the machined surface, and (4) the power consumption per unit volume of material removed. However, in production, tool life is generally considered the most important factor and, so, most of the investigators have related machineability with tool life. Higher the tool life, the better is the machineability of a work material. The various materials have been given machineability ratings, which are relative. Supposing a material is given the rating of 100. Those materials which Machining Process 425 have a better machineability will have higher ratings and those materials with lower machineability have a lower one. According to one investigator, the machineability may be evaluated as given below : 1. Long tool life at a given cutting speed. 2. Lower power consumption per unit volume of metal removed. 3. Maximum metal removal per tool resharpening. 4. High quality of surface finish. 5. Good and uniform dimensional accuracy of successive parts. 6. Easily disposable chips. The machineability rating or index of different materials is taken relative to the index which is standardised. The machineability index of free cutting steel is arbitrarily fixed at 100 per cent. For the other materials, the index is found as below : Cutting speed of material for 20 min. tool life Machineability index, %  Cutting speed of free cutting steel for 20 min. tool life  100 The machineability indexes for some common materials are given below : C - 20 steel = 65 C - 45 steel = 60 Stainless steel = 25 Table 6.3 Problems Remedies and Satuters wear resistant Select a more cutting speed cutting speed Increase the Increase the depth of cut depth of cut nose radius Reduce the Reduce the Reduce the geometry Increase the feed positive Select a Select a Select a tougher smaller Grade grade feed Flank Wear Notch wear Cratering Plastic Deformation Built-up edge (BUE) Small cracks normal to the cutting edge Small cutting edge fracture (frittering) Insert Breakage Curling of Long Chips Vibrations 426 A Textbook of Production Technology Copper = 70 Brass (red) = 180 Aluminium alloys = 300 – 1500 Magnesium alloys = 600 – 2000 Table 6.3. gives the various problems encountered during a machining process and the possible remedies and solution. PROBLEMS 1. Define machining process 2. With the help of a sketch, explain the machining process. 3. Explain the various elements of a single point cutting tool, with the help of a neat sketch. 4. What is meant by ‘hand’ of a single point cutting tool ? 5. With the help of a sketch, discuss the principal surfaces and planes in metal cutting. 6. Name the two systems of designating the cutting tool. 7. Sketch a single point cutting tool under ASA system. Define various tool angles and discuss their importance. 8. Why negative rake angle is normally employed for cutting hard and strong materials ? 9. What is meant by ‘tool designation’ or ‘tool signature’ ? 10. What is Orthogonal Rake System ? 11. Show the ORS of tool angles with the help of a sketch. 12. Write the relations between ASA and ORS of tool angles. 13. Sketch and explain the two methods of metal machining. 14. Discuss the various types of chips produced during metal machining. 15. Why are discontinuous type chips preferred over continuous type ? 16. Explain, how built up edge on a cutting tool is undesirable ? 17. What is the use of a ‘Chip breaker’ ? 18. Name and discuss the principal elements of metal machining. 19. Define Tool life. 20. Discuss the variables affecting tool life. 21. What is meant by orthogonal cutting and oblique cutting ? 22. Explain the term ‘‘Machinability’’. 23. Explain why large amounts of frictional heat are produced when machining very ductile materials ? 24. How does the rake angle affect the life of the cutting tool ? 25. What two pressure areas of the cutting tools are subjected to wear ? 26. With the help of a sketch, show crater wear and flank wear on a cutting tool. 27. Name the factors that contribute to flank wear. 28. Name the factors that contribute to crater wear. 29. Name the factors that contribute to the formation of discontinuous chips. 30. Name the factors that contribute to the formation of built-up edge. 31. Differentiate between positive and negative rake angles. 32. Discuss the various methods of meassuring temperature at the cutting zone. 33. How is the nose radius of a cutting tool selected ? 34. What is machining time ? Find the time required for one complete cut on a piece of work 350 mm long and 50 mm in diameter. Cutting speed is 35 m/min and feed is 0.5 mm/rev. (PTU Ans.  minutes)  Chapter 7 Cutting Tool Materials and Cutting Fluids 7.1. GENERAL We have seen in the last chapter, that the cutting tool is subjected to: static and dynamic forces, high temperatures, wear and abrasion. To get a reasonable tool life, the tool material should meet the following requirements: 1. Hot hardness, so that the tool does not loose its hardness and strength at the high temperatures developed during machining. It ha been shown that the tool material must be at least 35% to 50% harder than the work material. 2. Wear and abrasion resistance, so that the tool retains its shape and cutting efficiency for a reasonably long time before it is reconditioned or replaced. 3. Impact toughness, so that the fine cutting edge of the tool does not break or chip, when the tool is suddenly loaded. In addition to the above basic requirements, the tool material should possess the following properties: increased thermal conductivity, lower co-efficient of thermal expansion, lower chemical and mechanical affinity for the work-material and it should be easy to form, grind and sharpen to the desired tool geometry, high specific heat and low co-efficient of friction between work and the tool. It should also be easy to cveld/braze or fix to the tool holder. The selection of a proper tool-material depends upon a number of factors such as: type of cutting operation, material of the work piece, machine tool to be used and surface finish required. Usually, a compromise has to be made in the selection of tool-material, since the requirements to be met by tool-material are often contradictory in nature. Over the years, a wide variety of cutting tool materials have been developed to meet the ever increasing demand of machining harder and harder materials. The various cutting tool-materials can be grouped as follows: 1. Plain Carbon Steels. 2. Medium Alloy Steels. 3. High Speed Steels (H.S.S.). 4. Non-ferrous Cast Slloys 5. Cemented Carbides. 6. Ceramics or Oxides. 7. Cermets. 8. Diamond. 427 428 A Textbook of Production Technology 9. Cubic Boron Nitride (CBN). 10. UCON. 11. Sialon. 12. Coronite Plain carbon steels, Medium alloy steels and High speed steels are known as ‘‘Tool Steels’’. Medium alloy steels and high speed steels contain one or more alloying elements to impart the desired properties to the cutting tools. The function of each alloying element is given below. (i) Carbon. Carbon combines with iron to form carbide which makes it respond to hardening, thus increasing the hardness, strength and wear resistance. The percentage of carbon varies from 0.6 to 1.4%. (ii) Manganese. It is added to steels as a deoxidizing and desulphurizing agent. It lowers the critical range of temperature. It increases the time required for transformation, so that, oil quenching, becomes practicable. Its content is about 0.5 to 2%. (iii) Chromium. The addition of chromium results in the formation of various carbides of chromium which are very hard, yet the resulting steel is more ductile than a steel of the same hardness produced by a simple increase in carbon content. Chromium also refines the grain structure so that, these two combined effects result in both increased toughness and hardness. The addition of chromium increases the critical range of temperature and raises the strength at high temperatures. Alloy of chromium resists abrasion and wear. Its content ranges from 0.25 to 4.5%. (iv) Molybdenum. Molybdenum is a strong carbide forming element and its action is very much like chromium but is more powerful. It increases strength, wear resistance, hardness penetration and hot hardness. It is always used in conjunction with other alloying elements. Its content ranges upto about 10%. (v) Cobalt. Cobalt is commonly used in high speed steels to increase the hot hardness so that the cutting tools can be used at higher cutting speeds and temperatures and still they retain their hardness and a sharp cutting edge. Its content ranges from 5 to 12%. (vi) Vanadium. It increases hot hardness and abrasion resistance. As vanadium has a very strong tendency to form carbides, hence, it is used only in small amounts (0.2 to 0.5% in alloy carbon tool steels and 1 to 5% in H.S.S). (vii) Tungsten. It is widely used in tool steels because the tool maintains its hardness even at red heat. Tungsten produces a fine dense structure and adds both toughness and hardness. Its effect is similar to molybdenum except that it must be added in greater quantity(1.5 to 20%). Note : Most of the tool-steels contain two or three alloying elements, as the combined action of several elements is more effective than that of one element even when its content in steel is considerable. 7.2 CUTTING TOOL MATERIALS 7.2.1. Plain Carbon Tool Steels. These are the oldest type of tool steels. The material is inexpensive, can be easily formed and ground. The properties of the material will depend upon the percentage of carbon content. Low carbon steels are tough and shock resistant, whereas, high carbon steels are abrasion resistant. They are basically water hardening materials (Type W tool steels), that is, they are hardened by heating followed by quenching in water to obtain hardness of HRC 60 – 67. Their hardness decreases rapidly above 200°C and so they are useful for low speed operations — drilling, tapping, reaming, broaching etc. They are also used to manufacture; woodworking tools, Cutting Tool Materials and Cutting Fluids 429 cold chisels, hammers, knives and punches. These materials are prone to deformation and cracking when hardened. 7.2.2. Alloy Tool Steels. The various alloying elements (discussed above) are added to plain carbon steel to impart the desired properties to the tool steel. These alloying elements slow down the transformation rates. Due to this, the materials can be hardened in oil or air, and the alloy steels become less susceptible to cracks while quenching. These steels have greater wear resistance and hot hardness than the plain carbon steels. These are widely used for drills, taps, reamers etc., but do not have sufficient hot hardness to be used in high speed turning or milling. Depending upon the method of quenching, these steels are of two types. (a) Type – O Tool Steels. These steels are hardened by quenching in oil. Type O – 1 is most commonly used which has the following composition : C: 0.90%, Mn = 1.00%, W = 0.5%, Cr = 0.5% These steels find use in cold-working applications, such as punching and blanking, shearing, forming and drawing dies. (b) Type – A Tool Steel. These steels are hardened by cooling in air. In these steels, the content of alloying elements is higher as compared to type O steels. Usually, C is 1.0% and Cr is 5%. Chiefly used for cold working applications, for example thread rolling dies, coining dies and gauges. The alloy tool steels can operate upto cutting temperature of about 300°C. 7.2.3 High Speed Steel (H.S.S.). This tool material is basically high carbon steel, to which the various alloying elements (Tungsten W, Molybdenum Mo, Chromium Cr, Vanadium V and Cobalt Co) have been added in larger amounts as compared to alloy tool steels, to improve hardness, toughness and wear resistance properties. These materials are deep hardening and can be quenched in oil, air, or salt. They are capable of retaining their hardness upto 600°C and so can be operated at much higher cutting speeds as compared to alloy tool steels, hence the name “high speed steel”. This tool material was developed in 1905. There are two basic types of high speed steels: 1. Tungsten type steels (T series) which has tungsten as the major alloying element (12 – 20%). 2. Molybdenum type steels (M Series) in which tungsten is partially or completely replaced by molybdenum. The molybdenum type of H.S.S. is cheaper than the tungsten based material and is more readily sharpened. Also, it generally has a slightly greater toughness at the same level of hardness. The popular 18-4-1 H.S.S. contains 18% tungsten, 4% chromium and 1% vanadium. Cobalt (5 to 8%) may also be added to increase red hardness. Carbon is about 0.75%. This material is designated as T-4. The ISI designation of this steel is (T 75 W 18 Co 5 Cr 4 V 1 Mo 70). The most commonly used types of H.S.S. steels are listed in Table 7.1, with their designations and compositions. The most commonly used grades of high speed steels are: M – 1, M – 2, M – 7, M – 10, T – 1 and T – 2. M – 2 tool steel has the applications of: turning carbon and alloy steels of hardness up to 375 BHN, turning nitriding steels of hardness upto 350 BHN, turning ultra high-strength steels of hardness upto 300 BHN, turning tool steels, cast steels of hardness upto 300 BHN, turning armour plate of hardness upto 325 BHN, and turning non-ferrous materials such as copper, brass, aluminum, magnesium and plastics. Broaches to cut ferrous materials of hardness upto 260 BHN are made of M – 2 grade. Milling cutters for cutting ferrous materials of hardness upto 350 BHN are made of M – 2 and M – 7 grades. Gear hobbing, shaping and shaving tools are also made of M – 2 and M – 7  430 Table 7.1 High Speed Steels Designation Percentage of Constituents Relative properties from 1 (low) to 10 (high) C W M0 Cr V Co Wear Resistance Toughness Hardness Cost M–1 0.80 1.75 8.50 3.75 1.15 – 4 10 5 3 M–2 0.85 6.00 5.00 4.00 2.00 – 5 10 5 3 M–3 1.05 6.00 5.00 4.00 2.40 – 6 7 6 4 M–4 1.30 6.00 4.50 4.00 4.00 – 9 5 6 4 M–7 1.00 1.75 8.75 4.00 2.00 – 6 8 5 3 M – 10 0.90 – 8.00 4.00 2.00 – 5 8 5 3 M – 33 0.88 1.75 9.50 3.75 1.15 8.25 5 5 8 5 M – 36 0.85 6.00 5.00 4.00 2.00 8.25 5 5 8 5 M – 41 1.10 6.75 3.75 4.25 2.00 5.00 6 4 8 5 M – 42 1.10 1.50 9.50 3.75 1.15 8.25 6 4 9 5 M – 43 1.20 2.75 8.00 3.75 1.60 8.25 6 4 9 5 M – 44 1.15 5.25 6.50 4.25 2.00 12.00 6 3 10 6 M – 45 1.25 8.25 5.00 4.25 1.60 5.50 6 3 8 5 M – 46 1.25 2.00 8.25 4.00 3.20 8.25 8 3 9 5 T–1 0.75 18.00 – 4.00 1.00 – 4 8 5 5 T–2 0.85 18.00 – 4.00 2.00 – 5 6 5 5 T–4 0.75 18.00 – 4.00 1.00 5.00 5 4 7 6 T–5 0.80 18.00 – 4.25 2.00 8.00 – 4 8 6 T–6 0.80 20.00 – 4.50 1.75 12.00 5 2 9 8 T – 15 1.50 12.00 – 4.50 5.00 5.00 10 3 9 6 A Textbook of Production Technology Cutting Tool Materials and Cutting Fluids 431 grades. Drills and reamers for steels of hardness upto 325 BHN are made of M – 1, M – 2, M – 7 and M – 10 grades. Taps are mostly made from M – 1 grade. Form tools are usually made from M – 2 or T – 2 grade of H.S.S. High speed steels with cobalt (for example grades: M – 33, M – 36, T – 4, T – 5 and T – 6) have high hot hardness and wear resistance but lower toughness as compared to M – 2 H.S.S. Tool bits for planing and heavy duty turning are made from T – 4, T – 5 and T – 6 grades. Grades M – 33 and M – 36 are used for: drilling and milling hard alloy steels, stainless steel, titanium and heat resistant materials. T – 6 grades are also called “Super H.S.S”. They have a high W (18 to 22%) and high Co (10 to 12%) content. They are less tough than other types and need to be well supported on a very rigid machine. This type is the most expensive H.S.S. H.S.S. with high percentage of vanadium (For example M – 3, M – 4, and T – 15 grades) possess increased red hardness and wear resistance but their toughness and grind ability is reduced. Due to increased red hardness and wear resistance, these steels have greater tool life as compared to conventional H.S.S. They are used for machining conventional alloys, super alloys and refractory materials. Single point lathe tools, flat and form cutters, broaches and drills are made of these steels. H.S.S. with cobalt (grades M – 41, M – 42, M – 43, M – 44, M – 45 and M – 46) have high hot hardness and wear resistance. Their grind ability is better as compared to H.S.S. with vanadium. These steels are used for machining: heat treated steels, titanium alloys and aerospace materials of high hardness (cobalt and nickel base alloys). Drills, milling cutters, form tools, broaches, hobs, shavers, taps and tool bits are made of these steels. In the conventional H.S.S., there is every possibility of large dispersion and segregation of carbides. Due to this, there will be local variations in the chemical composition and structure of the material, resulting in difficulty in producing the tool and also poor performance of the tool. These drawbacks are overcome by using powder metallurgy (P/M) for the manufacture of tool steel. The powder of the steel alloy is made by atomizing the molten metal. The powder is then compacted under pressure in dies, to produce billets. These billets are then transformed to the desired shape and size by conventional hot forging. In this tool steel, there is better and more uniform distribution of carbides and the alloying elements. H.S.S. produced by P/M prossesses higher wear resistance greater toughness, greater impact strength, better hot workability, improved grindability, higher material removal rates and good dimensional stability. 7.2.4 Non - Ferrous Cast Alloys (Stellite). This material which was introduced in 1915 is an alloy of Cobalt, Chromium and Tungsten with composition: Cobalt, 38 to 53 per cent, Chromium, 30 to 33 per cent, Tungsten, 10 to 20 per cent and Carbon, 1 to 3 per cent. 1 per cent Carbon content gives a relatively soft and strong tool and 3 per cent Carbon content gives a hard and more wear resistant grade. Cast alloy tools are cast and ground to any desired shape. Cast alloys bridge the gap between H.S.S. and Carbides (the next tool material). They have properties intermediate between H.S.S. and cemented Carbides. This material maintains great hardness at high temperatures and has good wear resistance but is not as tough as H.S.S. and is sensitive to shock loading. The tool material is available in the form of tool bits for use in holders and in the form of tips brazed to a medium carbon steel shank. The material is recommended for deep roughing operations at relatively high speed and feed rates and it can machine more difficult materials such as high tensile steels, stainless steels and heat resistant steels, and C.I. This material is used at surface speeds above those of H.S.S. and below those of Carbides, and can withstand a cutting temperature in the range of 900°C. The introduction of Cast alloys as cutting tool materials somewhat overlapped that of the Tungsten carbide and since these carbides were, in general, superior cutting tools, the cast non- ferrous alloys never caught on to the extent anticipated. 432 A Textbook of Production Technology 7.2.5 Sintered or Cemented Carbides. The first sintered carbide cutting tool (tungsten carbide) was marketed in 1926. Since then lot of research has been done and many types of sintered carbide materials have been produced to improve their performance. This has been achieved by improving the methods of their manufacture and their composition. For example, carbides of titanium, tantalum, niobium and columbium etc. can be added to straight tungsten carbide to extend the range of their application. Sintered carbides are produced by P/M technique and have the following properties. Very high red hardness (of about 1000°C), very high wear resistance, high modulus of elasticity, low thermal expansion and high thermal conductivity. There are three general groups of cemented carbides in use : 1. Straight tungsten carbide with cobalt as a binder. 2. Tungsten carbide with cobalt as a binder and having large percentages of carbides of Ti, Ta and Nb, which along with WC form a solid solution of WC – TiC – TaC – NbC. 3. Titanium carbide with nickel or molybdenum as the binding material. Manufacture. The cemented carbides are manufacture by P/M technique involving the following steps: 1. Firstly, we should get the ingredients. Tungsten oxide is reduced in hydrogen to get tungsten metal powder. Similarly, to obtain titanium and tantalum, titanium oxide, and tantanlum oxide are reduced in hydrogen atmosphere, respectively. 2. Tungsten powder is then mixed with lamp black and the mixture is heated at about 1600°C to form carbide of tungsten, and so on. 3. The lump of the carbide so produced is crushed to a powder. Then, powdered cobalt metal which acts as a binder to hold together the particles of carbide powder, is thoroughly mixed with the latter. 4. To increase the mouldability of WC – Co mixture, it is mixed with a lubricant such as paraffin, ethylene glycol or camphor. Sometimes, only water is used. 5. After drying and refining, the mixture is compacted in a press to get the desired blocks. 6. The strength of the green compacts so obtained is quite low. To give the necessary strength to the compact, it is heated at 1300°C to 1865°C (sintered process) depending upon the composition and the cobalt content. As a rule, the sintering temperature is 90°C to 100°C below the melting point of pure cobalt. The sintering process is done for about 1.5 to 2 hours and it should be done preferably in an inert atmosphere to avoid oxidation or decarburization. During sintering, there is about 15 to 25% shrinkage and this factor must be taken into account while designing the product shape. 7. The product is finally ground or lapped depending on the requirements. Properties and Uses. The properties of the carbide tool material depend upon: (i) Chemical composition (ii) Method of manufacture (iii) Micro-structure of the tool material. The finer the grain size, the higher is the hardness. Out of these, the major effect is of the chemical composition (the method of manufacture and the grain size can be properly taken case of). Straight tungsten carbide contains cobalt from 3 to 20%. With increase in the percentage of cobalt, the hardness, the brittleness and compressive strength of the material decreases. However, with increase in cobalt content, the transverse rupture strength of the tool material increases. This material has high abrasive wear resistance and high strength in respect of a given hardness. The main drawback of straight tungsten carbide is its affinity for steel. Due to this, the steel chips tend Cutting Tool Materials and Cutting Fluids 433 to weld to the tool surface resulting in crater wear. Hence, this tool material is not suitable for cutting steel, but gives superior performance with non-ferrous, non-metallic material and cast iron. This material is called as ‘C’ grade cemented carbide material. To reduce the tendency of the metal chips to weld to the tool and to decrease the diffusion of the tool material to the chips, TiC is added to WC – Co system. However, with increase in the percentages of TiC, compressive, transverse and impact strengths and also the thermal conductivity and the modulus of elasticity of tool material decrease. These drawbacks of TiC are overcome by adding TaC, which increases the transverse strength and the hot hardness of the tool material. The effect of adding NbC is similar to that of TaC. These mixed carbides are used for machining steel. This cemented carbide material is known by ‘S’ grade. It contains: about 16% TiC, O to 10% TaC. Cabalt content varies from 3 to 16%. With TiC tool material nickel and molytdenum are used as binding materials. Molybdenum is added to check the very high grain growth with TiC-Ni system. This material has low wettability. This improves its crater wear resistance and hence it is used when cutting temperatures are high, because of high cutting speeds or hard workpiece material. According to ISO (International Standards Organization), the various grades of carbide tool material have been grouped in three series: 1. Carbide tools used for cutting cast iron and non-ferrous metals are designated from: K01 to K40. 2. Grades of carbide tools used for machining steel are designated as: P01 to P60. 3. Grades of carbide tools used for general purpose applications are designated as: M10 to M30. In all the above three series, the harder and brittle materials have the lowest number and the less hard and most tough materials have the higher numbers.For example in K-series, K01 is the hardest and most brittle and K40 is the least hard and the most tough material. The K-grades of carbides are essentially straight tungsten carbides with cobalt as the binder, and are used for machining cast iron, non-ferrous metals, plastics and similar materials. P-grade carbides are combined carbide tool materials (carbides of W, Ti, Ta and Nb) with cobalt as the binder. These materials are similar in composition to the K-grade, but have different properties due to different manufacturing methods. These are used to cut heat resistant steels and stainless steels. All carbides, when finished, are extremely brittle and weak in their resistance to impact and shock loading. Due to this, vibrations are very harmful for carbide tools. The machine tools should be rigid, faster and more powerful. Light feeds, low speeds and chatter are harmful. Due to the high cost of carbide tool materials and other factors, cemented carbides are used in the form of inserts or tips which are brazed or clamped to a steel shank, Fig. 7.1. Shank Shank Clamp screw Insert Clamp Insert Braze Seat (a) Clamping (b) Brazing Fig. 7.1. Methods of Attaching Inserts to Tool Shanks.