Handbook of Lubrication part 9 ppsx

FIGURE 2. Interface pressures in upsetting a cylinder with (a) no friction, (b) high friction, and (c) high friction and larger d/h ratio. (From Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977. With permission.) 318 CRC Handbook of Lubrication FIGURE 1. Examples of the variation of frictional stress with normal pressure, (a) Variation of shear stress; (b) coef- ficient of friction; and (c) interface shear strength factor. (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 318 Copyright © 1983 CRC Press LLC where x is the distance from the edge, d is workpiece diameter, and h is the workpiece height. Both maximum and average pressures are a function of the d/h ratio (Figures 2b and c). Stresses set limitations: interface pressures cause an elastic deformation of the tool, of significant proportions when precise forgings are to be made; maximum die pressure may exceed the pressure rating of the tooling; total force required may be too high for any press or hammer of reasonable size. All these limitations are a function of τ i and of process geometry, characterized by the d/h ratio. For this reason, forging of relatively thin workpieces can become extremely difficult unless friction is kept very low with a suitable lubricant. Friction also affects the deformation process. With low friction, resistance to sliding at the interface results in barrelling (Figure 2b). When friction is high enough to immobilize part of the end face, deformation becomes highly inhomogeneous, and some of the end face is actually formed by a folding over of the original side surfaces (Figure 2c). Barreling and folding over generate tensile stresses on the barrel surface. These “secondary tensile stresses” may lead to surface cracking in moderately ductile materials. Upsetting a Slab In forging flat slabs, major material flow takes place in the direction of minimum resist- ance, i.e., in direction Lin Figure 3. If the workpiece is very wide or material flow in the w direction is hindered by a die element, pressure distribution remains constant along the entire width and, in the absence of friction, equals the plane-strain flow stress. With friction, a friction hill develops which is the same for identical L/h and d/h ratios. Materials flows away from the neutral center line. When friction is high enough to cause sticking over part of the interface, the neutral line broadens into a neutral zone and the workpiece changes shape by the folding out of side surfaces (as in Figure 2c). In a workpiece of finite width or unconstrained by die elements, some spread takes place and this increases with higher friction. Ring Upsetting In the absence of friction, the hole (and outer diameter) of a ring expand as though the workpiece were solid. Friction hinders free expansion and some material now flows towards the center; a flow-dividing “neutral circle” develops (Figure 4). With increasing friction this neutral circle moves towards the outer diameter and the internal diameter actually shrinks. This offers one of the easiest methods for studying friction and lubricant evaluation: higher friction results in a greater decrease of internal diameter. Impression Die Forging When forging in shaped dies, a flash is generated which contributes to die filling by preventing the free escape of material from the die cavity. Therefore, high friction in the Volume II319 FIGURE 3.Interface pressure in upsetting a flat, rectangular workpiece with friction. (From Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977. With permission.) 317-333 4/10/06 12:59 PM Page 319 Copyright © 1983 CRC Press LLC FIGURE 6. Geometry of roll pass and associated velocities. (From Schey, J. A., Ed., Metal Deformation Processes: Friction and Lubrication, Marcel Dekker, New York, 1970. With permission.) Volume II 321 FIGURE 5. Deformation modes and interface pressures in indenting (a) a semiinfinite body, (b) a thick workpiece (h/L > 1), and (c) a workpiece with h/L = 1. (From Schey, J. A., Introduction to Manufacturing Processes, McGraw-Hill, New York, 1977. With permission.) 317-333 4/10/06 12:59 PM Page 321 Copyright © 1983 CRC Press LLC Since the same material volume must enter and leave the rolls in unit time, the product of velocities and slab thicknesses must be the same everywhere in the absence of spread. The slab moves with the rolls at the neutral point (Figure 6) but moves slower (backward slip) towards the entry and faster (forward slip) towards the exit. Since the position of the neutral point is governed by friction in the roll gap, forward slip s f is a very sensitive measure of the efficiency of lubricants. With decreasing friction, forward slip diminishes: (5) where the neutral angle depends on µ in the following relation: (6) Interface pressures reach a maximum at the neutral point, and the friction hill is similar to that observed in the compression of slabs (Figure 3). When µp =σ f sticking sets in and the friction hill becomes rounded. Rolling of very thin strips presents difficulties because roll flattening (broken lines in Figure 6) becomes commensurate with strip thickness. Interface pressures can be reduced with small-diameter rolls and application of front and back tensions to the strip; even so, good lubrication is indispensible. Some minimum friction is still needed, otherwise the strip may skid in the rolls and lateral strip movement becomes difficult to control. Wire, Bar, and Tube Drawing As the product is pulled through, deformation is attained by compressive stresses at the stationary die (Figure 7). Most drawing operations are performed on round wire (Figure 7a), although shaped wire and bar are also drawn. All sliding is unidirectional, and the draw force is opposed by frictional stresses at the interface. The draw stress is (7) If the strength of the drawn product is insufficient to carry the draw force, the product will be torn off. This sets a limit of 30% or less reduction in cross-sectional area per pass, even with a good lubricant. Interface pressures are always below the compressive flow strength of the material. As in forging, a large h/L ratio increases inhomogeneity of deformation. Since this ratio increases and friction forces decrease with increasing die angles, an optimum angle exists at which draw force is a minimum. Inhomogeneous deformation results in greater elongation of the surface layers, thus putting the center of the wire in tension. These secondary tensile stresses, combined with the drawing stresses, can lead to internal fracture (centerburst or arrowhead defect) in materials of limited ductility. Friction promotes this defect by increasing the drawing stresses. In tube drawing without an internal die (Figure 7b), frictional conditions are the same as in wire drawing. More frequently, however, a short plug (British) or mandrel (U.S.) controls the internal diameter (Figure 7c). Friction on this plug increases drawing stresses, thus reducing the maximum attainable reduction. However, when the internal die element is a long mandrel (British) or bar (U.S.), this die element moves together with the drawn product (Figure 7d) and some of the drawing stresses are transmitted to it by interface friction. Higher friction on the internal surface actually increases the attainable reduction. 322 CRC Handbook of Lubrication 317-333 4/10/06 12:59 PM Page 322 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 [...]... Press, Cambridge, 197 9 Taylor, F W., On the art of metal cutting, Trans ASME, 28, 31, 190 7 Vilenski, D and Shaw, M C., The importance of workpiece softening on machinability, Ann CIRP, 18, 623, 196 9 McKenna, P W., U.S Patent 2,113,353, 193 8 Opitz, H and Koenig, W., Ind Anzieger, 87, 46 (Part I), 26, 196 5; Ind Anzeiger 87, 845 (Part II); 43, 196 5; Ind Anmger, 87, 1033 (Part III), 51, 196 5 Sata, T., Chairman,... Society of Precision Engineers, Tokyo, 196 9 Copyright © 198 3 CRC Press LLC 335-356 4/11/06 356 12:34 PM Page 356 CRC Handbook of Lubrication 12 Tipnb, V A and Joseph, R A., J Eng Ind., Trans ASME, 93 , 571, 197 1 13 Rao, S B., Kumar, K V., and Shaw, M C., Friction characteristics of coated tungsten carbide cutting tools, Wear, 49, 353, 197 8 14 Shaw, M C., Grinding fluids, Manuf Eng Trans., 1, 197 2 15... Examples of SRG are abrasive cut off processes, conditioning of slabs and billets in a steel mill and vertical spindle surface grinding The rate of wear of a grinding wheel is usually important to the economics and performance of the process In the case of SRG wear is usually expressed in terms of a grinding ratio (G) volume of work removed G = volume of work consumed In such cases, the rate of change of. .. cutting edge The rate of crater formation depends on the stability of the constituents of the tool, the rate of diffusion of the products of decomposition, and their influence on the strength of the work material When a tungsten carbide tool is used to cut a low-carbon steel, the rate of crater formation Copyright © 198 3 CRC Press LLC 335-356 4/11/06 12:34 PM Page 3 49 Volume II 3 49 calcium and ferrosilicon... 1, 7, 197 9 12 Schey, J A., in Proc 4th Int Conf Prod Eng., Japan Society of Precision Engineering, Tokyo, 198 0, 102 13 Kalpakjian, S and Jain, S C., Eds., Metalworking Lubrication American Society of Mechanical Engineers, New York, 198 0 Copyright © 198 3 CRC Press LLC 335-356 4/11/06 12:33 PM Page 335 Volume II 335 METAL REMOVAL Milton C Shaw INTRODUCTION A good deal of effort in the manufacture of hard... York, 197 7 7 Wilson, W R D., in Mechanics of Sheet Metal Forming Plenum Press, New York, 197 8, 157 8 Proc 1st Int Conf Lubr Challenges in Metalworking and Processing, IIT Research Institute, Chicago, 197 8 9 Proc 2nd Inst Conf Lubr Challenges in Metalworking and Processing IIT Research Institute Chicago, 197 9 10 Schey, J A., in Metal Forming Plasticity, Lippmann, H., Ed., Springer-Verlag, Berlin, 197 9,... 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 Copyright © 198 3 CRC Press LLC 335-356 4/11/06 338 12:33 PM Page 338 CRC Handbook of Lubrication FIGURE 5 Regions of importance in metal cutting... and Shaw, M C., Study of the finish produced in surface grinding II Analytical, Proc, Inst Mech Eng (London), 182(3K), 182, 196 7- 196 8 24 Shaw, M C., A New Theory of Grinding, Institution of Engineers, Australia, 197 2, 73 25 Shaw, M C and DeSalvo, G J., The role of elasticity in hardness testing, Metals Eng Quart., 12, 1, 197 2 26 Amarego, E J A., and Brown, R H., The Machining of Metals, Prentice-Hall,... 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 © 198 3 CRC Press LLC 335-356 4/11/06 344 12:34 PM Page 344 CRC Handbook of Lubrication Table 2 REPRESENTATIVE... tools is the same as two-dimensional tools An 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 © 198 3 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) . York, 197 7. With permission.) 318 CRC Handbook of Lubrication FIGURE 1. Examples of the variation of frictional stress with normal pressure, (a) Variation of shear stress; (b) coef- ficient of friction;. edge. The rate of crater formation depends on the stability of the constituents of the tool, the rate of diffusion of the products of decomposition, and their influence on the strength of the work. Application Volume II 3 29 317-333 4/10/06 12: 59 PM Page 3 29 Copyright © 198 3 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 © 198 3 CRC