Handbook of Lubrication Episode 1 Part 14 doc

In reverse or indirect extrusion the billet remains stationary in the container Figure 8a, and the magnitude of friction plays no role; die pressure pe is a function of extrusion ratio R

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 pe is a function of extrusion ratio

R = Ao/Al

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

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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.)

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 = 3σf 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

A special 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/L ratio 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 σfup to multiples of σf (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)

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Layer-lattice compounds such as MoS2and 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

In most 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 EP additives 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 A moderately coarse, nondirectional (such as bead-blasted) surface finish is desirable in maintaining graphite or MoS2 supply in hot forging

A smooth, 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 = σf, 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

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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

A second 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 for Staining

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

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Table 2 TYPICAL LUBRICANTS USED IN COLD WORKING

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).

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.

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METAL REMOVAL

Milton C Shaw

INTRODUCTION

A good 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 A wide 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.1This was produced by moving

a workpiece 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 Merchant2and Piispanen3first 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

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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

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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.