Handbook of Lubrication Episode 1 Part 14 doc

Extrusion Since the billet is pushed through the die (Figure 8), strength of the extruded product is immaterial and attainable reduction is limited only by the strength of the container and punch. In reverse or indirect extrusion the billet remains stationary in the container (Figure 8a), and the magnitude of friction plays no role; die pressure p e is a function of extrusion ratio R = A o /A l p e = σ f (a + b 1nR) (8) where a and b are constants for a given die geometry. In forward extrusion (Figure 8b), the stresses necessary to overcome friction add to the die pressure, often limiting the length of billet that can be extruded. When a lubricant is used, the die entry must be tapered to facilitate material flow along the die face. Alternatively, one can extrude without any lubricant whatsoever; the die has Volume II 323 FIGURE 7. Basic bar and tube drawing operations. (From Schey, J. A., Ed., Metal Deformation Processes: Friction and Lubrication, Marcel Dekker, New York, 1970. With permission.) FIGURE 8. Basic forms of extrusion. (From Schey, J. A., Ed., Metal Deformation Processes: Friction and lubrication, Marcel Dekker, New York, 1970. With permission.) 317-333 4/10/06 12:59 PM Page 323 Copyright © 1983 CRC Press LLC a flat face and the material, in seeking a minimum-energy path, shears at some angle determined by the extrusion ratio. Material behind the shear surface forms a dead-metal zone (Figure 8c). To minimize friction in reverse extrusion of tubes, the container is kept as short as possible and a short land is formed on the punch (Figure 8d). Lubricant between the workpiece and punch end face must be gradually metered out to protect the freshly formed, highly extended surfaces. The extrusion ratio and die pressures diminish as the punch diameter decreases. With a small diameter, however, the process changes to indentation and punch pressures can never be less than p = 3σ f . As with all inhomogeneous deformation, lubrication is relatively ineffective in reducing punch pressure; nevertheless it is still desirable to prevent metal pickup and punch wear. Aspecial case is hydrostatic extrusion in which the extrusion pressure is supplied by a high-pressure fluid; container friction is eliminated and die friction reduced but the cycle time is long. As in wire drawing, a high h/Lratio can lead to centerburst defects. In contrast to drawing, friction increases the pressure in the deformation zone, reduces secondary tensile stresses, and delays the onset of the defect. Sheet Metalworking Sheet metaiworking is always a secondary process on previously rolled flat products such as sheet, strip and plate. The first such operation is usually shearing (or slitting, blanking, or punching). Separation of adjacent metal parts occurs through highly localized plastic deformation followed by shear failure (Figure 9a) and seems unaffected by friction. Never- theless, lubrication is necessary to protect against rapid wear and die pickup. Many parts are formed by bending, and bending forces are affected by friction when the workpiece slides over some die element (Figure 9b). Friction becomes extremely important when shapes of three-dimensional geometry are formed through stretching, deep drawing, or their combination. In pure stretching the sheet is firmly clamped (Figures 10a and b). In the absence of friction, thinning is most severe and fracture occurs at the apex of the stretched part. With increasing friction, free thinning over the punch nose is hindered and the fracture point moves further down the side of the part. In deep drawing (Figure 11), blanks of large diameter-to-sheet thickness ratio would buckle (wrinkle) and must be kept flat by applying pressure through a blankholder. Frictional stresses increase the force required for deformation, and when the force exceeds the strength of the partially drawn product, fracture occurs. Friction must be minimized to reach the maximum possible draw. Cups of large depth-to-diameter ratio must be produced with several redrawing steps. When the cup wall is to be reduced substantially, the drawn cup is pushed through an ironing die, and wall thickness reduction takes place under high normal pressures and severe sliding, calling for a much heavier-duty lubricant. Friction on the punch surface is again beneficial. Many sheet metal parts in the automotive and appliance industries are of complex shapes produced by simultaneous stretching and drawing. Lubrication is essential to prevent die pickup and surface damage. To restrict free drawing-in of the sheet, a draw bead is incor- porated (Figure 10b); this imposes severe conditions on the lubricant. LUBRICATING MECHANISMS Lubricants in deformation processes have to survive under an extremely wide range of conditions. Interface pressures range from a fraction of flow stress σ f up to multiples of σ f (up to 4 GPa or 500 kpsi). Sliding velocities range from zero (at sticking friction) to 50 m/s (10,000 fpm) sometimes combined with approach velocities of up to 20 m/s (66 ft/sec). 324 CRC Handbook of Lubrication 317-333 4/10/06 12:59 PM Page 324 Copyright © 1983 CRC Press LLC Layer-lattice compounds such as MoS 2 and graphite are effective if deposited in a continous film. Their shear strength is also pressure-sensitive. In common with other solid films, they cannot protect surfaces if the lubricant film breaks down. Boundary Films Very thin films formed on the die or workpiece may prevent adhesion and also reduce friction. Extreme-pressure (EP) compounds rely on reactions that take place only if sufficient time is available at temperature and if substrate composition is favorable. Contact time under metalworking conditions is usually too brief, but if contact is repeated (as in cold heading, wire drawing, sheet metal working, and sometimes rolling), reaction is possible with the die. Boundary films form almost instantaneously and are among the most important lubricants for reactive metals, particularly aluminum, copper, and to a lesser extent, steel. Their shear strength is pressure- and temperature-dependent. Breakdown at elevated temperatures limits them to cold working. Full-Fluid Film Lubrication In this regime, tool and workpiece surfaces are separated by a liquid film of sufficient thickness to avoid asperity interaction. Plastohydrodynamic theory can account for the effects of process geometry, sliding speed, and lubricant properties in maintaining such a film. The pressure and temperature sensitivity of viscosity must be considered, together with the possibility of the lubricant becoming a polymer-like solid at higher pressures (Figure 12). Mixed Lubrication Inmost practical situations, only some portion of the total contact area is separated by a thick lubricant film. Other parts of the contact area are in boundary contact, making boundary or EPadditives a necessity in almost all metalworking fluids. Depending on process con- ditions and lubricant viscosity, the deformed surface may be smoother than before defor- mation. It may also be roughened by the formation of entrapped lubricant pockets. Surface Roughness Effects Although asperities piercing through the lubricant film can create adhesion problems, tool and workpiece surfaces need not always be very smooth. To avoid skidding in rolling, the roll surface is kept somewhat rough. Amoderately coarse, nondirectional (such as bead- blasted) surface finish is desirable in maintaining graphite or MoS 2 supply in hot forging. Asmooth, polished die surface is, however, desirable for liquid lubricants; any remaining roughness is preferably oriented in the direction of material flow. Moderate roughness of the workpiece surface helps carry liquid lubricants into the inter- face, especially if the roughness is perpendicular to the direction of feeding. If sliding is multidirectional, a random (bead-blasted) finish is preferable (as on automotive body sheets). Lubricating Regimes The range for various lubricating mechanisms is summarized in Figure 13. For an interface pressure of p = σ f , friction cannot attain a value higher than that corresponding to sticking. The presence of a solid film (other than metal) reduces the coefficient of friction to about 0.05 to 0.1, apparently irrespective of the nature of solid film. With a liquid, mixed lubri- cation is attained once velocity and viscosity combine to sustain the pressures required for plastic deformation. With increasing velocity and/or viscosity, the proportion of surface area lubricated by the fluid film increases and the coefficient of friction drops to typically 0.03 to 0.05. True plastohydrodynamic lubrication is rare. With increasing interface pressures, the apparent coefficient of friction drops even for Volume II 327 317-333 4/10/06 12:59 PM Page 327 Copyright © 1983 CRC Press LLC deformation makes the formulation of a single test technique virtually impossible. Standard laboratory bench tests (such as the four-ball test) occasionally give successful correlation with metal working practice; these cases are, however, exceptions and are perhaps fortuitous. Most attractive are tests in which friction balance leads to a readily measurable change in deformation. The ring compression test (Figure 4) is one which measures lubricant behavior under normal contact to simulate forging. Forward slip (Equation 5), which directly reflects friction balance in the roll gap gives similarly obvious results under rolling conditions. Asecond group of test measures interface shear strength. Thus, a sheet drawing test (Figure 14a) reveals the magnitude of friction under moderate pressures for light-duty sheet- metalworking lubricants. Higher interface pressures and testing for lubricant durability are possible using a hollow specimen with an annular end face pressed and rotated against a flat anvil (Figure 14b); reasonable correlation is found with severe cold working such as extrusion. In a third group of tests, the magnitude of friction is judged from the force required to perform deformation such as upsetting of a cylinder, extrusion, or wire drawing. Lubricant ranking is made simply by comparing the magnitudes of forces. No single test provides information for all conceivable metalworking conditions. For a broader evaluation at least three tests have to be performed, such as ring compression for normal approach, plane-strain compression for deformation with extensive sliding, and twist compression for lubricant starvation situations. Die and workpiece material, surface prep- aration and roughness, interface velocity, and entry zone geometry should be the same or as closely scaled as possible to the actual process. Testing forStaining Lubricants are sometimes left on the deformed workpiece surface for corrosion protection, and testing in typical industrial atmospheres is necessary. In other instances, the workpiece is subjected to subsequent operations, such as annealing, joining, etc. Testing for staining propensity can be done if air access is controlled to reproduce that typical of annealing in coils. Hot-Working Lubricants Typical hot-working temperatures are in excess of 400°C for aluminum and magnesium alloys, over 600°C for copper alloys, and over 900°C for steels and nickel-base alloys. Lubricants are limited to those resisting the workpiece (or interface) temperature, such as recommended in Table 1. Oxides formed during heating the workpiece can fulfill a useful parting function, provided they are ductile at the interface temperature. This condition is partially satisfied only by iron oxides, cuprous oxide, and refractory metal oxides. Other harder oxides (such as ZnO) are effective only when they can break up into a powdery form. Yet others (such as aluminum and titanium oxides) are hard, brittle, cannot follow surface extension at all, and do not protect once broken up. Glasses of proper viscosity (typically 20 Pa·sec at the mean of the die and workpiece surface temperatures) can act as true hydrodynamic lubricants. If the process geometry is favorable (such as in extrusion), a thick glass mat may gradually melt off to provide a continuous coating on the deformed product. Glass may be applied either as glass fiber or powder to the die or hot workpiece, or in the form of a slurry with a polymeric bonding agent to the workpiece prior to heating. In the latter case it may also protect from oxidation and other reactions during the heating period. Graphite is effective in forging steel or nickel-base alloys, if uniformly deposited on the die surfaces from an aqueous or sometimes oily base. Wetting a hot surface is difficult but special formulations (sometimes with polymeric binders) have been developed. Application Volume II 329 317-333 4/10/06 12:59 PM Page 329 Copyright © 1983 CRC Press LLC Volume II 331 Table 2 TYPICAL LUBRICANTS USED IN COLD WORKING 317-333 4/10/06 12:59 PM Page 331 Copyright © 1983 CRC Press LLC 332 CRC Handbook of Lubrication Table 2 (continued) TYPICAL LUBRICANTS USED IN COLD WORKING Note: a Chlorine is the most effective EP agent on stainless steel. b Chlorine is avoided for Ti. c Sulfur is avoided for Ni because of reaction and for Cu because of staining. d Magnesium alloys are usually worked warm (above 200°C). e Usually conducted hot. Hyphenation indicates that several components are used in lubricant. CL = chlorinated paraffin; EM = emulsion (the listed lubricants are emulsified and 1 to 20% dispersed in water); EP = “extreme pressure” compounds (containing S, Cl, and/or P); FA = fatty acids, alcohols, amines, and esters; FO = fats and fatty oils, e.g., palm oil, synthetic palm oil; GR = graphite: MO = mineral oil (viscosity in units of centistoke [ = mm 2 /sec] at 40°C); PH = phosphate surface conversion; PC = polymer coating; and SP = soap (powder, or dried aqueous solution, or as a component of an EM). 317-333 4/10/06 12:59 PM Page 332 Copyright © 1983 CRC Press LLC for copper-base alloys, and is gaining some acceptance for steel rolling. Chemical solutions (of rust preventatives, etc.) are often adequate when a protective oxide is present. The water fulfills the important function of heat extraction. Cold-Working Lubricants The relatively low temperatures attained during cold deformation permit use of a wide range of lubricants such as indicated in Table 2. Solid films are of particular value for severe deformation. Although soft metals have declined tin importance, tin on mild steel sheet is employed in the production of drawn and ironed tin cans. Polymeric films, interposed as a separate film or deposited on the workpiece surface, find growing but still limited application. Of greatest importance are surface con- version coatings such as produced by phosphating of steel. They present a strongly adhering film of sufficient porosity or surface detail to provide a mechanical key for the superimposed lubricant layer, typically a soap. Film attachment is further enhanced by reacting the soap with the phosphate film. Layer-lattice compounds are mostly used as additives to other lubricants to provide a last defense in case of lubricant breakdown. Oil-based lubricants represent a large proportion of the total used. The viscosity of natural or synthetic oils is chosen to give mixed-film or occasionally almost full-fluid film lubrication, but not so high to induce excessive surface roughening or to drop friction below an acceptable limit. Because of the impossibility of avoiding all asperity contact, lubricants always contain boundary additives: typically fatty acids, alcohols, esters, or natural fatty oils. When contact is repetitive, EP additives may also be useful, particularly for metals (stainless steel, titanium) on which fatty additives are ineffective. When conditions are unfavorable to developing hydrodynamic films, grease may be used. Whenever cooling is important, the oil is applied in a recirculating system. A flood of aqueous emulsions or dispersions is even more effective, but staining may be a problem (e.g., on Al or Mg). Removal of wear particles by filtration is an essential requirement in all recirculating systems. REFERENCES 1. Bastian, E. L. H., Metalworking Lubricants, McGraw-Hill, New York. 1951. 2. Schey, J. A., Ed., Tribology in Mctalworking: Friction, Lubrication and Wear. American Society of Metals, Metals Park. Ohio, 1983. 3. Tribology in Iron and Steel Works. Publ. No. 125, Iron and Steel Institute, London, 1969. 4. Rowe, G. W., Mech. Mach. Electr., 266, 20, 1972. 5. Schey, J, A., in Proc. Triboiogy Workshop Atlanta, F. F. Ling, Ed., National Science Foundation. Wash- ington, D.C., 1974,428. 6. Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977. 7. Wilson, W. R. D., in Mechanics of Sheet Metal Forming. Plenum Press, New York, 1978, 157. 8. Proc. 1st Int. Conf. Lubr. Challenges in Metalworking and Processing, IIT Research Institute, Chicago, 1978 9. Proc. 2nd Inst. Conf Lubr. Challenges in Metalworking and Processing. IIT Research Institute. Chicago, 1979 10. Schey, J. A., in Metal Forming Plasticity, Lippmann, H., Ed., Springer-Verlag, Berlin, 1979, 336. 11. Wilson, W. R. D., J. Appl. Metalworking. 1, 7, 1979. 12. Schey, J. A., in Proc. 4th Int. Conf. Prod. Eng., Japan Society of Precision Engineering, Tokyo, 1980, 102. 13. Kalpakjian, S. and Jain, S. C., Eds., Metalworking Lubrication. American Society of Mechanical En- gineers, New York, 1980. Volume II 333 317-333 4/10/06 12:59 PM Page 333 Copyright © 1983 CRC Press LLC METALREMOVAL Milton C. Shaw INTRODUCTION Agood deal of effort in the manufacture of hard goods is concerned with providing a desired shape and accuracy to machine parts and the removal of material represents one important way of doing this. The entire area of material removal may be divided into metal cutting in which relatively large chips of uniform geometry arc formed and abrasive proc- essing in which relatively small chips having a wide dispersion of geometries are produced. It has been estimated that about 10% of the gross national product are spent in material removal operations in the U.S. It is, therefore, important that such processes be understood and efficiently performed if productivity (effective use of labor and capital) is to be achieved. Since friction, wear, and lubrication play important roles in material removal operations, it is pertinent to consider these operations here. There are a wide variety of removal operations which use tools of different geometry and kinematic relationship between tool and work. Some of the more important operations include the following: 1.Turning to produce cylindrical surfaces 2.Milling to produce flat surfaces and surfaces of complex geometry 3.Drilling, boring, and reaming to produce round holes 4.Awide variety of grinding operations In addition to these, there are a host of more specialized operations designed to do a particular job more effectively or which are better suited to mass production. However, all removal operations involve tools that penetrate the work to peel off unwanted material. What goes on at the tip of these tools is essentially the same regardless of geometry or kinematics. CUTTING MECHANICS Orthogonal Machining Figure 1 is a photomicrograph of a partially formed chip. 1 This was produced by moving aworkpiece against a stationary two-dimensional tool, abruptly stopping the operation in midcut and then sectioning and metallographically polishing the “hip root”. The magnified view of the etched surface reveals a great deal concerning the action of a metal cutting tool when removing a chip. As the material approaches line AB there is essentially no plastic flow until AB is reached. At this point a sudden concentrated shear occurs and then the material proceeds upward along the tool face with essentially no further plastic deformation. In the case of Figure 1, the cutting edge was stationary and straight, extending perpendicular to the plane of the paper. This is called orthogonal cutting since the cutting edge is per- pendicular to resultant velocity vector (V) which is in the horizontal direction in Figure 1. Merchant 2 and Piispanen 3 first discussed orthogonal cutting in fundamental terms. Figure 2 is a diagrammatic representation of Figure 1. Line AB is the trace of the surface on which the concentrated shearing action occurs and is called the shear plane. The angle the shear plane makes with velocity vector (V) is the shear angle (φ) while the angle between the face of the tool (rake face) and the normal to the velocity vector is called the rake angle (α). Also shown in Figure-2 is the clearance angle (γ). The thickness of the layer removed is the undeformed chip thickness (t) and the width of cut perpendicular to the paper will be Volume II 335 335-356 4/11/06 12:33 PM Page 335 Copyright © 1983 CRC Press LLC fractures internally in shear leaving behind the stationary body of metal attached to the tip of the tool (BUE). Another reason is associated with the fact that metals (notably steel) exhibit minimum ductility (strain at fracture) somewhat above room temperature. This is called “blue brittleness”, since the minimum strain-at-fracture temperature for steel cor- responds to that where the thickness of surface oxide produced gives rise to a blue interference color. At relatively low-cutting speeds (low-tool face temperature), the temperature along CD will be closer to the blue brittle temperature than along AC and then the strain at fracture along CD will be less than along AC. The inherent instability of a large BUE is very troublesome relative to surface finish. As cutting proceeds, BUE tends to grow slowly until it reaches a critical size and then it leaves abruptly with the chip. Since the BUE grows downward as well as outward (point D below A in Figure 4), the surface produced by the periodic change in size of the BUE is as shown in Figure 4. This is one of the sources of surface roughness when cutting at relatively low speed. An unstable BUE can also decrease tool life due to abrasive action of pieces of BUE on the tool face and wear land; on the other hand, a small, stable BUE can be beneficial in protecting the tip of the tool from wear. The most important way of avoiding a BUE is to increase cutting speed. Above a certain Volume II 337 FIGURE 3. Photomicrograph showing large BUE and portions of BUE along finished surface and along face of chip. AISI 1020 steel cut dry at 90 fpm (27.4 m/min). Undeformed chip thickness = 0.005 in. (0.125 mm). FIGURE 4. Diagrammatic representation of Figure 3 showing BUE- induced roughness on finished surface. 335-356 4/11/06 12:33 PM Page 337 Copyright © 1983 CRC Press LLC [...]... Copyright © 19 83 CRC Press LLC 335-356 4 /11 /06 342 12 :34 PM Page 342 CRC Handbook of Lubrication (Figure 10 a) (11 ) (Figure 10 b) (12 ) As already mentioned slip does not occur on every atomic plane but only on bands of planes in the vicinity of a defect This results in the strain in a chip being very inhomogeneous, being small within a slip band and large between It also results in the strain rate of chip... example of a three-dimensional cutting tool is the face milling cutter shown in Figure 11 In this case the inclination angle is the helix angle of the cutting edge Copyright © 19 83 CRC Press LLC 335-356 4 /11 /06 346 12 :34 PM Page 346 CRC Handbook of Lubrication FIGURE 12 Types of fool wear depending on value of product (Vt0.6) 1 2 3 4 5 6 High speed steels (HSS) Sintered tungsten carbide (WC) Sintered titanium... chemistry and hardness Values of u given in Table 1 are useful in estimating the power component of cutting force (Fp) as well as the power required for a specific cutting operation The component of the feed force may be estimated to a first approximation by assuming it to be half the value of FP Copyright © 19 83 CRC Press LLC 335-356 4 /11 /06 344 12 :34 PM Page 344 CRC Handbook of Lubrication Table 2 REPRESENTATIVE... in the strain rate of chip formation being very large since (13 ) where Δy is the width of a slip band which is normally about 10 –4 in (25 × 10 –4 mm) For example, for a moderate cutting speed of 10 0 fpm (0.5 m/sec), a rake angle (α) of 0°, a shear angle (φ) of 20°, and Δy = 10 –4 in (2.5µm), the strain rate is found to be approximately 0.2 × 10 6/sec This is recognized as a very high value when it is realized... thermal softening and a relatively large real area of contact for the tool face The crater that forms has its maximum depth at the point of maximum temperature and, hence, the crater develops as shown in Figure 14 , with its maximum point at approximately a constant distance from the cutting edge The rate of crater formation depends on the stability of the constituents of the tool, the rate of diffusion of. .. cutting data Copyright © 19 83 CRC Press LLC 335-356 4 /11 /06 340 12 :33 PM Page 340 CRC Handbook of Lubrication FIGURE 8 Construction for use in deriving expression connecting shear angle (φ) and cutting ratio (r) (5) The value of mean chip thickness in Equation 4 is not easily measured directly because most chips are relatively rough on the free surface A more convenient method of obtaining r is from...335-356 4 /11 /06 338 12 :33 PM Page 338 CRC Handbook of Lubrication FIGURE 5 Regions of importance in metal cutting FIGURE 6 Forces acting on tool for orthogonal cut cutting speed (~ Im/sec or 200 ft/min) a large unstable BUE will not tend to form Another way of decreasing BUE formation is to increase the rake angle of the tool This causes angle DCA in Figure 2 to... single-point cutting tool are 0 .10 , 0 .17 , and 0.25, respectively Wear Land Tool Wear Wear land tool wear (Figure 12 c) is probably the most important type of wear for singlepoint tools that are well matched to the material being machined When the extent of the wear land is plotted vs cutting time at a constant rate of metal removal, curves such as those shown in Eigure 13 are obtained Wear rate is high... other areas of mechanics The total specific energy (u) is (14 ) where (Vbt) is the volume rate of material removal The specific cutting energy (u) has only two significant components — us, specific shear energy, and uF, specific friction energy, where (15 ) (16 ) Since specific surface energy and specific momentum energy are negligible, in all metal removal operations including grinding u= us + uF (17 ) In... major cause for the strong dependance of tool life (T) on cutting speed (V) is due to tool temperature (θ) By use of Equation 20a, the Taylor equation may be generalized as follows for cases where both V and t vary (22) where n2 > n1 This equation may in turn be generalized to include a variable depth of cut (b) as follows: (23) where n1 < n2 < n3 Typical values for n1, n2, and n3, for a high-speed steel . Application Volume II 329 317 -333 4 /10 /06 12 :59 PM Page 329 Copyright © 19 83 CRC Press LLC Volume II 3 31 Table 2 TYPICAL LUBRICANTS USED IN COLD WORKING 317 -333 4 /10 /06 12 :59 PM Page 3 31 Copyright © 19 83 CRC Press. (r). 335-356 4 /11 /06 12 :33 PM Page 340 Copyright © 19 83 CRC Press LLC (Figure 10 a) (11 ) (Figure 10 b) (12 ) As already mentioned slip does not occur on every atomic plane but only on bands of planes in. Figure 11 . In this case the inclination angle is the helix angle of the cutting edge. 344 CRC Handbook of Lubrication Table 2 REPRESENTATIVE VALUES 335-356 4 /11 /06 12 :34 PM Page 344 Copyright © 19 83