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Metal cutting and the tribology of seizure: II movement of work material over the tool in metal cutting
Wear, 128 (1988)
41 - 64
METAL CUTTING AND THE TRIBOLOGY OF SEIZURE: II MOVEMENT OF WORK MATERIAL OVER THE TOOL IN METAL CUTTING E. M. TRENT* Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2!lT (U.K.) (Received November 17,1987;revised
May 19,1988;
accepted June 8,1988)
Summary The predominance of seizure at the tool-work interface in metal cutting operations, demonstrated in the first paper, appears inconsistent with the observed continuous flow of the chip and the main body of the work material over the tool surface. This contradiction has led many investigators to reject the concept of seizure in metal cutting. This paper presents observed features of movement of work material over the tool under seizure conditions. 1. Plastic deformation
on the shear plane
Plastic deformation takes place in the region of the shear plane OB and in the region of the tool-work contact area OC (Fig. 1). The amount of shear strain in the shear plane region is seldom less than two when the chip is thin but may be five or greater when the chip is much thicker than the undeformed chip thickness DE. In spite of these very large amounts of strain, fracture on the shear plane is inhibited by compressive stress acting on this plane. This stress results from the impeding force on the rake face contact area OC, which constrains the flow of the chip. Figures 2 and 3 show sections through quick-stop specimens when cutting medium carbon steel and cupro-nickel. The cupro-nickel chip is much thicker than the steel chip and the change in structure between the body of the work material and the chip shows that the amount of strain in the cupro-nickel is proportionately greater. In both cases all the plastic deformation occurs as the work material moves a very short distance (about 0.1 mm) through the shear plane. The main body of the chip moves across the seized contact area (O-C, Fig. l), a distance of about 1.5 mm for the steel chip and about 3 mm for the cupro-nickel, as a rigid body with no further plastic deformation. Transmission electron microscopy examination of the body of chips shows that shear strain takes place by dislocation movement with the forma*Hon. Research Fellow, Department of Metallurgy and Materials, University of
Birmingham.
0043-1648/88/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
48
Fig. 1. Simple model of metal cutting action.
Fig. 2. Section through quick-stop after cutting low alloy engineering steel at 100 m min-‘, at 0.25 mm rev-’ feed. S. Barnes, M.Sc. Thesis, University of Birmingham, 1986. Fig. 3. Section through quick-stop after cutting cupro-nickel at 35 m mini.
of cells or subgrains elongated in the direction of shear strain. The body of the chip is strain hardened, e.g. an increase from about 30 HV in the bar to about 80 HV in the chip for copper, and about 150 HV to about 250 HV in the steel chip. tion
49
In a few ductile metals and alloys, strain on the shear plane is modified by a mechanism by which strain is concentrated in a narrow zone. Within this zone, recovery processes halt the strain hardening and large amounts of strain can take place at constant or lowered yield stress. These are plastic instabilities, and may be “thermoplastic instabilities” within which temperature rise promotes recovery processes. Any such instability exists for a very short period of time, e.g. a few milliseconds, during which the large amount of local strain relieves stress in the oncoming material. This then deforms by dislocation movement with strain hardening until the next instability is generated; a cyclical process reflected in the shape of the chip. Figure 4 shows such a chip in a titanium ahoy. Titanium and its alloys are particularly likely to deform by plastic instabilities because of their low thermal conductivity, [I]. Austenitic stainless steel chips are also segmented by this mechanism but less frequently. The segmented chips remain continuous and the body of the chip moves across the seized contact area without further deformation.
Fig. 4. Section through strain in chip body.
segmented
titanium
chip showing
narrow bands of intense shear
Fig. 5. Section through quickstop when cutting flake graphite cast iron. Segmented with fractures separating most segments.
chip
Less ductile materials such as cast iron may not be able to sustain shear strains of the order of two without fracture on the shear plane. Figure 5 is a section through a quick-stop specimen when cutting flake graphite cast iron. Chip formation is achieved by dislocation movement and periodic fracture. Fracture plays a part in chip formation in some more ductile materials if compressive stress acting on the shear plane is reduced. This is achieved if the constraining force on the tool rake face is reduced so that the chip can flow freely away from the cutting edge, for example by the action of lead at the interface when cutting leaded brass. Figure 6 shows a discontinuous chip of free-machining brass. Fracture occurs periodically near the shear
Fig. 6. Section chip.
through
quick-stop
after cutting
60140 lea lded brass showing
discontinuous
plane and the chip body then moves across the tool face with no further deformation. 2. Plastic deformation at the tool rake face In the work material very close to the tool-work interface a region of shear strain much more intense than on the shear plane is observed, Figs. 2 and 3. This is a consequence of seizure of the work material to the tool, demonstrated in the first paper. Under seizure conditions two major patterns of flow of work material close to the interface are observed: (1) the formation of a built-up edge and (2) the formation of a flow zone. The former will be considered first.
3. Built-up edge formation At relatively low cutting speeds dislocation movement, with strain hardening, is the main mechanism of plastic deformation near the seized tool-work interface. Figure 7 shows a section through a quick-stop specimen with a characteristic built-up edge formed when cutting steel at low speed. The built-up edge is seen to be continuous with the work material from which it was formed; it is not a separate body of hardened work material over which the chip slides. The new work surface forms by fracture at A and the under surface of the chip by fracture at B (Fig. 7). During the quick-stop, separation occurred at the interface between the tool and the built-up edge, bonding at this interface being the “weakest link in the chain” under tensile stress. During cutting, under high compressive stress, bonding at the interface was strong enough to prevent slid.ing.Movement of the chip over the tool took place by plastic deformation between A and B and by fracture at A and at B.
51
Fig. 7. Section through quick-stop after cutting low carbon steel at 15 m min-’ showing built-up edge.
The first material to bond to the tool surface is strain hardened and its yield stress greatly increased but the shear stress is insufficient to break the bond to the tool. Strain then continues in the adjacent metal further from the tool surface until this also becomes intensely strain hardened. By repetition of this process a succession of layers forms the built-up edge. On the shear plane the amount of strain can be calculated, and it can also be measured from the change in shape of structural features in the work material (such as the pearlite colonies in the chip in Fig. 7) which are elongated in the direction of shear strain. In the built-up edge, shear strain cannot be calculated and the amount of strain is so great that structural features such as pearlite colonies cannot be resolved by optical microscopy. Even with transmission electron microscopy pearlite areas could not be positively identified, so well had they been dispersed by intense strain, which was orders of magnitude greater than on the shear plane [2]. The extreme strain results in a structure of very thin cells or microbands elongated in the direction of shear strain, Fig. 8. This is a very rigid structure. Microhardness tests in the built-up edge of a medium carbon steel give values up to 600 HV, compared with 200 - 250 HV in the body of the chip. In practice the size and shape of the built-up edge vary greatly with the work material and with the cutting conditions. Figure 9 shows another built-up edge from a low carbon steel. For any material and cutting conditions the built-up edge seems to reach an equilibrium size and shape. It has to support the high compressive and shear stress imposed by the cutting
52
Fig. 8. Transmission electron micrograph of built-up edge structure microhands characteristic of extreme strain hardening [ 21.
in steel, showing
Fig. 9. Section through quick-stop after cutting low carbon steel. Built-up edge and fractures forming new surfaces both along lines of flow and across lines of flow.
process and therefore it cannot grow in height indefinitely. As it grows in height and changes in shape (see Figs. 7 and 9) the stress system changes and parts of the built-up edge are broken away. The paths of fracture which form the new work surface at A and the chip underside at B (Fig. 7) change in such a way that work-hardened material at the top of the built-up edge is constantly being carried away on the new surfaces. The lower parts of the built-up edge often remain rigid and unchanged for long periods of cutting time. At the top of the built-up edge (between A and B in Fig. 7) intense work hardening must be accompanied by piling up of dislocations at inclusions and other discontinuities in the structure. In the centre of this region the microcracks, which would be formed under tensile stress, are inhibited by the high compressive stress but, as the work-hardened material flows towards A or B, compressive stress is reduced, microcracks develop and join to initiate fracture. The new surfaces formed by this fracture often follow
53
extremely elongated plastic inclusions such as sulphides in steel. The path of the fracture generally foyws the flow lines in the deforming structure but frequently moves from one line of microcracks to another, producing a typically rough surface. The presence or absence of plastic inclusions is very important, affecting both surface finish and the shape of the built-up edge. The built-up edge tends to grow in the feed direction (towards the left in Figs. 7 and 9) so that it overhangs the tool edge. This structure becomes unstable and a stable built-up edge form is preserved by shearing away of segments across rather than along the flow lines. This action can be seen in Fig. 9, and less emphatically in Fig. 7. A different fracture mechanism is involved, probably initiated at a thermoplastic instability in a very thin zone of extreme shear strain, a process described in relation to shear on the shear plane. This action leads to “sawtooth” irregularities on the surfaces generated, as shown in the scanning electron micrograph, Fig. 10. The smooth facets, appearing dark in Fig. 10, but often reflecting brightly, are generated by fracture along these instabilities.
Fig. 10. Scanning electron micrograph of undkr surface of &eel chip after cutting with a built-up edge. Rough and smooth areas result from fracture along and acrow lines of flow respectively (Fig. 9). S. Barnes, MA. Theeis,University of Birmingham, 1986.
Thermoplastic instabilities have a greater role in forming the built-up edge when cutting metals and alloys susceptible to this mode of deformation, e.g. titanium alloys, nickel-based alloys and austenitic stainless steel. Figure 11 shows part of a forming chip of austenitic stainless steel [ 31. The etched structure shows both flow lines, indicated by elongation of (Y and y phases, and thermoplastic instabilities, which appear as displace-
Fig. 11. Section through steel tool and bonded austenitic stainless steel chip. showing flow lines and thermoplastic shear bands. Fig. 12. Section through built-up edge and chip of nickel-based creep-resistant alloy after cutting at low speed. Numerous thermoplastic shear bands.
ments of flow lines across the flow direction, like geological faults. Figure 12 shows a section through a built-up edge formed when cutting at low speed a creep-resistant, nickel-based alloy. The white lines are numerous thermoplastic instabilities. This also demonstrates the complex nature of built-up edge formation during continuous cutting. Both internal instabilities and instabilities involved in fracture are short lived, lasting for only a few milliseconds or less before relieving stress so that strain continues by mechanisms of dislocation movement. Large built-up edges are formed only when cutting alloys with more than one phase, e.g. steel, cast iron and some nickel-based, copper-based and aluminium-based alloys. Pure metals do not normally form large built-up edges even at low cutting speed. Like their two-phased alloys, pure metals are seized to the tool rake face during cutting, but only a thin intensely strained layer, often about 50 - 100 pm thick, is observed adjacent to the tool face. Figure 13 shows such a “built-up layer”. The under surface of the chip is often formed by fracture initiated at the top of the layer.
Fig. 13. Section through quickatop up layer”.
after cutting commercially
pure iron, showing “built-
A large built-up edge is formed when cutting alloys with more than one phase because intense strain achieves much higher yield stress by strain hardening than is possible in pure metals. The yield stress is maintained at higher temperatures in poly-phased alloys than in pure metals. In a large built-up edge (Figs. 7 and 9) the cutting forces are supported on a small area at the top. With pure metals such a structure would collapse under the compressive stress. The result is a much thinner built-up layer with pure metals, with a much larger area supporting the stress.
4. Flow zone formation At higher rates of metal removal, i.e. at higher speed or feed, a built-up edge is no longer observed. In its place, at the rake face-work material interface, there is usually a “flow zone” as, for example, that shown in Fig. 2 when cutting steel. A series of cutting tests on steel over a range of cutting speeds and feeds will demonstrate the transition from built-up edge to flow zone. This transition is not absolutely sharp but the change takes place over a narrow range of speed. Charts such as that shown in Fig. 14 for one steel, machined under a standard set of cutting conditions, show the regions of built-up edge and flow zone formation as a function of speed and feed.
mm
-001
,002
'003 Feed
,005
per
,007
- inches
Fig. 14. Chart showing transition steel.
rev
.Ol per
.02
.03
-05
rev.
from built-up edge to flow zone when cutting 0.4% C
56
Fig. 15. Proposed model of strain within a flow zone seized to tool rake face.
With speed and feed coordinates plotted on a logarithmic scale the line for the transition is usually approximately straight [ 41. The transition is strongly influenced by both speed and feed and occurs in a range of cutting conditions commonly encountered in industrial machining operations. The flow zone is usually more strongly bonded to the tool than is the built-up edge. When subjected to the tensile separation of a quick-stop, part of the chip or the whole chip may remain bonded to the tool as was demonstrated by Figs. 7 - 9 in the first paper. Continuous flow of the chip across the tool is not by sliding but by plastic strain confined to the flow zone, usually between 10 and 100 pm thick, within which the amount and rate of strain are extremely high. Figure 15 is a simplified model of a flow zone, exaggerated in thickness to demonstrate the characteristics of shear strain in this situation. The material at the top of the flow zone a-a’ is moving at the speed of the chip, while the flow zone is bonded to the tool at the interface O-Y and the speed of the work material relative to the tool approaches zero at this interface. To describe shear strain in the flow zone, assume a unit cube at the tool edge OabX which is uniformly strained as the top of the cube moves from a-b to a’-b’ at the position C where the chip breaks contact with the tool. Considering a position at the mid-plane of the flow zone, an element c-d is subjected to the same amount of strain when it has reached c’-d’, half way along the seized interface. When it has reached the position c”-d” it has been subjected to twice the amount of strain as a’-b’. At a position one-quarter of the flow zone thickness from the interface, the element e-f of the unit cube has been subjected to four times the amount of strain as a’-b’ when it reaches e”‘-f”’ where the chip breaks contact with the tool. Thus, making the simplifying assumption that the strain is uniform within the flow zone, the amount of strain sustained is inversely proportional to the distance from the flow zone-tool interface. Table 1 gives an example of the amount of strain in such a flow zone at different distances from the interface [5]. The amount of strain becomes extremely high near the interface but the rate of strain remains constant; lo4 s-l in this example
57 TABLE 1 Shear strain in the flow zone according to the model in Fig. 15 (chip speed, 60 m min-I; flow zone thickness, 80 m; contact length, 1.6 mm) Distance from tool rake face Mm)
Shear strain 7 over contact length
80 40 20 10 5 2.5
20 40 80 160 320 640
in which the chip body passed over the contact area in 1.6 ms. The time for any small element of material in the flow zone to cross the contact area increases as the interface is approached but, at a distance of 2.5 pm (commensurate with the surface roughness of many tools) the time was only 55 ms. The amount of strain in the flow zone is orders of magnitude higher than in the chip body. In spite of the extreme strain, fracture in the flow zone over the contact area is inhibited by the very high compressive stress. The pearlite-ferrite structure, clearly visible in the chip body (Figs. 2 and 16), is completely destroyed in the flow zone, where no structure can be resolved with an optical microscope (Fig. 16). Transmission electron microscopy investigations of the flow zone have shown that the basic structure, in all cases studied, is equi-axed grains or cells of very small size (0.2 - 0.5 pm) containing few dislocations [6] (Fig. 17). Pearlite in the flow zone of steel chips is completely disrupted and cementite is present as dispersed particles.
Fig. 16. Flow zone in medium carbon steel chip. Fig. 17. Transmission electron micrograph of flow zone structure in steel chip; very small, equi-axed cells or grains [ 61.
58
Although this structure may not be that which was present during the cutting action, it is clear that the very rigid structure of the buiit-up edge (Fig. 8) has been superceded. The mechanism of plastic deformation has changed from dislocation movement and strain hardening to one III which recovery processes accompany dislocation movement, resulting in the formation of very small equi-axed grains and many new grain boundaries. Recent investigations of metals subjected to extreme strain [ 7, 81 have shown similar structural changes, resulting in a condition where grain boundary sliding plays an important role and plastic deformation takes place without strain hardening. In the flow zone many metals and alloys behave like very viscous liquids; unlimited deformation occurs at constant or reducing yield stress. The flow zone is a “thermoplastic instability”. As discussed above, such instabilities, occurring on the shear plane or in the built-up edge are of very short duration because they provide stress relief, after which plastic deformation reverts to a dislocation mechanism. It is the seizure condition at the interface with the rigid tool which anchors the thermoplastic instability to the tool for an indefinite period of time, as long as cutting continues. The flow zone is thus a “stable instability”, a contradictory term which suggests “thermoplastic shear band” as a preferred description. The material of the flow zone changes continually as new material is fed in at the cutting edge and leaves at the end of the contact area but the flow zone persists with a characteristic thickness, depending on the work material and cutting conditions. The behaviour of a work material in thermoplastic shear bands is probably the most important property governing its “machinability” in high speed machining. So far there have been very few studies of the influence of composition and structure of the work material on the stresses and temperatures generated in the flow zone and on its thickness. Since the flow zone is strongly bonded to the tool, making them one piece of material, the new surface generated in machining can be formed only by fracture. The fracture forming the underside of the chip may take place at the interface, as shown diagrammatically in Fig. 18(a). This is a shear fracture and forms a smooth shiny surface. It may also take place at the top of the flow zone (Fig. 18(b)) giving a rougher surface. Evidence for the formation of these surfaces comes from scanning electron microscopy studies after quick-stops. Figure 19(a) shows the under surface of a steel chip after quick-stop. The location of Fig. 19(a) is shown diagrammatically in Fig. 19(b). At the right (C -+ R) the smooth ridged surface had been formed just before quick-stop by shear fracture nucleated at C. At the left the dimpled fracture (P - C) was formed during quick-stop by largely tensile stress. The mating half of this fracture remained as a thin layer of work material adhering to the tool as shown diagrammatically in Fig. 19(b). Figure 20(a) is a scanning electron micrograph from a quick-stop after cutting steel with a steel tool coated by chemical vapour deposition. The location is shown diagrammatically in Fig. 20(b). In this example small fragments of tool material were detached at the cutting edge and remain
59
Fig. 18. Diagrams showing bcation of fractures forming under surface of chips with flow zone seized to tool. (a) New surface formed at the interface, (bf New surface formed near top of flow zone.
embedded in the quick-stop at 0. The replica of the cutting edge is on the line Q-0. The replica of the rake face is at the right U-X, the new ma&ined surface is at the left (O-N). This smooth ridged surface was formed by continuous shear fracture nucleated at the cutting edge of the tool or just below the edge. With a flow zone, the surfaces formed by machining are usually much smoother than those where a built-up edge is present (Figs. 7, 9 and 10). The machined surface is a shear fracture and a thin sheared layer a few micrometres thick remains on the surface. There is not a sharp de~eation between the built-up edge and the flow zone regimes and surfaces showing features of both are frequently seen.
5. Sliding at the tool-wurk
interface
Seizure in the contact area is responsible for the most important charaoteristic features of the machining process but, as demons~at~ in the first paper, sliding and seizure may occur simultaneously, sliding most commonly
60
(a)
Fig. 19. (a) Scanning electron micrograph of under surface of chip and fracture at contact area after quick-stop. (b) Diagram showing location of (a).
in peripheral regions of the contact area. Compressive stress is lower in these regions and the surrounding atmosphere has some access to the interface The outer edge of the chip is often segmented while the remainder of the chip is continuous, as shown in Fig. 21, which is a scanning electron micrograph of a steel chip. A section through the segmented outer edge, parallel with the direction of movement, shows that it was sliding over the tool surface by a periodic slip-stick action (Fig. 22). This sliding action is limited to a few tenths of a millimetre from the outer edge of the chip. A se&ion only 0.5 mm from the edge of the same chip shows the flow zone, characteristic of seizure, at the under surface of the chip (Fig. 23). Wear features on cutting tools in peripheral, sliding regions are often very different from those in regions of seizure. The rate of wear in sliding regions is often many times greater [9]. When cutting steel with cemented carbide or steel tools, the rate of sliding wear is sensitive to the surrounding atmosphere. It is accelerated by directing a jet of oxygen towards the cutting
61
(b)
(4
Fig. 20. (a) Scanning electron micrograph of unde r surface of quick-stop showing edge (O-O), new surface (O-N) and replica of t 001 rake surface (O-X). (b) Diagram showing location of features in (a).
Fig. 21. Scanning electron micrograph showing segmented outer edge of steel chip. Fig. 22. Section action.
through outer edge of steel chip (Fig. 21) showing stick-slip
sliding
edge of the tool and retarded by a jet of nitrogen or argon. In the seized parts of the contact area, however, the rate of wear is not influenced by the surrounding atmosphere.
6. Tool materials The observations reported above concern movement of work material over steel or cemented carbide tools, i.e. where both tool and work material are of metallic character and strong atomic bonding at the interface is predicted and confirmed by observation. Few comparable observations have
62
Fig. 23. Section through same chip, at 0.5 mm from the outer edge, showing flow zone, characteristic of seizure.
been reported when machining with other tool materials such as alumina, sialon, diamond and cubic boron nitride. Evidence has been presented by Doyle et al. (ref. 14, paper I; [lo]) in Cambridge who observed the toolwork interface by looking through transparent, polished sapphire tools during cutting. This shows sliding at the interface under some cutting conditions, particularly close to the cutting edge where seizure is most pronounced with steel and carbide tools. Using tools coated with a layer of alumina by chemical vapour deposition (CVD), however, the evidence of quick-stop specimens demonstrates seizure at the interface and movement of the chip over the tool by a flow zone mechanism [ 11, 121. CVD tool surfaces are much rougher on a micro scale than the polished sapphire and surface topography may influence seizure at the interface. So far the range of work materials and cutting conditions studied, using tools of non-metallic character, is small. Conclusions regarding the character of the interface during cutting, and the mechanism by which the work material moves over the surface of these tools in metal cutting, must await much more extensive study.
7. Conclusions The apparent contradiction of observed seizure at the tool-work interface and continuous flow of chip and work material over the tool surface, has been resolved by the observation of intense localized shear strain within the work material close to the tool surface and fracture which forms the new surfaces. This takes place by two different modes of action. (1) At relatively low speed and feed the mechanism of intense localized plastic deformation is dislocation movement, forming a highly strainhardened layer of work material seized to the tool surface. A sequence of extremely strain-hardened layers forms the built-up edge, which is a dynamic
63
structure, continuous with the main body of work material from which it is formed. The built-up edge maintains, roughly, an equilibrium form by continuous accretion of strain-hardened layers and continuous fractures which carry away some of the strain-hardened material and form the new machined surface and the under surface of the chip. (2) At higher rates of metal removal, movement over the seized interface takes place by even more intense shear strain confined to a very thin thermoplastic shear band seized to the tool surface and termed a “flow zone”. Strain is concentrated in the flow zone because, at a critical condition, strain hardening ceases and yield stress is reduced by dynamic recovery processes which occur as temperature rises, at increasing cutting speed and feed. The structures observed in the flow zone are very fine equi-axed grains or cells (0.2 - 0.5 pm) with few dislocations. Deformation in the flow zone probably involves a large element of grain boundary sliding without strain hardening. An essential feature of this mechanism is that continuous shear fracture takes place to separate the work material from the tool. The fracture is nucleated at, and propagates from the work-tool interface or very close to it within the flow zone, This fracture forms the new machined surface and the under surface of the chip. The behaviour of work materials and of their constituent phases in thermoplastic shear bands is very important in relation to machinability when cutting at high rates of metal removal. While seizure at the interface controls the mode of action in most industrial machining operations, sliding also is observed in peripheral regions of the contact area, where it it may take place by a stick-slip process. Sliding and seizure occur simultaneously at different positions on the tool-work interface. These modes of action at the interface must influence models constructed for analysis of forces, stresses, strain and energy dissipation in metal cutting. It would be very difficult to construct a satisfactory model for the built-up edge mode because of its great complexity. A model should be possible for the flow zone mode, which is of greater importance for problems where such analysis is required. Treatment of the subject as a conventional problem of frictional action must be abandoned. Under conditions of seizure there is no significant relationship of frictional to normal force corresponding to the coefficient of friction. Stress-strain relationship on the shear plane is directly related to values determined for the work material by room temperature mechanical tests at appropriate amounts and rates of shear strain. These relationships cannot be used to analyse stress and strain in the flow zone where strain hardening is absent and the work material behaves like a very viscous fluid. For this, knowledge of the stress strain-temperature relations in thermoplastic shear bands is required and few data are available. Extensive research in this area is necessary. A major factor to be studied is the temperatures generated, and the third paper in this series opens up this subject by presenting results of studies of temperature in metal cutting operations.
64
References 1 2 3 4 5 6 7 8 9 10 11 12
R. F. Recht, J. Appl. Me&., 31 (1964) 189. J. WaIIbank, Met. TechnoZ., 6 (4) (1979) 145. P. K. Wright, Ph.D. Thesis, University of Birmingham, 1971. E. M. Trent, Inst. Prodn. Eng. J., 38 (1959) 105. E. M. Trent, Metal Cutting, Butterworths, London, 2nd edn., 1984, p. 60. A. Shelboum, W. T. Roberts and E. M. Trent, Mater. Sci. Technol., 1 (1985) 220. I. Saunders and J. Nutting, Met. Sci., 18 (1984) 571. 0. D. Sherby and J. Wadsworth, Mater. Sci. Technol., I (1985) 925. E. M. Trent, Metal Cutting, Butterworths, London, 2nd edn., 1984, pp. 108, 144. P. K. Wright, Met. Technob, 8 (4) (1981) 150. J. 0. Fowler, Ph.D. Thesis, The Polytechnic, Wolverhampton, 1980. P. A. Dearnley and E. M. Trent, Met. Technol., 9 (1982) 60.
41 - 64
METAL CUTTING AND THE TRIBOLOGY OF SEIZURE: II MOVEMENT OF WORK MATERIAL OVER THE TOOL IN METAL CUTTING E. M. TRENT* Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2!lT (U.K.) (Received November 17,1987;revised
May 19,1988;
accepted June 8,1988)
Summary The predominance of seizure at the tool-work interface in metal cutting operations, demonstrated in the first paper, appears inconsistent with the observed continuous flow of the chip and the main body of the work material over the tool surface. This contradiction has led many investigators to reject the concept of seizure in metal cutting. This paper presents observed features of movement of work material over the tool under seizure conditions. 1. Plastic deformation
on the shear plane
Plastic deformation takes place in the region of the shear plane OB and in the region of the tool-work contact area OC (Fig. 1). The amount of shear strain in the shear plane region is seldom less than two when the chip is thin but may be five or greater when the chip is much thicker than the undeformed chip thickness DE. In spite of these very large amounts of strain, fracture on the shear plane is inhibited by compressive stress acting on this plane. This stress results from the impeding force on the rake face contact area OC, which constrains the flow of the chip. Figures 2 and 3 show sections through quick-stop specimens when cutting medium carbon steel and cupro-nickel. The cupro-nickel chip is much thicker than the steel chip and the change in structure between the body of the work material and the chip shows that the amount of strain in the cupro-nickel is proportionately greater. In both cases all the plastic deformation occurs as the work material moves a very short distance (about 0.1 mm) through the shear plane. The main body of the chip moves across the seized contact area (O-C, Fig. l), a distance of about 1.5 mm for the steel chip and about 3 mm for the cupro-nickel, as a rigid body with no further plastic deformation. Transmission electron microscopy examination of the body of chips shows that shear strain takes place by dislocation movement with the forma*Hon. Research Fellow, Department of Metallurgy and Materials, University of
Birmingham.
0043-1648/88/$3.50
@ Elsevier Sequoia/Printed
in The Netherlands
48
Fig. 1. Simple model of metal cutting action.
Fig. 2. Section through quick-stop after cutting low alloy engineering steel at 100 m min-‘, at 0.25 mm rev-’ feed. S. Barnes, M.Sc. Thesis, University of Birmingham, 1986. Fig. 3. Section through quick-stop after cutting cupro-nickel at 35 m mini.
of cells or subgrains elongated in the direction of shear strain. The body of the chip is strain hardened, e.g. an increase from about 30 HV in the bar to about 80 HV in the chip for copper, and about 150 HV to about 250 HV in the steel chip. tion
49
In a few ductile metals and alloys, strain on the shear plane is modified by a mechanism by which strain is concentrated in a narrow zone. Within this zone, recovery processes halt the strain hardening and large amounts of strain can take place at constant or lowered yield stress. These are plastic instabilities, and may be “thermoplastic instabilities” within which temperature rise promotes recovery processes. Any such instability exists for a very short period of time, e.g. a few milliseconds, during which the large amount of local strain relieves stress in the oncoming material. This then deforms by dislocation movement with strain hardening until the next instability is generated; a cyclical process reflected in the shape of the chip. Figure 4 shows such a chip in a titanium ahoy. Titanium and its alloys are particularly likely to deform by plastic instabilities because of their low thermal conductivity, [I]. Austenitic stainless steel chips are also segmented by this mechanism but less frequently. The segmented chips remain continuous and the body of the chip moves across the seized contact area without further deformation.
Fig. 4. Section through strain in chip body.
segmented
titanium
chip showing
narrow bands of intense shear
Fig. 5. Section through quickstop when cutting flake graphite cast iron. Segmented with fractures separating most segments.
chip
Less ductile materials such as cast iron may not be able to sustain shear strains of the order of two without fracture on the shear plane. Figure 5 is a section through a quick-stop specimen when cutting flake graphite cast iron. Chip formation is achieved by dislocation movement and periodic fracture. Fracture plays a part in chip formation in some more ductile materials if compressive stress acting on the shear plane is reduced. This is achieved if the constraining force on the tool rake face is reduced so that the chip can flow freely away from the cutting edge, for example by the action of lead at the interface when cutting leaded brass. Figure 6 shows a discontinuous chip of free-machining brass. Fracture occurs periodically near the shear
Fig. 6. Section chip.
through
quick-stop
after cutting
60140 lea lded brass showing
discontinuous
plane and the chip body then moves across the tool face with no further deformation. 2. Plastic deformation at the tool rake face In the work material very close to the tool-work interface a region of shear strain much more intense than on the shear plane is observed, Figs. 2 and 3. This is a consequence of seizure of the work material to the tool, demonstrated in the first paper. Under seizure conditions two major patterns of flow of work material close to the interface are observed: (1) the formation of a built-up edge and (2) the formation of a flow zone. The former will be considered first.
3. Built-up edge formation At relatively low cutting speeds dislocation movement, with strain hardening, is the main mechanism of plastic deformation near the seized tool-work interface. Figure 7 shows a section through a quick-stop specimen with a characteristic built-up edge formed when cutting steel at low speed. The built-up edge is seen to be continuous with the work material from which it was formed; it is not a separate body of hardened work material over which the chip slides. The new work surface forms by fracture at A and the under surface of the chip by fracture at B (Fig. 7). During the quick-stop, separation occurred at the interface between the tool and the built-up edge, bonding at this interface being the “weakest link in the chain” under tensile stress. During cutting, under high compressive stress, bonding at the interface was strong enough to prevent slid.ing.Movement of the chip over the tool took place by plastic deformation between A and B and by fracture at A and at B.
51
Fig. 7. Section through quick-stop after cutting low carbon steel at 15 m min-’ showing built-up edge.
The first material to bond to the tool surface is strain hardened and its yield stress greatly increased but the shear stress is insufficient to break the bond to the tool. Strain then continues in the adjacent metal further from the tool surface until this also becomes intensely strain hardened. By repetition of this process a succession of layers forms the built-up edge. On the shear plane the amount of strain can be calculated, and it can also be measured from the change in shape of structural features in the work material (such as the pearlite colonies in the chip in Fig. 7) which are elongated in the direction of shear strain. In the built-up edge, shear strain cannot be calculated and the amount of strain is so great that structural features such as pearlite colonies cannot be resolved by optical microscopy. Even with transmission electron microscopy pearlite areas could not be positively identified, so well had they been dispersed by intense strain, which was orders of magnitude greater than on the shear plane [2]. The extreme strain results in a structure of very thin cells or microbands elongated in the direction of shear strain, Fig. 8. This is a very rigid structure. Microhardness tests in the built-up edge of a medium carbon steel give values up to 600 HV, compared with 200 - 250 HV in the body of the chip. In practice the size and shape of the built-up edge vary greatly with the work material and with the cutting conditions. Figure 9 shows another built-up edge from a low carbon steel. For any material and cutting conditions the built-up edge seems to reach an equilibrium size and shape. It has to support the high compressive and shear stress imposed by the cutting
52
Fig. 8. Transmission electron micrograph of built-up edge structure microhands characteristic of extreme strain hardening [ 21.
in steel, showing
Fig. 9. Section through quick-stop after cutting low carbon steel. Built-up edge and fractures forming new surfaces both along lines of flow and across lines of flow.
process and therefore it cannot grow in height indefinitely. As it grows in height and changes in shape (see Figs. 7 and 9) the stress system changes and parts of the built-up edge are broken away. The paths of fracture which form the new work surface at A and the chip underside at B (Fig. 7) change in such a way that work-hardened material at the top of the built-up edge is constantly being carried away on the new surfaces. The lower parts of the built-up edge often remain rigid and unchanged for long periods of cutting time. At the top of the built-up edge (between A and B in Fig. 7) intense work hardening must be accompanied by piling up of dislocations at inclusions and other discontinuities in the structure. In the centre of this region the microcracks, which would be formed under tensile stress, are inhibited by the high compressive stress but, as the work-hardened material flows towards A or B, compressive stress is reduced, microcracks develop and join to initiate fracture. The new surfaces formed by this fracture often follow
53
extremely elongated plastic inclusions such as sulphides in steel. The path of the fracture generally foyws the flow lines in the deforming structure but frequently moves from one line of microcracks to another, producing a typically rough surface. The presence or absence of plastic inclusions is very important, affecting both surface finish and the shape of the built-up edge. The built-up edge tends to grow in the feed direction (towards the left in Figs. 7 and 9) so that it overhangs the tool edge. This structure becomes unstable and a stable built-up edge form is preserved by shearing away of segments across rather than along the flow lines. This action can be seen in Fig. 9, and less emphatically in Fig. 7. A different fracture mechanism is involved, probably initiated at a thermoplastic instability in a very thin zone of extreme shear strain, a process described in relation to shear on the shear plane. This action leads to “sawtooth” irregularities on the surfaces generated, as shown in the scanning electron micrograph, Fig. 10. The smooth facets, appearing dark in Fig. 10, but often reflecting brightly, are generated by fracture along these instabilities.
Fig. 10. Scanning electron micrograph of undkr surface of &eel chip after cutting with a built-up edge. Rough and smooth areas result from fracture along and acrow lines of flow respectively (Fig. 9). S. Barnes, MA. Theeis,University of Birmingham, 1986.
Thermoplastic instabilities have a greater role in forming the built-up edge when cutting metals and alloys susceptible to this mode of deformation, e.g. titanium alloys, nickel-based alloys and austenitic stainless steel. Figure 11 shows part of a forming chip of austenitic stainless steel [ 31. The etched structure shows both flow lines, indicated by elongation of (Y and y phases, and thermoplastic instabilities, which appear as displace-
Fig. 11. Section through steel tool and bonded austenitic stainless steel chip. showing flow lines and thermoplastic shear bands. Fig. 12. Section through built-up edge and chip of nickel-based creep-resistant alloy after cutting at low speed. Numerous thermoplastic shear bands.
ments of flow lines across the flow direction, like geological faults. Figure 12 shows a section through a built-up edge formed when cutting at low speed a creep-resistant, nickel-based alloy. The white lines are numerous thermoplastic instabilities. This also demonstrates the complex nature of built-up edge formation during continuous cutting. Both internal instabilities and instabilities involved in fracture are short lived, lasting for only a few milliseconds or less before relieving stress so that strain continues by mechanisms of dislocation movement. Large built-up edges are formed only when cutting alloys with more than one phase, e.g. steel, cast iron and some nickel-based, copper-based and aluminium-based alloys. Pure metals do not normally form large built-up edges even at low cutting speed. Like their two-phased alloys, pure metals are seized to the tool rake face during cutting, but only a thin intensely strained layer, often about 50 - 100 pm thick, is observed adjacent to the tool face. Figure 13 shows such a “built-up layer”. The under surface of the chip is often formed by fracture initiated at the top of the layer.
Fig. 13. Section through quickatop up layer”.
after cutting commercially
pure iron, showing “built-
A large built-up edge is formed when cutting alloys with more than one phase because intense strain achieves much higher yield stress by strain hardening than is possible in pure metals. The yield stress is maintained at higher temperatures in poly-phased alloys than in pure metals. In a large built-up edge (Figs. 7 and 9) the cutting forces are supported on a small area at the top. With pure metals such a structure would collapse under the compressive stress. The result is a much thinner built-up layer with pure metals, with a much larger area supporting the stress.
4. Flow zone formation At higher rates of metal removal, i.e. at higher speed or feed, a built-up edge is no longer observed. In its place, at the rake face-work material interface, there is usually a “flow zone” as, for example, that shown in Fig. 2 when cutting steel. A series of cutting tests on steel over a range of cutting speeds and feeds will demonstrate the transition from built-up edge to flow zone. This transition is not absolutely sharp but the change takes place over a narrow range of speed. Charts such as that shown in Fig. 14 for one steel, machined under a standard set of cutting conditions, show the regions of built-up edge and flow zone formation as a function of speed and feed.
mm
-001
,002
'003 Feed
,005
per
,007
- inches
Fig. 14. Chart showing transition steel.
rev
.Ol per
.02
.03
-05
rev.
from built-up edge to flow zone when cutting 0.4% C
56
Fig. 15. Proposed model of strain within a flow zone seized to tool rake face.
With speed and feed coordinates plotted on a logarithmic scale the line for the transition is usually approximately straight [ 41. The transition is strongly influenced by both speed and feed and occurs in a range of cutting conditions commonly encountered in industrial machining operations. The flow zone is usually more strongly bonded to the tool than is the built-up edge. When subjected to the tensile separation of a quick-stop, part of the chip or the whole chip may remain bonded to the tool as was demonstrated by Figs. 7 - 9 in the first paper. Continuous flow of the chip across the tool is not by sliding but by plastic strain confined to the flow zone, usually between 10 and 100 pm thick, within which the amount and rate of strain are extremely high. Figure 15 is a simplified model of a flow zone, exaggerated in thickness to demonstrate the characteristics of shear strain in this situation. The material at the top of the flow zone a-a’ is moving at the speed of the chip, while the flow zone is bonded to the tool at the interface O-Y and the speed of the work material relative to the tool approaches zero at this interface. To describe shear strain in the flow zone, assume a unit cube at the tool edge OabX which is uniformly strained as the top of the cube moves from a-b to a’-b’ at the position C where the chip breaks contact with the tool. Considering a position at the mid-plane of the flow zone, an element c-d is subjected to the same amount of strain when it has reached c’-d’, half way along the seized interface. When it has reached the position c”-d” it has been subjected to twice the amount of strain as a’-b’. At a position one-quarter of the flow zone thickness from the interface, the element e-f of the unit cube has been subjected to four times the amount of strain as a’-b’ when it reaches e”‘-f”’ where the chip breaks contact with the tool. Thus, making the simplifying assumption that the strain is uniform within the flow zone, the amount of strain sustained is inversely proportional to the distance from the flow zone-tool interface. Table 1 gives an example of the amount of strain in such a flow zone at different distances from the interface [5]. The amount of strain becomes extremely high near the interface but the rate of strain remains constant; lo4 s-l in this example
57 TABLE 1 Shear strain in the flow zone according to the model in Fig. 15 (chip speed, 60 m min-I; flow zone thickness, 80 m; contact length, 1.6 mm) Distance from tool rake face Mm)
Shear strain 7 over contact length
80 40 20 10 5 2.5
20 40 80 160 320 640
in which the chip body passed over the contact area in 1.6 ms. The time for any small element of material in the flow zone to cross the contact area increases as the interface is approached but, at a distance of 2.5 pm (commensurate with the surface roughness of many tools) the time was only 55 ms. The amount of strain in the flow zone is orders of magnitude higher than in the chip body. In spite of the extreme strain, fracture in the flow zone over the contact area is inhibited by the very high compressive stress. The pearlite-ferrite structure, clearly visible in the chip body (Figs. 2 and 16), is completely destroyed in the flow zone, where no structure can be resolved with an optical microscope (Fig. 16). Transmission electron microscopy investigations of the flow zone have shown that the basic structure, in all cases studied, is equi-axed grains or cells of very small size (0.2 - 0.5 pm) containing few dislocations [6] (Fig. 17). Pearlite in the flow zone of steel chips is completely disrupted and cementite is present as dispersed particles.
Fig. 16. Flow zone in medium carbon steel chip. Fig. 17. Transmission electron micrograph of flow zone structure in steel chip; very small, equi-axed cells or grains [ 61.
58
Although this structure may not be that which was present during the cutting action, it is clear that the very rigid structure of the buiit-up edge (Fig. 8) has been superceded. The mechanism of plastic deformation has changed from dislocation movement and strain hardening to one III which recovery processes accompany dislocation movement, resulting in the formation of very small equi-axed grains and many new grain boundaries. Recent investigations of metals subjected to extreme strain [ 7, 81 have shown similar structural changes, resulting in a condition where grain boundary sliding plays an important role and plastic deformation takes place without strain hardening. In the flow zone many metals and alloys behave like very viscous liquids; unlimited deformation occurs at constant or reducing yield stress. The flow zone is a “thermoplastic instability”. As discussed above, such instabilities, occurring on the shear plane or in the built-up edge are of very short duration because they provide stress relief, after which plastic deformation reverts to a dislocation mechanism. It is the seizure condition at the interface with the rigid tool which anchors the thermoplastic instability to the tool for an indefinite period of time, as long as cutting continues. The flow zone is thus a “stable instability”, a contradictory term which suggests “thermoplastic shear band” as a preferred description. The material of the flow zone changes continually as new material is fed in at the cutting edge and leaves at the end of the contact area but the flow zone persists with a characteristic thickness, depending on the work material and cutting conditions. The behaviour of a work material in thermoplastic shear bands is probably the most important property governing its “machinability” in high speed machining. So far there have been very few studies of the influence of composition and structure of the work material on the stresses and temperatures generated in the flow zone and on its thickness. Since the flow zone is strongly bonded to the tool, making them one piece of material, the new surface generated in machining can be formed only by fracture. The fracture forming the underside of the chip may take place at the interface, as shown diagrammatically in Fig. 18(a). This is a shear fracture and forms a smooth shiny surface. It may also take place at the top of the flow zone (Fig. 18(b)) giving a rougher surface. Evidence for the formation of these surfaces comes from scanning electron microscopy studies after quick-stops. Figure 19(a) shows the under surface of a steel chip after quick-stop. The location of Fig. 19(a) is shown diagrammatically in Fig. 19(b). At the right (C -+ R) the smooth ridged surface had been formed just before quick-stop by shear fracture nucleated at C. At the left the dimpled fracture (P - C) was formed during quick-stop by largely tensile stress. The mating half of this fracture remained as a thin layer of work material adhering to the tool as shown diagrammatically in Fig. 19(b). Figure 20(a) is a scanning electron micrograph from a quick-stop after cutting steel with a steel tool coated by chemical vapour deposition. The location is shown diagrammatically in Fig. 20(b). In this example small fragments of tool material were detached at the cutting edge and remain
59
Fig. 18. Diagrams showing bcation of fractures forming under surface of chips with flow zone seized to tool. (a) New surface formed at the interface, (bf New surface formed near top of flow zone.
embedded in the quick-stop at 0. The replica of the cutting edge is on the line Q-0. The replica of the rake face is at the right U-X, the new ma&ined surface is at the left (O-N). This smooth ridged surface was formed by continuous shear fracture nucleated at the cutting edge of the tool or just below the edge. With a flow zone, the surfaces formed by machining are usually much smoother than those where a built-up edge is present (Figs. 7, 9 and 10). The machined surface is a shear fracture and a thin sheared layer a few micrometres thick remains on the surface. There is not a sharp de~eation between the built-up edge and the flow zone regimes and surfaces showing features of both are frequently seen.
5. Sliding at the tool-wurk
interface
Seizure in the contact area is responsible for the most important charaoteristic features of the machining process but, as demons~at~ in the first paper, sliding and seizure may occur simultaneously, sliding most commonly
60
(a)
Fig. 19. (a) Scanning electron micrograph of under surface of chip and fracture at contact area after quick-stop. (b) Diagram showing location of (a).
in peripheral regions of the contact area. Compressive stress is lower in these regions and the surrounding atmosphere has some access to the interface The outer edge of the chip is often segmented while the remainder of the chip is continuous, as shown in Fig. 21, which is a scanning electron micrograph of a steel chip. A section through the segmented outer edge, parallel with the direction of movement, shows that it was sliding over the tool surface by a periodic slip-stick action (Fig. 22). This sliding action is limited to a few tenths of a millimetre from the outer edge of the chip. A se&ion only 0.5 mm from the edge of the same chip shows the flow zone, characteristic of seizure, at the under surface of the chip (Fig. 23). Wear features on cutting tools in peripheral, sliding regions are often very different from those in regions of seizure. The rate of wear in sliding regions is often many times greater [9]. When cutting steel with cemented carbide or steel tools, the rate of sliding wear is sensitive to the surrounding atmosphere. It is accelerated by directing a jet of oxygen towards the cutting
61
(b)
(4
Fig. 20. (a) Scanning electron micrograph of unde r surface of quick-stop showing edge (O-O), new surface (O-N) and replica of t 001 rake surface (O-X). (b) Diagram showing location of features in (a).
Fig. 21. Scanning electron micrograph showing segmented outer edge of steel chip. Fig. 22. Section action.
through outer edge of steel chip (Fig. 21) showing stick-slip
sliding
edge of the tool and retarded by a jet of nitrogen or argon. In the seized parts of the contact area, however, the rate of wear is not influenced by the surrounding atmosphere.
6. Tool materials The observations reported above concern movement of work material over steel or cemented carbide tools, i.e. where both tool and work material are of metallic character and strong atomic bonding at the interface is predicted and confirmed by observation. Few comparable observations have
62
Fig. 23. Section through same chip, at 0.5 mm from the outer edge, showing flow zone, characteristic of seizure.
been reported when machining with other tool materials such as alumina, sialon, diamond and cubic boron nitride. Evidence has been presented by Doyle et al. (ref. 14, paper I; [lo]) in Cambridge who observed the toolwork interface by looking through transparent, polished sapphire tools during cutting. This shows sliding at the interface under some cutting conditions, particularly close to the cutting edge where seizure is most pronounced with steel and carbide tools. Using tools coated with a layer of alumina by chemical vapour deposition (CVD), however, the evidence of quick-stop specimens demonstrates seizure at the interface and movement of the chip over the tool by a flow zone mechanism [ 11, 121. CVD tool surfaces are much rougher on a micro scale than the polished sapphire and surface topography may influence seizure at the interface. So far the range of work materials and cutting conditions studied, using tools of non-metallic character, is small. Conclusions regarding the character of the interface during cutting, and the mechanism by which the work material moves over the surface of these tools in metal cutting, must await much more extensive study.
7. Conclusions The apparent contradiction of observed seizure at the tool-work interface and continuous flow of chip and work material over the tool surface, has been resolved by the observation of intense localized shear strain within the work material close to the tool surface and fracture which forms the new surfaces. This takes place by two different modes of action. (1) At relatively low speed and feed the mechanism of intense localized plastic deformation is dislocation movement, forming a highly strainhardened layer of work material seized to the tool surface. A sequence of extremely strain-hardened layers forms the built-up edge, which is a dynamic
63
structure, continuous with the main body of work material from which it is formed. The built-up edge maintains, roughly, an equilibrium form by continuous accretion of strain-hardened layers and continuous fractures which carry away some of the strain-hardened material and form the new machined surface and the under surface of the chip. (2) At higher rates of metal removal, movement over the seized interface takes place by even more intense shear strain confined to a very thin thermoplastic shear band seized to the tool surface and termed a “flow zone”. Strain is concentrated in the flow zone because, at a critical condition, strain hardening ceases and yield stress is reduced by dynamic recovery processes which occur as temperature rises, at increasing cutting speed and feed. The structures observed in the flow zone are very fine equi-axed grains or cells (0.2 - 0.5 pm) with few dislocations. Deformation in the flow zone probably involves a large element of grain boundary sliding without strain hardening. An essential feature of this mechanism is that continuous shear fracture takes place to separate the work material from the tool. The fracture is nucleated at, and propagates from the work-tool interface or very close to it within the flow zone, This fracture forms the new machined surface and the under surface of the chip. The behaviour of work materials and of their constituent phases in thermoplastic shear bands is very important in relation to machinability when cutting at high rates of metal removal. While seizure at the interface controls the mode of action in most industrial machining operations, sliding also is observed in peripheral regions of the contact area, where it it may take place by a stick-slip process. Sliding and seizure occur simultaneously at different positions on the tool-work interface. These modes of action at the interface must influence models constructed for analysis of forces, stresses, strain and energy dissipation in metal cutting. It would be very difficult to construct a satisfactory model for the built-up edge mode because of its great complexity. A model should be possible for the flow zone mode, which is of greater importance for problems where such analysis is required. Treatment of the subject as a conventional problem of frictional action must be abandoned. Under conditions of seizure there is no significant relationship of frictional to normal force corresponding to the coefficient of friction. Stress-strain relationship on the shear plane is directly related to values determined for the work material by room temperature mechanical tests at appropriate amounts and rates of shear strain. These relationships cannot be used to analyse stress and strain in the flow zone where strain hardening is absent and the work material behaves like a very viscous fluid. For this, knowledge of the stress strain-temperature relations in thermoplastic shear bands is required and few data are available. Extensive research in this area is necessary. A major factor to be studied is the temperatures generated, and the third paper in this series opens up this subject by presenting results of studies of temperature in metal cutting operations.
64
References 1 2 3 4 5 6 7 8 9 10 11 12
R. F. Recht, J. Appl. Me&., 31 (1964) 189. J. WaIIbank, Met. TechnoZ., 6 (4) (1979) 145. P. K. Wright, Ph.D. Thesis, University of Birmingham, 1971. E. M. Trent, Inst. Prodn. Eng. J., 38 (1959) 105. E. M. Trent, Metal Cutting, Butterworths, London, 2nd edn., 1984, p. 60. A. Shelboum, W. T. Roberts and E. M. Trent, Mater. Sci. Technol., 1 (1985) 220. I. Saunders and J. Nutting, Met. Sci., 18 (1984) 571. 0. D. Sherby and J. Wadsworth, Mater. Sci. Technol., I (1985) 925. E. M. Trent, Metal Cutting, Butterworths, London, 2nd edn., 1984, pp. 108, 144. P. K. Wright, Met. Technob, 8 (4) (1981) 150. J. 0. Fowler, Ph.D. Thesis, The Polytechnic, Wolverhampton, 1980. P. A. Dearnley and E. M. Trent, Met. Technol., 9 (1982) 60.