A Textbook of Production Technology PDF

434 A Textbook of Production Technology In the case of a brazed tip, when it is worn out, it is resharpened with the help of special grinding wheels on a Tool and Cutter grinder. The main drawbacks of a brazed tip are : For resharpenin...

434 A Textbook of Production Technology In the case of a brazed tip, when it is worn out, it is resharpened with the help of special grinding wheels on a Tool and Cutter grinder. The main drawbacks of a brazed tip are : For resharpening purposes, the tool will have to be removed from the machine involving a resetting operation. Also, because of the difference in co-efficients of expansion of tip material and tool shank material, the brazing has to be done very carefully. Mechanically clamped tips are known as ‘‘Indexable tips or inserts”, because these have more than one cutting edge which are used one by one by indexing the tip. These tips are also known as “Throw-away” or “disposable”, because once all the edges of the tip have been used, the tip or insert is removed from the tool shank seat and thrown away or disposed-off. In this way, a rectangular tool bit (tip or insert) can be used upto eight times before disposal and requires no resharpening cost. The inserts are available in a variety of shapes, such as square, triangle, diamond and round, as shown in Fig. 7.2 90° 60° 87° 35° 35° Fig. 7.2. Different Shapes of Inserts. The various parameters depend upon the shape of the insert. For example, (a) Higher the cutting edge angle, higher is the cutting edge strength. That is, cutting edge strength increases from right to left in Fig. 7.2 (b) Power reguirement decreases as the cutting edge angle decreases. (Left to Right) (c) Tendency to vibration increases with increase in cutting edge angle. (Right to Left). (d) Versatility and accessibility increases as thecutting edge angle decreases. (Left to Right) There are certain rules of thumb to be followed when selecting an insert for a particular Job:- (i) For strength and economy, the largest possible cutting edge angle should be selected. (ii) When there is a tendency to vibration during operation, strength versus versatility and assessibility through the use of smaller cutting edge angles should always be considered. To inprove the cutting edge strength, the insert edges are usually honed and chamfered or produced with a negative land. The radius of honing may be about 0.025 mm. When using ceramics, re-chamfering is recommended to minimize the risk of burrs when the insert exits the cut. It also has a positive effect on the insert while entering. Laminated and Coated Carbides. We have seen above that the resistance of straight tungsten carbide to crater wear can be increased by the addition of TiC. The same result can be obtained by laminated and coated carbides. In laminated carbides, laminates consisting of a hard thin surface layer of TiC and in the form of throw-away tips, are bonded by epoxy resin to the rake face of a tip body of WC. Coated carbides have a thin coating of TiC on all faces of the tip. The coating thickness is of the order of a few microns (0.0025 to 0.005 mm). These tools resist the diffusion wear on the crater and give a tough shock resistant tool. Laminated and coated carbides are shown in Fig. 7.3. Other common coating coating materials are titanium nitride, titanium carbonitride, aluminium oxide and diamond. Coating is done by Chemical Vapour Deposition (CVD). Oxide Coating of Carbide tools. The diffusion of atoms between the tool and chip material (which is the major cause of carbide tools) can be retarded by coating the tool surface of the Cutting Tool Materials and Cutting Fluids 435 carbide tools with oxides of aluminium and zirconium. This considerably increases the tool life. Coated Carbides are used for machining super alloys. Largely Tungsten Carbide & Cobalt Surface Layers of TC i Layer of pure Ultra-Fine Grain TiC (a) Laminate (b) Coated Fig. 7.3. Laminated and Coated Carbides. These tools operate at cutting speeds which are about 5 times of that for H.S.S. Cemented carbide tool materials also include: nitrides, borides and silicides which are compounds of nitrogen, boron and silicon with such metals as Tungsten, Titanium, Tantalum, Niobium and Molybdenum. 7.2.6 Ceramics and Oxides. Ceramics or sintered oxides were developed as cutting tool materials during 1950 – 1960. These materials are basically aluminium oxide (Al2O3) containing additions like MgO, NiO, Cr2O3, TiO and TiC etc. to improve the grain structure, cutting properties and sintering. These materials are produced in the same manner as sintered carbides, that is, by P/M technique. Th aluminium powder is ball milled to a suitable grain size. Water is added and the ingredients are thoroughly mixed to make a stiff paste. This is then pressed into pallets of required form in punch and die. The pallets are dried and then sintered at temperature of 1500°C to 1700°C. The grain particles then get bonded to one another by the process of diffusion. The pallets are then cut and ground to the required shape, size and surface finish, with the help of resinoid bonded grinding wheels. Ceramics are always used in the form of indexable inserts of standard shapes and sizes. At present there is no satisfactory method of brazing the tool tips to steel shank. So, the ceramic tool tips are clamped mechanically to the steel tool shank. Ceramic tool materials have a very high abrasion resistance, are harder than cemented carbides and H.S.S. and have less tendency to weld to metals during machining. However, they lack impact toughness, so, vibration and chatter are fatal to these tools. Due to this, the tool mountings should be rigid and the machine tools should be rigid. These materials are especially effective at very high cutting speeds (2 to 5 m/s) and for uninterrupted cutting operations. These materials are particularly successful for machining cast iron, and high tensile materials, at cutting speeds which are 2 to 3 times of that of comented carbides. These tools can retain cutting edge hardness upto about 1400°C and exhibit uniform strength upto 1200°C. Another ceramic tool material is Silicon nitride (Code named S-8), which is particularly used for cast iron machining applications. These ceramic tools have a tool life that was effective over 1500 C.I. pieces, where coated tungsten carbide tools lasted only 250 pieces before dulling. Ceramics are less expensive than carbides and the trend now is to replace the latter with the former whenever possible and reasonable. Ceramics are mainly used for finishing and super-finishing. Poor results are obtained if Al2O3 tool material is used to machine Al or Ti alloys, because strong bonds tend to form between the chip and the tool. 7.2.7 Cermets: The cermets are combinations of ceramics and metals, bonded together in the same manner in which P/M parts are produced. They combine some of the high refractoriness of ceramics and toughness and thermal shock resistance of metals. For cutting tool materials, the 436 A Textbook of Production Technology usual combination is Al2O3 plus metal additions (W, Mo, Boron, Ti etc.) in an amount upto 10%. These additions reduce the brittleness to some extent, but they reduce the wear resistance of the material as well. 7.2.8. Diamonds. Diamond is the hardest of all the cutting tool materials. Diamond has the following properties: extreme hardness, low thermal expansion, high heat conductivity, and a very low co-efficient of friction. This is used when good surface finish and dimensional accuracy are desired. The work-materials on which diamonds are successfully employed are the non-ferrous one, such as copper, brass, zinc, aluminium and magnesium alloys. On ferrous materials, diamonds are not suitable because of the diffusion of carbon atoms from diamond to the work-piece material. Diamond tools have the following applications: single point turning and boring tools, milling cutters, reamers, grinding wheels, honing tools, lapping powder and for grinding wheel dressing. Due to their brittle nature, the diamond tools have poor resistance to shock and so, should be loaded lightly. For fine turning, the following values are recommended: cutting speed 200 to 500 m/min, feed 0.01 to 0.05 mm/rev. and depth of cut 0.10 to 0.15 mm. Diamonds are available either as naturally or as man made (synthetic). The natural diamonds are, however, of low grade. For metal cutting applications, polycrystalline diamond instead of single crystal diamond, has been recently introduced. This tool material known as ‘‘Compacts’’ has increased strength and shock resistance. This material is made of diamond powder by sintering into moulded shapes by high pressure and temperature technology. Polycrystalline diamond has been successfully used for machining tough, abrasive non-ferrous materials, plastics, ceramics and glass. The material can be moulded into standard shaped inserts and used as conventional indexable inserts. Indexable inserts can also be made by brazing compacts of polycrystalline diamond to each cover of a carbide insert. This material can also be used by bonding its thin layer (about 0.5 mm thick) to a WC substrate. These blanks are brazed on to a steel shank in position, ground and then used. WC being tougher than diamond will increase the shock resistance of the tool. such tools are called as ‘‘Compax’’ tools. Diamond tools can with stand heat upto 2000°C with highest tool life (50 to 100 times more than that of WC). 7.2.9 Cubic Boron Nitride (CBN): Next to diamond, CBN is the hardest material currently available. This material, which consists of atoms of nitrogen and boron, was produced in the early 1970's by high pressure, high temperature processing. As a cutting tool material, CBN is used in the polycrystalline form. CBN has high hardness and high thermal conductivity. It has much higher tensile strength (1000 N/mm2) as compared to diamond (300N/mm2). CBN being chemically inert, is used as a substitute for diamond for machining steel. Other applications are: as a grinding wheel on H.S.S. tools, for machining high temperature alloys, Titanium, Nimonic, Stainless steel, Stellites and Chilled C.I. In dealing with iron-based alloys and hardened steels, the life of a CBN tool is 4 to 5 times higher than that of a diamond tool. CBN, as a cutting tool material, can be used in different ways: In ‘‘Compax’’ tools, a 0.5 mm thick layer is bonded to a cemented carbide substrate at high temperature and pressure. These tools combine the high hardness and wear resistance of CBN with the high shock resistance and toughness of WC. CBN can also be made in the form of indexable inserts and blanks of standard shape and size. The blanks can be brazed on to steel shanks, form ground and then used. This material is known by the trade name ‘‘Borazon’’, in U.S.A. and ‘Elbor’ in Russia. 7.2.10 UCON. This is also new cutting material developed by Union Carbide, U.S.A. Its constituents are: Columbium 50%, Titanium 30%, and Tungsten 20%. This tool material is manufactured according to the following steps: 1. The powders of columbium, Titanium and Tungsten are thoroughly mixed and blended. Cutting Tool Materials and Cutting Fluids 437 2. The mixture is compacted in a punch and a die. 3. The compact is then melted in an electric arc furnace and the alloy is cast into ingots. 4. The ingot is rolled into sheets which are then cut into strips. 5. The strips are then cut into blank of desired shape and size. 6. The blanks are then can ground and honed subsequently by tumbling to produce a radius of 0.05 to 0.075 mm at cutting points. 7. Lastly, the blanks are nitrided in a nitrogen atmosphere at a very high temperature. UCON has the following properties : High hardness, High toughness, excellent shock resistance and excellent resistance to diffusion and adhesion wear. This is a basically steel cutting material and is not preferred for cutting cast iron, stainless steel and super alloys containing Ni, Co and Ti as base materials. Cutting operations recommended for UCON are: roughing, semi-roughing and finishing, turning, facing and boring operations. It permits 60% increase in cutting speed when compared with WC. 7.2.11 Sialon (SiAlON): The research on this tool material has been going on for the last about 14 years. The material is produced by milling together Si3N4, Aluminum oxide, Al2O3 and yttria. The powder is dried, pressed to shape and sintered at a temperature of about 1800°C. This material has been found to be considerably tougher than ceramics, and thus can be successfully used for machining with interrupted cuts. Cutting speed can be 2 to 3 times, those with carbides. At present, the field of application of this tool material (in the form of tips) is for machining aerospace alloys, Ni-based gas turbine blades etc. at cutting speed in the range of 3.3 to 5 m/s. 7.2.12. Coronite : It is a new cutting tool material whose properties lie in between those of H.S.S. and cemented carbides. It combines the toughness of H.S.S. with hardness and wear resistance of cemented carbides. This improves tool life, reliability and surface finish. Cutting tools made from this material are mainly endmills used for machining grooves, pockets and for profiling in majority of the workpiece materials. The material consists of fine grains of Ti N evenly dispersed in a material of heat treatable steel. The hard grains of Ti N form about 35 to 60% of material’s volume. The properties of the material are attributed to : very small size of hard grains of Ti N (about 0.1 micron) as compared to 1 to 10 microns in H.S.S. and Cemented carbides and the proportion of hard grains in the material (which is higher than in H.S.S. but less than in cemented carbides). The material is producced by particle metal technology. Majority of the tools are not produced from solid coronite but by compound and coating technology as follows :- 1. A core of H.S.S or spring steel. 2. A layer of about 15% of diameter of core is created over the core by extrusion process at about 540°C. The bar thus produced is the raw material for coronite cutting tools. 3. A thin coating (about 2 microns) of TiCN or TiN is created on the material by PVD method. 7.3. CUTTING FLUIDS In any metal cutting operation lot of heat is generated due to: plastic deformation of metal, friction at the rake face of the tool between the tool and the chip and also the friction between the workpiece and the flank of the tool. This increases the temperature both of the workpiece and the tool point, resulting in decrease in hardness and hence life of the tool. The machined surface will also be rough and the possibility of built up edge increases. So, the use of a cutting fluid during a 438 A Textbook of Production Technology machining operation is very essential. Its application at the workpiece-tool interface produces the following effects: 1. Friction at the workpiece -tool interface is reduced, due to lubricating action. 2. Heat is reduced due to cooling action at the interface. 3. Chips are washed off. So, the cutting fluid performs the following functions: 1. Reduces heat generation. 2. Provides lubricating action. 3. Carries away the heat generated and so provides cooling action, thus, reducing workpiece temperature and distortion. 4. Provides flushing action in washing off the chips and swarf. 5. Reduces friction and wear, which improve tool life and surface finish. 6. Reduces force and energy consumption. 7. Protects the newly machined surface from environmental corrosion. 8. Prevents surface welding of points at high presssure, thus, controlling formation of B.U.E. 9. Facilitates chip breaking in certain materials. Due to reduction in friction because of lubricating action: shear angle increases, chips produced are thin, cutting force is decreased, less heat is produced and there is low built up edge. Due to reduction in heat produced and cooling effect of the cutting fluid, the tool and the workpiece remain cooler. This results in: maintenance of tool hardness, less tool wear and longer tool life, less distortion and easy handling of the job. Washing off the chips helps in better surface finish and use of higher feed rate. 7.3.1. Lubrication and Cooling Action of Cutting Fluids. During metal cutting, the area of contact between tool and job is very large and also the ratio of real to apparent area of contact is very nearly equal to unity. Again, the contact pressure at the tool-workpiece interface is very high. Due to the above two factors, the type of lubrication in metal cutting can never be full fluid film lubrication. It can only be boundary lubrication. Because of this, the chemical properties of a cutting fluid are more important than its physical properties. Additives like chlorine, phosphorous, sulphur and fatty acids in the cutting fluid react with workpiece material and form a layer of solid lubricant. This lubricant has a low shear strength, and can withstand high temperature. Due to this lubrication action, the amount of heat generated is considerably reduced, the formation of weld between the tool and the chip is prevented and the chips shear easily in sliding. However, at cutting speeds of more than 60m/min, the lubricating effect of chemically active cutting fluids is only marginal and for such high speed cutting operations, the cooling action of the cutting fluid is more predominant than its lubricating action. 7.3.2. Requirements of A Cutting Fluid. A cutting fluid should have the following properties: 1. It should wet the surfaces of cutting tool and workpiece for better cooling and lubricating effects. 2. Good lubricating property. 3. High heat absorbing capacity. 4. High flash point. 5. It should not damages or react with the materials of machine tool parts. 6. It should not stain or leave residues on the workpiece surface. 7. It should not emit toxic vapours. 8. It should be stable, that is, it should not get oxidised or decomposed when left in air. 7.3.3 Types of Cutting Fluids. There are basically two main types of cutting fluids: Cutting Tool Materials and Cutting Fluids 439 1. Those which are mixed with water, such as, soluble oils and soaps. These are emulsions of oil and water or soap and water. 2. Those which are not mixed with water, called cutting oils, which can be pure oils or a mixture of two or more oils. Soluble Oils. Water increases the cooling effect and oil provides the best lubricating properties. By mixing various proportions of water with soluble oils or soaps, cutting fluids with a wide range of cooling and lubricating properties, can be obtained. The ratio of oil to water depends upon the application of the cutting fluid and ranges from about 1: 5 to 1: 50. The usual ratios for the various machining operations can be: Turning : 1 : 25; Milling : 1 : 10 Drilling and reaming : 1 : 25; Grinding : 1 : 50 Cutting Oils. These are fixed oils and mineral oils. Fixed oils consist of animal, fish and vegetable oils. Chiefly used fixed oils are: lard oil, sperm or whale oil, and olive, cotton seed and linseed oil. Turpentine oil distilled from vegetable oils is also used. Fixed oils have greater oiliness than mineral oils but are not so stable as mineral oils and tend to become gummy and decompose when heated. Mineral oils come from crude petroleum oils, for example, paraffin. To combine stability of the mineral oils with good lubricating properties of fixed oils, they are often mixed. Sometimes chlorine and sulphur is added to give the property of ‘‘Wetting’’ the metal with a highly adhesive oil film. This imparts the antiwelding properties which help to prevent the formation of built up edge on the cutting tool. Such cutting oils are known as ‘‘sulphonated oils’’. To prevent the growth of bacteria and fungi which makes the cutting fluid to become injurious to the human skin, a phenol disinfectant is added. During machining of heat resistant and stainless steels, nimonic alloys etc. the pressure and temperature at the cutting edge can be very heavy. The normal cutting oil may not be able to support this heavy presure at the cutting edge, resulting in welding of metal chips of the tool face. To prevent this, extreme pressure (EP) additives are added to the cutting oils. The most commonly used EP additive is sulphur and next is chlorine. These additives form solid films of iron chloride and iron sulphide between the tool face and the chips. These films are easily sheared and have high melting points. Thus, they help in preventing the chips from welding to the nose of the tool where the pressure is maximum. Chlorine will form a film between surfaces at points of high pressure, preventing their contact, but at the same time allowing them to slide over one another. So, depending upon the working conditions, four types of cutting oils can be had: (i) Straight mineral oils. These are suitable for light duty and high speed work. (ii) Mixture of mineral oils and fixed oils. These are suitable for light and medium duty. (iii) Mineral oils with EP additives suitable for heavy duty. (iv) Mixture of mineral oils and fixed oils with EP additives suitable for the heaviest duty. 7.3.4. Application of Cutting Fluids. The cutting fluid may be applied to the cutting tool in the following ways: 1. By hand, using a brush. 2. By means of a drip tank attached to the machine body. 3. By means of a pump. For the effective use of cutting fluid and for heavy and continuous cutting, the fluid should penetrate into the cutting zone. For this, the third method (using a pump) of supplying the cutting 440 A Textbook of Production Technology fluid is the most common. This is known as ‘‘Flood application’’. Here, a continuous stream of cutting fluid is directed at the cutting zone with the help of a nozzle or jet, (Fig. 7.4). This is also known as ‘‘Hi-jet Method’’. The used cutting fluid drops into a tank at the bottom. Before it, is recirculated by the pump, it passes through many filters to remove chips and dirt. To avoid excessive splashing and vapourization of the fluid, it is supplied to the cutting zone from the top also, (Fig. 7.5). For some applications, the cutting fluid is supplied through the tool itself and directed along the flank face of the tool. Under suitable circumstances, the ‘high-jet’ method gives a significant increase in tool life. However, the apparatus for applying the cutting fluid is expensive and also the high-presure just may be of some danger to the operator. Thus, it may be an economical procedure of coolant application, it is not universally adopted. Top Nozzle Tool Work J piece O Tool B Cooling Jet Bottom Nozzle Fig. 7.4. Flood Application of Cutting Fluid. Fig. 7.5. Use of Two Nozzles. Mist method of coolant application. The application of the cutting fluid as a fine atomized mist (combination of the carrier air and very fine drops of fluid) has received considerable attention in the last 25 years. The size of the fluid drops is of the order of 10 to 25 μ m. The mist is sprayed onto the cutting zone at high velocities of about 300 mps and more and under pressures of the order of 0.28 to 0.42 N/mm2. The beneficial cooling effects of mist cooling result from the following facts: 1. Due to the high velocity of fluid application the dispersal of heat by convection currents, is intensified. This maintains the desirable high temperature gradient near the tool surface. 2. The surface area of the coolant is much greater in the form of mist than in flood applications. This increases the cooling capacity of the coolant. 3. Due to the expansion of the mist in the issuing nozzle, its temperature falls down considerably. Mist cooling can be applied to almost all cutting operations, but it is generally more useful with high-hardness work materials. It has been proved by research workers that mist cooling is less effective than flood cooling at significantly higher and lower temperatures (480° and 150°C). At low temperatures, the cooling is by heat conduction into the coolant and in this case, the stream of coolant is more effective than the mist. At very high temperatures, the mist produces a substantial blanket of vapours around the cutting zone. Its heat-transfer co-efficient becomes poor and its cooling efficiency descreases. Thus, mist cooling is more effective than flood cooling within a limited temperature range where the flood cooling loses its effectiveness due to the formation of a vapour layer under it. Cutting Tool Materials and Cutting Fluids 441 The process of producing mist of the cutting fluid is based on the venturi principle, (Fig. 7.6). The high pressure air flowing by a siphon tube, draws the fluid into the nozzle. The shape and the density of the mist is then controlled by the nozzle. The basic components of a mist producing system are: air pump with an air storage, the cutting fluid container, piping and the spray nozzle. On-Off Mist Valve Nozzle Pressurised Air Cutting Siphon Fluid Fig. 7.6. Production of Mist. 7.3.5. Selection of a Cutting Fluid. The type of cutting fluid to be employed depends upon the work material and the characteristics of the machining process. For some machining process, a cutting fluid which is predominantly a lubricant is desirable, while with other machining processes, a cutting fluid which is predominantly a coolant should be used. For machining free cutting steels and ‘‘yellow’’ metals, soluble oils are used as cutting fluids. For expensive cutting tools, such as, form tools, gear cutters, broaches and milling cutters, a carefully selected cutting oil which does not effect the material of the cutting tool is selected. For machining difficult materials and for severe cutting conditions, a cutting oil is preferred to a soluble oil. Cast iron is usually machined dry, because, the usual cutting fluid forms a sort of slurry or mud with the powdery chips and this can enter the grooves and recesses in the machine tools. The graphite flakes within gray cast iron provide excellent lubrication. Brass, bronze and aluminum may be machined dry or wet with water soluble oils. Steel should always be machined with a lubricant. Magnesium and zincalloy die castings are generally machined dry when light cuts are taken or when cutting is done at low speeds. For heavy stock removal and for cutting at high speeds, cutting oils are used for magnesium and for zinc, kerosene lard oil mixture (50: 50) is most efficient. By far, the most common of all cutting fluids is the use of soluble oils which are emulsions of mineral oil with an emulsifier such as soap in water. These are combination of fatty oils, fatty acids, wetting agents, softening agents, emulsifiers, sulphur, chlorine, rust and foam inhibitors, germicides and water. Water as a cutting fluid is the best cooling medium and is most effective fluid for high speed cutting, but it has little lubricating value, it does not spread well over a surface to wet it because of high surface tension, it causes rust and corrosion. To improve it as a cutting fluid, small proportions of inorganic additives are mixed with it. These additives include amines and nitrites to prevent rust, nitrates for nitrite stabilization, phosphates and borates to act as water softeners, wetting agents, and in some cases phosphorous, chlorine, or sulphur to prevent extreme pressure lubrication. The general working rules for cutting-fluid selection are: 1. Select oil with extreme pressure properties for low cutting speeds (less than 30 to 60 mpm). At low cutting speeds, lubrication is the important factor in order to reduce the tendency to produce B.U.E. 2. At higher cutting speeds, temperatures increases substantially and cooling is the important consideration. So, select coolants (oil-in-water emulsions or water with rust inhibitor) for medium to high speed cutting. 442 A Textbook of Production Technology Recently, pure compounds like carbon tetrachloride (CCl 4 ), chloroform (CHCl 3), trichloroethane and certain other chlorinated hydrocarbons have been found to be efficient lubricants when cutting many metals at low speeds (few cms. per min). However, their main drawback is their toxicity and their use involving high temperatures is out of question, since, when heated they give off poisonous gases. Brass like C-I can be machined without a cutting fluid. Paraffin is sometimes used as a basis for cutting fluid when machining aluminium and is normally mixed with 50% mineral oil. Aluminium, being very soft and ductile, becomes welded to the tool cutting edge, particularly when using high cutting speeds and feeds. Cutting becomes inefficient resulting in rough surface. Paraffin provides a high level of lubrication between tool and Al to prevent this situation from occurring. Table 7.2. gives types of cutting fluids used with different work materials and for different machining operations. Table 7.2. Type of Cutting Fluids Material being Machining operation machined Turning Drilling Tapping Milling Cast Iron Dry Dry Dry or 25% Dry Lard oil + 75% Mineral oil Alloy Steels 25% sulphur base oil Soluble oil 30% Lard oil + 70% 10% Lard oil + 75% mineral oil mineral oil + 90% mineral oil Low carbon 25% Lard oil + 75% Soluble oil 25–40% Lard oil with Soluble oil and Tool steels Mineral oil mineral oil Malleable Iron Soluble oil Soluble oil Soluble oil Soluble oil Bronze Soluble oil soluble oil 30% Lard oil with Soluble oil mineral oil Copper Soluble oil Soluble oil Soluble oil Soluble oil Effect of Coolants on Cutting Variables, i.e., cutting speed, feed and depth of cut :– Tool life is a direct function of cutting temperature (temperature at the work – tool interface). Excessive temperature is the most serious limitation to tool life, because cutting tool materials markedly soften at sufficiently high temperature, thus adversely affecting the tool life. The cutting temperature increases with increase in three process variables. Increased speed and feed and depth of cut magnify the thermal problems of the tools and the surface finish of the job. However, the work-tool interface temperature can be sufficiently reduced and hence the tool life can be enhanced by the effective use of a cutting fluid. Reduced average temperature at the tool-work interface permits a sufficiently high cutting speed and feed. This inhibits the formation of build-up edge, increases the metal removal rate and shortens the cycle time. Also, higher cutting speed leads to a higher shear-plane angle and a reduced co-efficient of friction, both advantageous with respect to optimum cutting conditions. PROBLEMS 1. Enumerate the essential requirements of a tool-material. 2. Name the various cutting tool materials. 3. Discuss the role of carbon, manganese, chromium, molybdenum, cobalt, vanadium and tungsten in tool steels. Cutting Tool Materials and Cutting Fluids 443 4. Give the field of application of type ‘O’ tool steels and type ‘A’ tool steels. 5. What are the significant characteristics of high-speed steels ? 6. Discuss the T- series and M- series high-speed steels. 7. Enumerate the advantages of high-speed steel produced by powder metallurgy. 8. How are carbide tools made? Describe the process. 9. Which sintered carbides are employed for machining steels and for machining non-ferrous metals ? 10. Name the various binding materials used in the manufacture of sintered carbides. 11. What do you understand by ‘‘laminated’’ and ‘‘coated carbides’’ ? 12. Give the advantages of coating the face of tungsten carbide tools with a thin coating of titanium carbide. 13. Enumerate the advantages of ceramic cutting tools over tungsten carbide tools. 14. Give the constituents of stellite tool material. How this is manufactured? 15. How ‘UCON’ is produced ? 16. List the various tool materials used in industry. State the advantages and disadvantages of each material. 17. State the optimum operating temperature of each of the tool materials used in industry. 18. What are the main functions of a cutting fluid ? 19. List the essential characteristics of a cutting fluid. 20. Discuss the various types of cutting fluids. 21. Discuss the various methods of applying the cutting fluid at the cutting zone. 22. Give the constituents of ceramics. How these materials are produced ? 23. What are cermets ? Give their composition and properties. 24. Give the advantages of C.B.N. as a cutting tool material. 25. What are EP additives ? Why are these added to a cutting fluid ? 26. Write the note on ‘‘selection of a cutting fluid’’. 27. Why were cutting tool inserts developed ? 28. What are ‘‘Indexable Inserts’’ and ‘‘Throw away inserts’’ ? 29. Discuss the two methods of attaching inserts to tool shanks. 30. What is the composition of Sialon ? 31. Explain the application and limitation of ceramic tools ? 32. Why are tools coated ? 33. What is 18-4-1 type of cutting tool material ? 34. What are “Compax tools” ? 35. Write on “Oxide Coating” of carbide tools. 36. What is “Compact” diamond tool material ? 37. Why C.I. is usually machined dry ? 38. Why gray C.I. does not need any lubrication during machining ? 39. List the additives added to water to improve its properties as a cutting fluid. 40. What is “Coronite” cutting tool material ? 41. List the major elements in cast-cobalt tools. 42. List the advantages of Cermets. 43. Which is the hardest known material, next to diamond ? 44. Write about the use of H.S.S. in the form of inserts and also about coated H.S.S. tools. 444 A Textbook of Production Technology Note :– Till recently, H.S.S. tools have been used as solid tools. However, of late, there has been an increasing tendency to use H.S.S. in the form of inserts just like inserts of WC, Al2O3, Sialon etc. This has resulted due to the high cost of H.S.S. tool material. These inserts are attached to the tool shank (Carbon or low alloy steel) by clamping or by brazing (See Fig. 7.1), or welded to it. For example, H.S.S. cutting ends of drills are welded to the steel shank by ‘‘Friction Welding’’ (see Chapter 5 Art. 5.5 (3)). Similarly, in band saws, narrow H.S.S. strips are welded to the steel bands by Electron Beam Welding (EBW). Coated H.S.S. tools :– Just like coated carbides, coated H.S.S. tools have recently been introduced in the market. The cutting tool (in the form of insert) is given a thin coating (2 to 6 μm thick) of a refractory carbide or nitride, for exaple TiC, TiN, Hafnium nitride and Alumina (Al2O3). Cost of coated tool = 2 to 5 times the cost of uncoated tool. However, life of coated tool = 5 to 10 times the life of uncoated tool. Coated tools perform much better while cutting general material as well as hard to machine alloys such as Cr – Mo steels. 45. Write about whisker reinforced tool materials. Whisker reinforced tool materials, Kyon : The preformance of cutting tool materials has been further enhanced by the development of whisker reinforced cutting tool materials (composite Materials). Whiskers have been used as reinforcing fibres in composite cutting tool materials. For example, SiC whiskers are used as reinforcement in Al2O3 matrix (See Chapter 12). The resultant material is wear resistant and very tough. The material is used in the form of inserts. This cutting tool material is superior to coated carbides, because, it is resistant to chipping. Cutting speed = 2 to 3 times that for plain carbide tools. Such cutting tool materials are suitable for intermittent cutting and machining of Ni-based alloys. 46. Why most of the tool-steels contain 2 or three elements ? 47. Write the benefits of "Coronite" cutting tool material. 48. How is the "Coronite" cutting tool material produced ? 49. What is a "Substrate"? 50. How is the "Compact" diamond cutting tool material produced? 51. What are "Borazon" and "Elbor" cutting tool materials? 52. Sketch the various shapes of inserts used in metal cutting tool industry. 53. What are "Sulphonated Oils"? 54. Write the benefit of adding chlorine to a cutting fluid. 55. Can Brass be machined without a cutting fluid? 56. How Paraffin helps in the machining of Aluminium? 57. Discuss the four types of cutting oils used in a machine shop. 58. What is the main drawback of using carbon tetrachloride and chloroform as cutting fluid? 59. Write the drawbacks of "Mist method" of applying coolants. 60. Discuss the effect of coolants on Cutting Variables.  Chapter 8 Machine Tools 8.0. GENERAL The readers should understand the difference between a machine and a machine tool. A machine is or device which converts some form of input into output, e.g., Air coolers, Air conditioners and refrigerators etc., where the input is electric energy and the output is the cooling effect. On the other hand, machine tools are machines that produce the various articles. Thus, whereas, all the machine tools are machines, but all the machines are not machine tools. All the machine tools used in manufacturing can be categorized into two groups :- 1. Metal Forming Machine tools. 2. Metal Cutting Machine tools. Metal forming machine tools produce the various articles by displacing material from one place of workprice to another place by its plastic deformation (See Chapter 4 on “Mechanical Working of Metals”). A machine tool is a power driven machine for making articles of a given shape, size and accuracy (according to the blueprints) by removing metal from workpieces in the form of chips. Machine tools are factory equipment for producing machines, instruments and tools of all kinds. So, it can be said that the machine tool is the mother of all machines. Hence, the size of a country's stock of machine tools and their technical quality and condition largely characterizes its industrial potential. Most machine tools perform the following four functions: 1. Hold the job. 2. Hold the cutting tools. 3. Move one or both of these (rotary motion or reciprocating motion). 4. Provide a feeding motion for one of these. Classification of Machine Tools. Machine tools can be classified in various ways. From the point of view of their field of application, machine tools are classified as: 1. General Purpose Machine Tools : General purpose or universal machine tools are used for performing a great variety of machining operations on a wide range of workpieces. These are employed chiefly in piece and small-lot production and for repair work. Machine tools used for a particularly wide range of work are known as multipurpose machine tools. Especially versatile machine tools are also called “Omniversal’’. General purpose machine tools include : plain turning lathes, turret lathes, milling machines, drilling machines, grinding machine etc. 445 446 A Textbook of Production Technology 2. Single Purpose Machine Tools : These machine tools are designed to perform a single definite machining operation, e.g., broaching, thread cutting, gear shaping and hobbing machines, machines for machining pistons, crank shafts, camshafts and for turning the cam contours on camshafts etc. 3. Limited Purpose Machine Tools : These machine tools are capable of a narrow range of operations on a wide variety of workpieces, e.g., automatic cutting off machines. 4. Production Machine Tools : These are mainly used in batch and mass production and feature high power and rigidity. These machine tools include : multi-tool lathes, single-and multi- spindle automatics, and semi- automatic lathes, plunge-cut cylindrical grinders, centreless, planer- type milling machines, thread rolling machines for tap production, numerically controlled machine tools etc. 5. Specialized Machine tools : These are used for machining articles similar in shape but different in size. This group includes unit built machine tools. These machine tools allow the machining of several surfaces in different planes. Their advantage is that they are readily changed over from one job to another. This is done by mounting additional unit heads, positioning them at an angle to a horizontal or vertical plane or other-wise. These machines are used mainly in large-lot production. 6. Special Machine Tools : These machine tools are designed and manufactured individually and are intended for performing a certain definite operation in machining a certain definite workpiece. These machine tools include : machines for sharpening round threading dies, for grinding relief surfaces at the chamfer of round threading dies, for marking round threading dies and shank-type tools, for threading by die taps, for grinding flutes on taps and reamers, tap chamfers, flutes on twist drills etc. These machines find applications in large-lot and mass production. The general purpose machine tools have the following characteristics: (i) Usually less initial investment in equipment. (ii) Greater machine flexibility. (iii) Fewer machines may be required. (iv) Less maintenance cost. (v) Less set up and debugging time. (vi) Less danger of obsolescence. The special purpose machines have the following characteristics: (i) Uniform product flow. (ii) Reduced in-process inventory. (iii) Reduced manpower requirements. (iv) Reduced factory floor space. (v) Higher output. (vi) Higher product quality. (vii) Reduced inspection cost. (viii) Reduced operator skill requirements. According to Accuracy the Machine Tools are Divided into Five Classes : (a) Normal Accuracy : Machine tools of normal or standard accuracy include the majority of the general purpose machine tools. (b) Higher Accuracy : Machine tools of higher accuracy are manufactured to the same drawings as the normal accuracy models, except that higher requirements are made to the accuracy with which the critical parts are manufactured, as well as to the quality of assembly and adjustments. (c) Precision : Machine tools have certain parts that have been specially designed with the aim of maintaining the high accuracy standards. In addition, narrow tolerances are Machine Tools 447 stipulated for the machining of all the parts, as well as for their assembly and adjustment as a whole. (d) High Precision : Machine tools are manufactured according to even more rigorous accuracy requirements than “Precision” class machine tools. (e) Super-high Precision : Master machine tools are intended for machining the parts which determine the accuracy of machine tools belonging to “precision” and “high precision” class. To ensure the required accuracy of their operation, the machine tools of the last three classes are to be installed in special constant-temperature rooms, with automatically controlled (Constant) temperature and humidity. According to Weight, machine tools can be classified as :- Light weight (upto 1 tonne), medium-weight (upto 10 tonnes), and heavy-weight (over 10 tonnes). The last group can be further divided into subgroups as: Large size (10 to 30 tonnes), Heavy (30 to 100 tonnes) and Extra-heavy (over 100 tonnes). According to the type of processing operations they perform or the tools they employ, all machines can be divided into nine groups, as under:– 1. Lathes : Engine and facing lathes, cutting-off lathes, multiple-tool lathes, Turret lathes, Automatics and Semi-automatics (Single spindle, multiple-spindle). Vertical turning and boring mills, and specialized machine tools. Vertical turning and boring mills are used to machine blanks of large diameter and relatively small height. Most of the larger machines (some can accommodate work 25 m in diameter) are called vertical boring mills and the smaller models are usually known as vertical turret lathes. 2. Drilling and Boring Machines : Upright drill presses, Semi-automatic single-spindle drilling machines, Semi-automatic multiple-spindle drilling machines, Jig borers, Radial drills, Boring machines, Precision boring machines, and horizontal drilling machines. 3. Planers, Shapers, Slotters and Broaching Machines : Open side Planers, Double-housing Planers, Shapers, Slotters, Horizontal broaching machines, Vertical broaching machines. 4. Milling Machines : Horizontal knee-type milling machines, Vertical knee-type milling machines, Tracer controlled milling and engraving machines, Continuous milling machines, Vertical- spindle compound-table milling machine, Fixed-bed and planer-type milling machines, and Ram- head milling machines. 5. Grinding and Micro-finishing Machines : Cylindrical grinders, Internal grinders, Snagging grinders, Specialized grinders, Tool and cutter grinders, Surface grinders, and Micro-finishing machines. 6. Gear and Thread Cutting Machines : Shapers and Planers for Spur and Helical gears, Bevel gear generators, Hobbers for Spur and helical gears and splined shafts, Worm Wheel and Worm cutting machines, Gear tooth Chamfering machines, Thread-milling machines, Gear finishing machines, Gear and thread grinders. 7. Combination Machine Tools : General purpose machines, Semi-automatic machines and Automatic machines. 8. Cutting-off Machines : Cutting off lathes, Abrasive cutting machines, Circular-saw friction cutting machines, Straightening and cutting off machines, Bond saw cutting-off machines, Circular Cold sawing machines, and Power hacksawing machines. 9. Miscellaneous Machine Tools : Coupling and pipe thread cutting machines, Saw-cutting machines, Centreless bar turning and straightening machine, Tool Testing machines, Dividing machines, and Balancing machines. Motions in Machine Tools : To obtain a finished workpiece on a machine tool, certain co- ordinated motions must be imparted to the work and the cutting tool. These motions are of two 448 A Textbook of Production Technology types : Primary or working motions and auxiliary motions. Primary motions consist of principal or cutting motions and feed motions. They serve the purpose of removing metal from the workpiece. The speed of the principal motion depends on the optimum cutting speed, while the speed of the feed motion depends on the required degree of surface finish. Auxiliary motions help in the completion of the machining process and include such motions as : handling and clamping the work in the machine, advance and withdrawl of the cutting tool, engagement and disengagement of working motions, and changing their speeds etc. Working motions are power driven. However, certain small machines have hand feeds. Auxiliary motions may be either hand or power operated. On automatic machine tools, practically all auxiliary motions are automated. Principal motions are of three types : rotary, reciprocating or a combination of these. This motion can be imparted either to the workpiece or to the tool. Rotary motion to work : Lathes Rotary motion to tool : Milling, grinding and drilling machines. Both simultaneously : In drilling small diameter holes. Reciprocating motion of tool : Shapers, slotters, broaching machines etc. Reciprocating motion of work : Planers Feed motions may be : Continuous, intermittent or compound. Continuous feed : lathes, milling, drilling machines etc. Intermittent feed : Shapers, planers etc. Compound feed : Gear cutter for cutting helical geacs. Combined feed : Cylindrical grinders. Methods of Machining a Shape 1. Forming. Here the shape of the tool is the finished shape of the workpiece and the tool is called “Form tool” to finish the job, all that is necessary, in addition to the relative movement required to produce the chip (primary motion) is to feed (plunge) the tool in depth, e.g., turning as shown in Fig. 8.1(a). Other examples are : drilling, plunge cut method of cylindrical grinding and plunge cut method of thread grinding. 2. Generating. Here, the required shape of the workpiece is obtained by combining several motions that not only accomplish the chip forming process (primary action) but also move the point of engagement (of the job and the tool) along the surface (called as feed motion), e.g., in cylindrical turning on a centre lathe, Fig. 8.1(b), the tool is set to cut a certain depth of cut and then traverses along the job to produce the chips. Other examples are : shaping and planning a flat, surface grinding, peripheral milling and so on. Workpiece Head stock Spindle Workpiece Feed Depth of cut Tool Tool (Form tool) Direction of feed Infeed (a) Forming (b) Generating Fig. 8.1. Producing a Shape. Machine Tools 449 3. Combined forming and generating: Thread cutting with a single point tool and hobbing etc. When discussing the various machine tools, the various motions of the tools, the various motions of the tool/workpiece will be represented as given below: (i) ReciprocatingMotion : Right : ; Left : Combined Right-Left :  Up : ; Down :  Combined Up-Down :  In ; Out :  (ii) Rotary Motion : About, Horizontal Axis : ; Vertical Axis : 8.1. LATHE MACHINES A lathe is one of the oldest and perhaps most important machine tools ever developed. The job to be machined is rotated (turned) and the cutting tool is moved relative to the job. That is why, the lathes are also called as “Turning machines”. If the tool moves parallel to the axis of rotation of the workpiece, cylindrical surface is produced, while, if it moves perpendicular to this axis, it produces a flat surface. A lathe was basically developed to machine cylindrical surfaces. But many other operations can also be performed on lathes, for example, facing, parting, necking, knurling, taper turning, thread cutting and forming etc., (Fig. 8.2). We also can perform operations of other machine tools on a lathe, for example, drilling, reaming, milling and grinding operations etc. No wonder, a lathe is called “the father of the entire machine tool family”. About one half of machine tools operating in engineering plants are of the lathe group. External Parting Tapering Knurling Threading Necking Turning Facing Turning or Radius Bevel Grooving Internal Threading Threading Threading Right Left-Cut Tool Tool Curved- Cut Tool Tool Knurling Form Cut-off Cutting or Right-cut Round-Nose Tool Tools or Parting Tool Side-Facing Tool Square Tool Left-cut Nose Side-Facing or Tool End-Cutting Tool Fig. 8.2. Common Lathe Operations. On the basis of their purpose, design, number of tools accommodated, degree of mechanisation and other factors, lathe-type machine tools may be classified as : 1. Limited or low-production Machines. The lathes included in this category are : engine lathe (centre lathe), bench lathe, tool room lathe and speed lathe. 2. Medium-production Machines. Turret lathes and duplicating (or tracer controlled) lathes. 3. High-production Machines. Semi automatic and automatic lathes. 450 A Textbook of Production Technology The construction and principle of lathe-type machine tools will be illustrated by the example of the most common representative of this class-the engine lathe. 8.1.1. Engine Lathe. It is so called because the first of this type of lathe was driven by a steam engine. It is also called “Centre lathe”, because, it has two centres between which the job can be held and rotated. A very high percentage of all lathe work is turned between centres. The main parts of a centre lathe are : Bed, Head stock, Tail stock, Carriage and the Electric drive, (Fig. 8.3). Chuck (face plate/collet) Live Centre Dead Centre Tail Stock Spindle Head Stock Tool Post Cross Slide Bed Saddle Lead Screw Carriage Feed Rod Apron Legs Fig. 8.3. Block Diagram of an Engine Lathe. 1. Bed. The bed is the base or foundation of the lathe. It is a massive (heavy) and rigid casting made in one piece to resist deflection and vibrations. It holds or supports all other parts, that is, head stock, tailstock and carriage etc. The top of the bed is planned to form “guides” or “ways”. Ways are accurate rails which support carriage and the tailstock. More expensive lathes have a combination of V ways and flat ways. Less expensive lathes have flat ways. Directly under the front way on the bed is a rack. A pinion gear meshes with the rack for moving the carriage when the handwheel is turned. The bed is usually fastened to steel legs so that the lathe can be bolted to the shop floor. 2. Head stock. The headstock assemebly is permanently fastened to the left hand end of the lathe. It serves to support the first operative unit of the lathe, that is, the spindle. The spindle revolves in bearings, one at each end of the headstock. The spindle is rotated by a combination of gears and cone pulleys or by gears alone. Present day lathes have individual motor drives and most of them have geared headstocks. The steel spindle is hollow to take long bar stock. The spindle has a definite taper at the front end for holding centres and other tools having a tapered shank. The hole through the spindle makes it possible to use a knockout bar to remove such tools. Work- holding attachments such as driving plate, face plate or various types of chucks may be mounted on the threaded spindle nose. Some type of work may also be held in a collet which is inserted into the hollow headstock spindle. A taper sleeve fits into the taper spindle hole. The headstock or live centre fits into the sleeve. This centre is called live centre because it turns with the work. The Machine Tools 451 centre is a tapered metal part with a pointed end. This supports the end of the workpiece as it turns between the centres. All centre points have a 60-degree included angle. 3. Tail stock. Tail stock is on the other end of the bed from the head stock. Its chief function is to hold the dead centre so that long workpieces (L/D > 4) can be supported between centres. It can be moved along the bed and clamped to the bed at the various desired locations to suit the length of the workpiece. Tailstock consists of two main parts. The lower part rests directly on the bed ways, and the upper part rests on the lower part. Adjusting screws hold the two parts together. The upper casting can be moved toward or away from the operator to offset the tailstock for taper turning and to re- align the tailstock centre for straight turning. The body of the tailstock has a bore for the hollow cylindrical sliding member, known as a “quill”. This quill is sometimes called as “tailstock spindle” even though it can not rotate. The quill moves in and out of the tailstock bore when the tailstock handwheel is turned. Once set, the quill may be clamped to remain in a desired position. The quill has a taper hole into which the dead centre is fitted. Drills, reamers, taps and other end cutting tools are held and fed to the work piece by the quill, the shanks of the tools being held in the tapered hole of the quill. 4. Carriage. In between the headstock and the tailstock is the carriage. It is movable on the bed ways and its purpose is to hold the cutting tool and to impart to it either longitudinal or cross feed. It has five major parts : (a) Saddle. The base of the carriage is the saddle which slides along the ways of the lathe bed. (b) Cross-Slide. The cross-slide is mounted on the saddle. It provides cutting tool motion which is perpendicular to the centre line of the lathe itself. The cross-feed movement may be controlled by manual (with cross slide handle) or by power feed. (c) Compound Rest. It is mounted on top of the cross-slide. The compound rest has a graduated base and can be swivelled around a vertical axis. In this way, its slide can be set at any angle with the axis of the workpiece. It can be clamped to remain at any angular setting. The range of compound rest is only limited and is used for obtaining angular cuts and short tapers, as well as convenient positioning of the tool to the work. Both the cross slide and the compound rest screws are equipped with micrometer collars. These are used in making accurate adjustments when turning workpieces to close measurements, and when cutting screw threads. There is no power feed for the compound rest. (d) Tool post. The tool post is mounted on the compound rest and slides in a T-slot. Cutting tool/tool holder is firmly held in it. The tool can be swivelled as well as tilted by means of a rocker and a concave ring collar, (Fig. 8.4). (e) Apron. The apron is secured underneath the saddle and hangs over the front of the bed. It contains the gears, clutches, and levers for operating the carriage by hand and power feeds. The apron hand wheel can be turned to move the carriage longitudinally by hand. This hand wheel is attached to a pinion that meshes with the rack under the front of the bed. The apron also contains friction clutches for automatic feeds and a splitnut or halfnut. The split nut can be closed over the lead screw threads and is used only when cutting screw threads. 8.1.2.Drive. The centre lathe has primarily two motions : the primary cutting motion and the feed motion. These motions are accomplished by means of corresponding drives, which are systems of mechanisms for transmitting power from its source (the electric motor) to the operative units of the lathe, that is, to the spindle for primary cutting motion (main drive) and to the carriage for feed motion (feed drive). 452 A Textbook of Production Technology Tool Post Screw Tool Holder & Tool Clamped ‘‘Short’’ Job Tool Post Tool Holder se er Ba Cutting Tool on Rock ollar Centre Line C Ring Compound Rest T-Slot Fig. 8.4. Tool Post. 1. Main Drive. The function of main drive is to drive the spindle and to change its speed so as to obtain the most expedient cutting speeds. Power for driving the spindle is provided by an electric motor. Two general types of main drives are used on lathes : Stepped drive and Step -less drive. In the first case, the maximum, minimum and a series of intermediate spindle speeds in definite steps are available. In the step-less system, or the infinitely variable spindle speed system, any spindle speed from the minimum to the maximum can be obtained. This feature allows the most suitable cutting speeds for each workpiece diameter to be set up, thereby maintaining the specified surface finish without decreasing the rate of production. Handle to engage Back Gear Back Gear Eccentric moves C & D D in when handle is turned C Spindle E A B (Keyed to Spindle) Cone Pulley (Free on spindle) Fig. 8.5. Step-cone Pulley with Back Gear Arrangement. Stepped spindle speed variation can be achieved in two ways : (a) Cone Pulley System. In the smaller lathes, we have the cone or step pulley arrangement. The power flows from the motor in the base to the cone pulley attached to the spindle, by means of Machine Tools 453 a belt. The spindle speed is changed by moving the belt to different positions on the step pulley. To obtain slower speeds and more power, the back gears are used. To understand how the back gears operate, (see Fig. 8.5). The gear B called the bull gear is fastened to the spindle. The spindle is a loose fit over a sleeve. The step pulley is firmly secured to the sleeve. The small end of the pulley has a pinion A attached to it. With the above arrangement, the bull gear can never turn free of the spindle and the pinion A always turns when the pulley turns. The step pulley and the pinion A are connected with the bull gear by a sliding pin E called the bullgear lockpin. At the back of the headstock are two gears C and D, called the back gears, mounted on the same shaft (an eccentric one). They are spaced to line up or mesh with the bull gear B and the pinion A. To make the back gears mesh with bull gear B and the pinion A, the back gear handle is pulled forward. When the back gears are not in mesh and the bull gear lock-pin is in place, we will get a direct drive. The speed of the spindle will then be the same as that of the cone. If a reduction in speed is desired, the bull gear lock-pin is disengaged and the back gear lever is pulled forward. Now the power from the step pulley is delivered to the bull gear through the pinion A and the back gears, which turns the spindle. So, there will be a direct as well as a back-gear speed for every step on the cone pulley. The cone pulley drive is very simple, and cheap in design, but it occupies lot of space and the number of speed steps are limited. The flat belt drive is not positive and shifting of belt from one step to the other takes time affecting the rate of production. (b) All Geared Drive. In this arrangement, we have a completely geared headstock with a combination of shifting levers. The power from the motor is delivered to the spindle through a belt drive and speed gear box. A geared headstock has got the following advantages : (i) It is more efficient and compact than the cone pulley drive. (ii) Possibility of transmitting high power. (iii) The available power on the spindle remains almost constant for the different speeds A Z1 5 I Z9 Z3 Z6 Spindle Z2 II II Z Z4 5 Belt drive B Z10 C III III Z7 Z8 Motor Fig. 8.6. Geared Main Drive. Fig. 8.6 shows a simple geared headstock. Power is transmitted from the motor through a belt drive to shaft I-I. This shaft carries three cluster gears Z1, Z2 and Z3. The cluster gear can be 454 A Textbook of Production Technology shifted with help of lever A, on key 5 along the shaft I-I, so that its three rims mesh in turn with the gears Z4, Z5 and Z6 which are rigidly mounted on sleeve B. This sleeve is freely mounted on the lathe spindle II-II. The gears Z9 and Z10, mounted directly on the spindle are constantly in mesh with gears Z7 and Z8 of the countershaft III-III. When the clutch C is shifted into engagement to the left, countershaft III-III is disengaged. The spindle will obtain one of the three speeds- n1, n2 and n3 – depending upon the position of the cluster gear. If n1 = Speed of the shaft I – I, then Z1 Z2 Z3 n1  n0  , n2  n0  , and n  n . Z4 Z5 3 0 Z6 When the clutch C is shifted to the right, the counter shaft is also included in the gear train. Its gearing ratio will be, Z10 Z7  Z8 Z9 With this, the spindle obtains three more speeds. Therefore, the countershaft enables additional speeds to be obtained, in the present case 6 spindle speeds can be obtained. Geared headstocks are commonly designed for 3, 4, 6, 8, 12, 16 and 24 spindle speeds. In properly designed, geared head stock, the values of the spindle speeds in the available range vary in a geometrical progression. The constant ratio has one of the following values : 1.06, 1.12, 1.26, 1.41, 1.58 and 2. The less the constant ratio, the less the difference will be between successive speeds and the more exactly the required speed can be approximated. In geometrical progression, the various spindle speeds will be given as : n1 ,  n1 , 2 n1 , n1 ,... z 1. n1 where n1  nmin.; Z 1. n1  nmax and  = step ratio and Z = number of spindle speed steps nmax  Z 1  nmin 1 ⎛n ⎞ Z 1    ⎜ max ⎟ ⎜n ⎟ ⎝ min ⎠ 1 f 2 4 3 5 6 Fig. 8.7. Feed Drive of Engine Lathe. Machine Tools 455 The “Step-less drive” is discussed in volume II (A Textbook of Production Engineering). 2. Feed Drive. The feed drive serves to transmit power from the spindle to the second operative unit of the lathe, that is, the carriage. It, thereby, converts the rotary motion of the spindle into linear motion of the carriage. It also enables the specific rate of feed as well as its direction to be selected. The feed drive not only provides the different feeds required in machining but also provides a wide range of pitches for thread cutting operations. The feed drive of an engine lathe consists of : the reversing mechanism (1), change gear quadrant (2), quick-change gear box (3), leadscrew (4), and feed rod (5), and apron (6), (Fig. 8.7). (i) Reversing mechanism. Its function is to reverse the direction or movement of the lead screw/feed rod. The most frequently used reversing mechanism for engine lathe comprises : four spur gears with consecutive engagement of the bracket holding the reverse gears, (Fig. 8.8). The gear Z1 is mounted on the spindle I and the gear Z4 - on shaft II which is usually a shaft of the change gear quadrant. Gears Z2 and Z3 are freely mounted on Z1 I studs III and IV of the bracket. P is the feed reverse lever. The bracket is mounted on shaft II and the feed reverse lever can be moved to three positions 1, 0, or 2. Gears Z2, Z3 and Z4 are constantly in mesh with each other. If the lever is shifted to position 1, gears Z1, Z2, Z3 and Z4 are Z2 IV put into consecutive mesh and with the automatic feed engaged, the shaft II begins to rotate in a direction opposite III Z3 to that of the spindle. With lever in position 2, only gears Z1, Z3 and Z4 will be in mesh and shaft II will rotate in the same direction as the spindle. With lever in the centre II position O, both gears Z2 and Z3 are pulled out of mesh with gear Z1 and no motion is transmitted from the spindle Z4 to shaft II. The reversing bracket is thus disengaged. 2 P (ii) The change gear quadrant. The change gear 0 quadrant of an engine lathe consists of a set of change 1 gears and the device known as the quadrant proper. It serves to set up the feed drive to different speeds of the lead screw and feed rod. The quadrant also transmits Fig. 8.8. Feed Reversing Mechanism. motion to the quick-change gear box or to leadscrew. Z1 Z1 D I Z2 Z2 Z3 Z3 Z4 E D E II Z4 Fig. 8.9. Change Gear Quadrant. 456 A Textbook of Production Technology A frequently used change-gear quadrant for an engine lathe is shown in Fig. 8.9. It is a two- pair arrangement. There can also be single-pair or three-pair arrangement. The two pair arrangement has four gears Z1, Z2, Z3 and Z4. Gear Z1 is mounted on shaft I of the reversing mechanism and the gear Z4 - on shaft II of the quick- change gear box. The intermediate gears Z2 and Z3 are keyed on a sleeve which, in turn, is mounted freely on stud E. The stud E can be adjusted and clamped as required along a straight slot in quadrant D. The latter is mounted on shaft II and can turn freely in reference to this shaft. The gearing ratio of the change gear quadrant is Z1 Z3  Z2 Z4 For new gearing ratios with new sets of change gears, the centre distances are changed as required by adjusting stud E along the straight slot and swivelling the quadrant about shaft II along the circular slot. (iii) Quick-change Gear Box. The quick-change gear box is located on the front of the lathe, directly below the headstock assembly. It allows a variety of feeds to be easily the rapidly selected by merely shifting the corresponding levers. This gear box contains a number of different- size gears, which provides a means to change (1) the rate of feed and (2) the ratio between revolutions of the spindle and the movement of the carriage for thread cutting. An index chart or plate is attached to the front of the gear box. It tells the position of the levers to use to obtain the desired feed or threads per cm. The most widely used design of quick- change gear box is the tumbler gear, (Fig. 8.10). 8 7 7 6 5 4 3 2 1 S2 Stop Intermediate Pin Gear Tumbler Gear S1 Swinging and Sliding Lever Fig. 8.10. Norton type Tumbler-gear quick-change Gear box. It comprises a cone of gears 1 to 8 mounted on shaft S2. The tumbler gear can slide on shaft S1. It can mesh with any gear on shaft S2 through an intermediate gear which is located on a swinging and sliding lever so that it can engage gears 1 to 8 of different diameters, on shaft S2. The lever can be fixed in any desired ratio position with the help of a stop pin. The drive is usually from the driving shaft S1 to the driven shaft S2. (iv) Apron. The function of the apron has already been explained. Power comes to the apron for feeding the carriage (from lathe spindle) through feed reversing mechanism, change- gear quadrant or quick-change gear box and the lead screw/feed rod. The apron encloses clutches, systems of Machine Tools 457 spur and worm gearing, Fig. 8.11. Its feed mechanisms convert rotary motion of the lead screw or feed rod into linear motion (feed) of the carriage. 11 7 8 6 10 1 9 2 5 4 3 Fig. 8.11. Lathe Apron Mechanism. The lead screw 1 is only used to move the carriage for cutting threads. For other lathe operations, feed rod 2 is used so as to save the lead-screw. Worm 3 with its key slides along the feed rod which has a key way along its whole length. When the feed rod rotates, the worm turns. The worm meshes with a worm wheel 4 and the motion is transmitted to spur gear 5 mounted on the same shaft. When the automatic feed knob is turned to the right, the clutch engages another gear (gear 5 meshes with gear 6) which transmits rotation to pinion 7 since the gear 6 and pinion 7 are mounted on the same shaft. Pinion 7 runs along the stationary rack 8, fastened to the bed, thus feeding the carriage 11, rigidly secured to the apron, along the bed. Thus, the rotary motion of the feed rod is converted into linear motion of the carriage. Other gearing arrangements in the apron convert the rotary motion into cross feed of the cutting tool by rotation of the cross-feed screw in the cross slide. For this, the feed-change lever on the apron is moved to the “down” position and the automatic feed knob is turned to the right. For thread cutting, the feed rod is disengaged from the apron gearing by placing the feed- change lever on the apron in the centre or neutral position. The lead screw feeds the carriage lengthwise through two half nuts 9 mounted on the rear wall of the apron. Upon engaging lever 10, the half nuts are closed and they engage the rotating lead screw. Then the half nuts and, with them, the carriage are fed along the bed. Disengagement of lever 10 spreads the half nuts to release the lead screw and the carriage stops. 8.1.3. Work-holding devices. The common work holding devices used for a centre lathe are discussed below : 1. Centres. A very high percentage of all lathe work is turned between centres. Long work pieces (shafts and axles) with L/D ratio >4, are turned length wise between centres. The work- piece, in whose ends centre holes have been previously drilled, and on which a driving dog has been clamped on one end, is mounted between the headstock (live) and tailstock (dead) centres. The workpiece is rotated by driving the lathe dog. The lathe dog is driven with the help of a drive plate. The drive plate is mounted on the threaded nose of the spindle. It has one open slot and three closed slots. A pin is inserted in the open slot which engages with the tail of the lathe dog, (Fig. 8.12 a). When the spindle rotates, the workpiece will rotate through this pin and the dog. If the dog has a bent tail, it will fit into one of the slots, (Fig. 8.12 b). 458 A Textbook of Production Technology Dog Plate Dog Pin Dog Job Drive Plate (a) (b) Fig. 8.12. Turning Between Centres. The two types of the lathe dogs are shown in Fig. 8.13. Fig. 8.13. Lathe Dogs. Since the tailstock centre does not rotate, it acts as a bearing. It, therefore, must be lubricated and excessive pressure should be avoided. To avoid this process of lubrication in production work, a revolving centre is used. (a) Three designs of lathe centres are illustrated in Fig. 8.14. An ordinary centre lathe is shown in Fig. 8.14 a. A ball- ended centre (Fig. 8.14 b) is used when the tail stock is set over to turn a taper. A half centre (Fig. 8.14 c) enables external straight turning to be combined (b) with facing the end of the workpiece. The wear resistance of the centres can be substantially increased by tipping the point with cemented carbide or metallizing it with a hard-facing alloy. (c) 2. Chucks. Work piece with a length L < 4D may be clamped in a chuck without the need of additional support of the free end. Three- Fig. 8.14. Types of Lathe and four-jaw chucks, screwed on the spindle nose, are employed for this Centres. purpose. The chuck may be adapted to the screwed nose spindle by using a back plate, whilst on the flanged spindle, it is bolted direct. Among the chucks used in a lathe, the most widespread are three- jaw universal self-centring chucks. These are used for holding symmetrical work as a rule. Here, the three jaws move together towards or away from the centre of the chuck. In the 4 -jaw chuck, each of the four jaws is moved by its own screw independent of the other jaws along radial slots of the chuck body. These chucks are used for holding complex and non-symmetrical work pieces. Pneumatically and hydraulically operated chucks are used to speed up and facilitate handling operations of certain parts in mass and large-lot production. 3. Collets. They are the most accurate of the chuck family. They are like three jaw chucks and are used primarily for bar stock or other sections upto about 63 mm, for example round, square, hexagonal etc. 4. Face Plate. A face plate is larger than a drive (dog) plate and is screwed on the spindle nose. It usually has four T-slots and a number of plain radial slots, (Fig. 8.15). It is highly efficient Machine Tools 459 in machining asymmetrical work or work of complex Counter-Weight and irregular shape which is inconvenient or even impossible to clamp in jaw-type chucks. The Work workpiece is clamped to the face plate with bolts Face Plate and straps. Sometimes, it is more convenient to mount an angle plate on the face plate and to clamp the work on the angle plate. 5. Mandrels. A mandrel is used to locate and held a work-piece with a central hole, such as, gear blanks, pulleys etc. A mandrel is a solid Angle Plate hardened bar, with centres and flats on each end. The mandrel is usually tapered so that the work Fig. 8.15. A face Plate. can be forced on it with a press fit and then Work removed after working. A mandrel is held between tapered centres and rotated with a lathe dog clamped on its flat, Fig. 8.16. The taper is about 0.005 mm per cm. length. 6. Steady Rests. When very long workpieces (L/D >10 or 12) or long slender workpieces of low rigidity are machined between Flat for Dog centres, steady rests are used to additionally (Both ends) support the workpiece and prevent it from bending Fig. 8.16. A Mandrel. due to the pressure of cut. There are two types of steady rests used on a lathe : (a) Fixed Steady Rest. This type of steady is fixed on both bed ways of the lathe, between the headstock and tailstock. The workpiece is supported by three adjustable jaws, (Fig. 8.17 a). Steady rests for high-velocity turning have ball or roller bearings contacting the rotating workpiece, the bearings being built into the jaws. This type of steady rest has the drawback that since the carriage can not pass it, the job will be turned in two stages by being reversed end for end after half its length has been machined. Work Locking pin Hinge Tool Cross slide Bed (a) Stedy rest (b) Follower rest Fig. 8.17. Steady Rests. (b) Follower or travelling Steady Rest. This steady is mounted on the saddle and moves together with the tool. It has two jaws which support the work opposite the tool, (Fig. 8.17b) This design does not have the drawback of fixed steady rest. 460 A Textbook of Production Technology 7. Milling Vise. Milling vise is mounted in place of the compound rest. The work is held in the vise and an end mill or stub-arbor mounted cutter is inserted in the lathe spindle. Work movement is limited. 8. Special Fixtures. When a job can not be held in one of the above discussed work holding devices, a fixture is designed, made and used for that job. 8.1.4. Lathe Operations. A large variety of operations can be performed on an engine lathe, as shown in Fig. 8.2. Turning is the operation to remove material from the outside diameter of a workpiece to obtain a finished surface. The finished surface may be of continuous diameter, stepped, tapered or contoured. The feed of the tool for turning operation is along the axis of the lathe, Facing is the operation of machining the end of a workpiece to make the end square with its own axis and that of the lathe. The tool moves perpendicular to the axis of the lathe. Reaming and Drilling. Drilling is the operation of making a hole in a work piece where none previously existed. Reaming is the operation of finishing the drilled hole. These operations are done on lathe by holding the drills and reamers in the tailstock quill. The shanks of these tools are held in the tapered hole. For smaller tools, drill chucks can be used to hold the tools, the drill chuck being held in the tapered hole of the quill by its shank. The job is held in a chuck and the tools are fed to the revolving workpiece by the quill by rotating the tailstock handle. Boring. Boring is the operation of enlarging the drilled hole. The workpiece is held in a chuck in the lathe spindle and the boring bar is mounted in the tool post. Boring is done by moving the carriage towards the headstock. Knurling. It is the operation of plastically displacing metal into a particular pattern for the purpose of creating a hand grip or roughened surface on a workpiece. The knurling tool is held in the tool post and is pressed against the surface of the workpiece by cross feed. Milling. For the milling operation, small milling cutters are held in the headstock and revolved while the work is clamped in a vise mounted over the top of the compound rest, instead of the tool post. The operation is used only for small work. Grinding. Cylindrical and internal grinding can be done on a lathe, with a tool-post grinder. This is a holder containing a spindle for mounting a grinding wheel and a motor for driving it. A small wheel is used for internal grinding and a larger wheel is used for external grinding. The tool post grinder is mounted on the compound rest in place of tool post. For external grinding, the workpiece is held in a chuck or between centres. For internal grinding, the workpiece is held in a chuck or mounted to the faceplate. Tool post grinder is used for simple jobs like grinding mandrels or reamers, truing chuck jaws, sharpening lathe centres and milling cutters etc. Taper turning. Tapered surfaces can be turned by employing one of the following methods: 1. Compound Rest Method. The compound rest is swivelled to the required angle, (Fig. 8.18 a). The angle is determined by the formula : D–d tan   2l where   Half taperangle. D = smaller diameter l = length of the taper After swivelling the compound rest to this angle about the vertical axis, it is clamped in position. The taper is turned by hand wheel by rotating the handle. The method can be employed for turning short internal and external tapers with a large angle of taper. The workpiece is commonly held in a chuck. Machine Tools 461 Chuck Axis of Lathe  D d  job d d l Fee D Too Form Feed Handle Tool Cross-slide Compound rest-slide (a) (b) 1 2 3 L l  4 D Bed  d h Feed Bed Feed 5 (c) (d ) Fig. 8.18. Taper Turning Methods. 2. With a Form tool. Short external tapers with various angles of taper can be turned with a form tool, Fig. 8.18 b, using cross feed. The width of the form tool slightly exceeds that of the taper being turned. The work is held in a chuck or clamped on a faceplate. 3. Setting over the Tail-stock. Long workpieces with a small angle of taper (not exceeding 8°) are usually turned by setting over the tailstock centre, (Fig. 8.18 c). Tailstock setting over or offset can be determined from the formula, L (D  d ) h mm 2l where, L = full length of the workpiece l = length to be taper turned 4. Taper turning attachment. Long tapered work is frequently turned with a taper turning attachment, Fig. 8.18 d. Bracket 1 is attached to the lathe bed. It carries guide bar 2 which can be turned to the required angle and clamped in place. Guide block or slide 3, linked to the lathe cross slide 5 by tie member 4, is free to slide along the guide bar. The cross slide is disengaged from the cross feed screw. Upon travel of the saddle along the bed ways, the guide block slides along the guide bar, and through the tie member, forces the cross slide with the cutting tool to travel parallel to the guide bar at the given angle and the work is turned to the specified taper. The angle of taper can be found out by the relation : Dd tan   2l In some taper turning attachments, the bracket carries divisions in mm, instead of in degrees. In such cases, the guide bar has to be swivelled through mm divisions, which can be found out as : 462 A Textbook of Production Technology Dd S  lg 2l where lg = Half of the total length of the guide bar or plate. Conicity or taper, T, is defined as the ratio of the difference in large and small diameters to the length of taper.  Dd T l In the case of compound rest method, T  2 tan  In the “Setting over tail stock” method, L T h , when the length of taper is less than length of workpiece