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and later, by Herbert (1928). Around this time, the cut- ting speeds were steadily improving with the arrival of new cutting tool materials, such as cemented carbide. In 1937, Piispanen introduced his so-called ‘Deck of Cards’ principle as an explanation of the cutting pro- cess (see Fig. 24 for Piispanen’s idealised model, with Fig. 25 depicting sheared chips at a range of cutting speeds). Here, Piispanen’s model depicts the workpiece material being cut in a somewhat similar manner to that of a pack of cards sliding over one another, with the free surface an angle, which corresponded to the shear angle (ϕ). So, as the tool’s rake face moves rela- tive to that of the workpiece, it ‘engages’ one card at a time, causing it to slide over its adjacent neighbour, this process then repeats itself ‘ad nitum’ – during the remainder of the cutting process. Some important Figure 25. Variations in chip morphological surfaces at dierent cutting speeds, giving an indication of the various shearing mechanisms. [Source: Watson & Murphy, 1979] . Turning and Chip-breaking Technology limitations are present with Piispanen’s model, namely that it: • exaggerates strain in homogeneity, • shows tool face friction as elastic rather than plastic in nature, • considers shearing takes place on a completely at plane, • assumes that BUE does not occur, • takes an subjectively assumed shear angle, • takes no account of either chip curling, or predic- tion of chip/tool length. NB Piispanen’s model is easily understood and does contain the major concepts in the chip-forming process – admittedly for simple shear in the main. By way of further information concerning chip mor- phology: the micrographs of chip surfaces illustrated in Fig. 26 show in these cases, that the morphology indicates a semi-continuous chip form. ese chip forms point towards the fact that noticeable periodic variations have occurred, perhaps as the result of the stress becoming unstable, rather than resulting from any vibrational eects produced by the machine tool. Any such instability, has the eect of causing minute oscillations (i.e. backward and forward motion) in the shear zone, while the machining takes place. e dierences in segment shapes shown and their fre- quency occurring at diering cutting data in these micrographs, are thought to be dependent upon the frequency of the shear plane’s oscillation relative to the cutting speed. A considerable volume of fundamental work on machining research has been undertaken over the last few years, but during World War Two (i.e. from a European perspective), Ernst and Merchant (1941) produced another signicant paper dealing with the mechanics of the machining process – some of these research ndings will be briey dealt with in the chap- ter on Machinability and Surface Integrity, along with other contributions to this subject. 2.3 Chip-Development Most metallic materials can be considered as rela- tively hard to machine and this is evident from all of the reported literature on the subject of metal cutting, indicating that shearing occurs in a concentrated re- gion between the chip and tool, this eect being de- picted schematically in Fig. 26. e overall machining process is well concealed behind a amalgamation of: workpiece material, high speeds and feeds, elevated temperatures and enormous pressures 18 . e actual cutting dynamics in contemporary machining opera- tions, utilises just a few millimetres of physical contact between the tool and the chip of a precisely-shaped cutting edge geometry in an exotic mixture of tool ma- terial to eciently machine the workpiece – this being an impressive occurrence worthy of note. In the early work on machining, it was thought that the chip was formed by deformation along a shear plane, elastically in the rst instance, then plastically as the evolving chip passed through a stress concentra- tion. e Piispanen model (i.e. Fig. 24) illustrates this point, where workpiece material is being cut by pro- gressive slip relative to the tool point, an angle which corresponded to that of the shear plane. Here (i.e. Fig. 24), it shows how each chip segment forms a small, but very thin parallelogram, with slippage occurring along its shear plane. In an orthogonal cutting process 19 , as the workpiece material approaches this ‘shear plane’ it will not be- gin to deform until it reaches the ‘shear plane’. Here, it is transformed from that of simple shear, as it moves across a thin shear zone, with the minute amount of secondary shear being virtually ignored, as is the case for tertiary shear – this being the equivalent of a slid- ing friction but having a constant coecient of fric- tion. Chip deformation in reality, is produced over a zone of nite width, usually termed the ‘primary shear zone’ (see Fig. 26). As the chip evolves, the back of the chip tends to be roughened, due to the plastic strain being inhomogeneous in nature (see Fig. 25). is shearing action creates a particular chip morphology as a result of the either, stress concentrations, or by presence of points of weakness in the workpiece be- 18 Interface pressures between the chip and the tool are nor- mally exceedingly high, typically of the order of 1,000 to 2,000 N mm –1 , with temperatures in certain instances at the tool’s face reaching approximately 1100°C. 19 Orthogonal machining, is when the cutting tool’s edge (i.e. rake face – see Fig. 19b) is presented ‘normal’ to the evolving chip and thus, to the workpiece, at 90° to the relative cutting motion. at is, little if any, side shearing action occurs, while the chip is be- ing formed as it progresses up the tool’s rake face – eectively created by two distinct cutting forces: tangential and axial. Chapter Figure 26. Schematic representation of a sing-point stock removal process, during the continuous cutting of ductile metals. Turning and Chip-breaking Technology ing machined 20 . Once the chip deformation begins, it will continue within this ‘zone’ , as though here in this vicinity, the workpiece material is exhibiting a form of negative strain-hardening. e oblique cutting process 21 presents a dierent and much more complex analytical problem, which has been the subject of a lot of academic interest over the years. Even here, the whole cutting dynamics change, when the tool’s top rake surface is not at, which is the normal status today, with the complex contoured chip- breaker geometries nowadays employed (typically il- lustrated in Figs. 4, 10 and 27a). Actual chips are normally severely work-hardened, in particular with any strain-hardening materials (for example: high-strength exotic alloys employed for heat-resistance/aerospace applications) as they evolve, by the combined action of: elevated interface tempera- tures, great pressures and high frictional eects. Such machined action of the combined eects of mechanical and physical work, produce a ‘compressive chip thick- ness’ 22 , which is on average, dimensionally wider than the original undeformed chip thickness (see Fig. 26). e rake angle depicted in Fig. 26 is shown as posi- tive, but its geometry can tend to the neutral, right through to the negative in its inclination. As the rake angle changes, so will the complete dynamic cutting behaviour also change, modifying the mechanical and 20 As the shear plane passes through a particular stress concen- tration point, it will deform more readily and at a lower stress value, than when one of these ‘points’ is not present. 21 Oblique machining, is when the rake face has a compound an- gle, that is it is inclined in two planes relative to the workpiece, having both a top and side rake to the face, creating a three- force model (see Fig. 19a), where the cutting force mathemati- cal dynamics are extremely complex and are oen produced by either highly involved equations, or by cutting simulations. is latter simulated treatment is only briey mentioned later and is outside the remit of this current book. However, this information on dynamic oblique cutting behaviour can be gleaned, from some of the more academic treatment given in some of the selected books and papers listed at the end of this chapter. 22 Compressive chip thickness is sometimes known as the: chip thickness ratio (r)* – being the dierence between the unde- formed chip thickness (h 1 ) and the width/chip thickness of the chip (h 2 ). *Chip thickness ratio (r) = h 1 /h 2 ** (i.e. illustrated in Fig. 26). ** h 2 = W/ρwl Where: W = weight of chip, ρ = density of (original) work- piece material – prior to machining, w = chip width (i.e D OC ), l = length of chip specimen. physical properties within the chip/tool region, as the various deformation zones are distinctly altered. In ef- fect, due to rake angle modication (i.e. changing the rake’s inclination), this can have a profound aect on the: cutting forces, frictional eects, power require- ments and machined surface texture/integrity. e chips formed during machining operations can vary enormously in their size and shape (see Fig. 35a). Chip formation involves workpiece material shearing, from the vicinity of the shear zone extending from the tool point across the ‘shear plane’ to the ‘free surface’ at the angle (ϕ) – see Fig. 26. In this region a consider- able amount of strain occurs in a very short time in- terval, with some materials being unable to withstand this strain without fracture. For example, grey cast iron being somewhat brittle, produces machined chips that are fragmented (i.e. termed ‘discontinuous’), con- versely, more ductile workpiece materials and alloys such as steels and aluminium grades, tend to produce chips that do not fracture along the ‘shear plane’ , as a result they are continuous. A continuous chip form may adopt many shapes, either: straight, tangled, or with dierent types of curvature (i.e. helices – see Fig. 35a). As such, continuous chips have been signicantly worked, they now have considerable mechanical strength, therefore eciently controlling and dealing with these chips is a problem that must be overcome (see the section on Chip-breaking Technology). Chip formation can be classied in a number of distinct ways 23 , these chip froms will now be briey reviewed: • Continuous chips – are normally the result of high cutting speeds and/or, large rake angles (see Figs. 26 and 27b). e deformation of workpiece mate- rial occurs along a relatively narrow primary shear zone, with the probability that these chips may de- velop a secondary shear zone at the tool/chip inter- face, caused in the main, by frictional eects. is secondary zone is likely to deepen, as the tool/chip friction increases in magnitude. Deformation can also occur across a wide primary shear zone with 23 One of the major cutting tool manufacturer classies chips in seven basic types of material-related chip formations, these are: Continuous, long-chipping – mostly steel derivatives, La- mellar chipping – typically most stainless steels, Short-chip- ping – such as many cast irons, Varying, high-force chipping – many super alloys, So, low-force chipping – such as alu- minium grades, High pressure/temperture chipping – typied by hardened materials, Segmental chipping – mostly titanium and titanium-based alloys. Chapter Figure 27. Chip-breaking inserts and chip control whilst turning – in action. [Courtesy of Iscar Tools]. Turning and Chip-breaking Technology curved boundaries, with the lower boundary being below the machined surface (Fig. 26), which may distort a soer workpiece’s machined surface – par- ticularly with small rake angles and at low speeds. Strain-hardening of this type of chip, results in it becoming harder than the bulk hardness of the original workpiece material (see Fig. 28c, where the bulk workpiece hardness is 230 H K and the work- hardened chip is ≈350 H K ). is increase in the chip’s strength and hardness will depend upon the shear strain (see Table 4 for details of the Rheololog- ical 24 status, related to the inclination of the tool’s rake angle). erefore as the rake angle decreases, the shear strain will increase, causing this con- tinuous chip to become both harder and stronger – behaving in a similar manner to that of a rigid, perfectly plastic body. In order to satisfactorily deal with long continuous work-hardened chips, that could either wrap around the machined workpiece, potentially spoiling the surface texture, or become ensnarled around tooling, or even, reduce ecient coolant delivery to the cutting edge, with integrated tool chip-breakers having been designed and devel- oped – see Fig. 27b. 24 Rheology is a branch of science dealing with both the ow and deformation of materials, with the shear strain rate, oen termed just the shear rate. (i.e usually quoted in Pascals-sec- onds ‘Pa-s’). • Continuous chips with a built-up edge (BUE) – when machining ductile workpiece materials, a built-up edge (BUE) can form on the tool’s tip. is BUE con- sists of gradually deposited material layers from the workpiece, hence the term ‘built-up’ (see Fig. 28). As cutting continues, the BUE becomes larger and more unstable, eventually partially breaking away, with some fragments being removed by the under- side of the chip, while the remainder is randomly deposited on the workpiece’s surface (Fig. 28a). is process of BUE formation, shortly followed by its destruction, is continuously repeated during the whole cutting operation. e BUE deposited on the workpiece will adversely aect the machined sur- face texture. e BUE modies the cutting geom- etry, creating a large cutting tip radius (Fig. 28a and b). Due to the BUE being severely work-hardened by the action of successive deposits of workpiece material, the BUE’s hardness signicantly increases by around 300% over the bulk component hardness (Fig. 28c). At this severely work-hardened level, the BUE becomes in eect a modied cutting tool. Nor- mally, an unstable BUE is undesirable, conversely, a thin stable BUE is as a rule, regarded as desirable, as it protects the top rake surface. e formation mechanism for the BUE is thought to be one of ad- hesion of workpiece material to the tool’s rake face, with the bond strength being a function of the af- nity of the workpiece to that of the tool material. is adhesion, is followed by the successive build- up of adhered layers forming the BUE. Yet another factor that contributes to the formation of a BUE, Table 4. Strength and hardness of chips when turning mild steel. Rake angle (γ°): 45 35 27 10 10 10 Feed (f - mm rev –1 ) 0.30 0.30 0.20 0.20 0.20 0.20 Cutting speed ( V C - m min –1 ) 50 50 168 168 168 76 Cutting uid: Soluble oil None None Soluble oil None None Tensile strength (UTS – kg mm –2 ) 75 84 91 92 93 95 Vickers hardness number (Hv) 272 289 302 320 314 325 H V /UTS 3.6 3.4 3.3 3.5 3.4 3.4 Shear strain (Pa-s) 1.1 1.7 2.1 2.9 3.1 4.0 [Source: Nakayama and Kalpakjian 1997] . Chapter Figure 28. The development of a continuous chip with Built-Up Edge (BUE), its typical hardness distribution and its aect on the machined surface . Turning and Chip-breaking Technology is the strain-hardening tendency of the workpiece material. erefore, the greater the strain-harden- ing exponent, the higher will be its BUE formation. From experiments conducted on BUE formation, it would seem that the higher the cutting speed, the less is the tendency for BUE to form. Whether this lack of BUE formation at higher cutting data is the consequence of increased strain rate, or the result of higher interface temperatures is somewhat open to debate. However, it would seem that a paradox exists, because as the speed increases, the tempera- ture will also increase, but the BUE decreases. e propensity for BUE formation can be lessened by: I. Changing the geometry of the cutting edge – by either increasing the tool’s rake angle, or de- creasing the D OC , or both, II. Utilising a smaller cutting tip radius, III. Using an eective cutting uid, or IV. Any combination of these factors. • Discontinuous chips – consist of adjacent work- piece chip segments that are usually either loosely attached to each other, or totally fragment as they are cut (Fig. 29). e formation of discontinuous chips usually occur under the following machining conditions: I. Brittle workpiece materials – these materials do not have the machining capability to un- dergo the high shear strains, II. Hard particles and impurities – materials with these in their matrix, will act as ‘stress-raisers’ and actively encourage chip breakage, III. Very high, or low cutting speeds – chip veloc- ity at both ends of the cutting spectrum, will result in lack of adherence/fragmentation of the chip segments, IV. Low rake angles/large D OC ’s – either small top rakes and heavy D OC ’s will decrease the adher- ence of the adjacent chip segments, V. Ineective cutting uid – poor lubricity, com- bined with a meagre wetting ability, will en- courage discontinuity of chip segments, VI. Inadequate machine tool stiness – creating vibrational tendencies and cutting instability, leading to disruption of the machining dynam- ics and loosening of chip segments. As mentioned in ‘Roman II’ above, the hard particles and impurities tend to act as crack nucleation sites, therefore creating discontinuous chips. Large D OC ’s increase the probability that such defects occur in the cutting zone, thereby aiding discontinuous chip for- mation. While, faster cutting speeds result in higher localised temperatures, causing greater ductility in the chip, lessening the tendency for the formation of dis- continuous chips. If the magnitude of the compressive stresses in the both the primary and secondary shear zones signicantly increase, the applied forces aid in discontinuous chip formation, this is because of the fact that the maximum shear strain will increase, due to the presence of an increased compressive stress. NB Due to the nature of discontinuous chip forma- tion, if the workpiece-tool-machine loop is not su- ciently sti, this will generate vibrational and chatter tendencies, which can result in an excessive tool wear regime, or machined component surface damage. • Segmented chips – are sometimes termed: in-, or non-homogeneous chips, or serrated chips. is chip form has the characteristic saw-toothed pro- le which is noted by zones of low and high shear strain (Fig. 30). ese workpiece materials possess low thermal conductivity, as such, when machined their mechanical strength will drastically decrease with higher temperatures. is continuous thermal cycle of both fracture and rewelding in a very nar- row region, creates the saw-toothed prole, being particular relevant for titanium and its alloys and certain stainless steel grades. For example, to ex- plain what happens in realistic machining situation, the specimen Fig. 30a is displayed, for an austenitic stainless steel quick-stop micrograph. is micro- graph being the result of a less than continuous ma- chining process (1), utilising a 5° top rake-angled turning insert. Here, variations in the cutting pro- cess have created uctuations in the cutting forces, resulting in waviness of the machined surface (2). Prior to the material yielding, then the shearing process occurring, the workpiece material has de- formed against the cutting edge (3). To explain how changing the top rake angle inuences the resul- tant chip formation for an identical stainless steel workpiece material, Fig. 30b is shown. Machining has now been undertaken with a 15° top rake, pro- moting a more continuous machining process than was apparent with the 5° tool (i.e. illustrated in Fig. 30a). is more ecient cutting process, results in smaller variations in the cutting forces (1 and 2). e chip is seen to ow over the rake face in a more Chapter Figure 29. Discontinuous chip formation. [Courtesy of Sumitomo Electric Hardmetal Ltd.]. Turning and Chip-breaking Technology consistent manner (3). It was found with this work- piece material in an experimental cutting proce- dure, that the tangential cutting force component, was closer to the actual cutting edge than when similar machining was undertaken on unalloyed steel specimens. NB e cutting data for machining the stain- less steel specimens in Figs. 30 a and b, were: 180 m min –1 cutting speed, 0.3 mm rev –1 feedrate, 3 mm D OC . 2.4 Tool Nose Radius e insert’s nose radius has been previously mentioned in Section 2.1.6, concerning: Cutting Tool holder/In- sert Selection. Moreover, the top rake geometry of the cutting insert will signicantly aect the chip forma- tion process, particularly when prole turning. In Fig. 31a, a spherically-shaped component is being ‘prole machined’ using a large nose-radiused turning insert. Here, as the component nears its true geometric cur- vature, the cutting insert forces will uctuate continu- Figure 30. Segmented chip formation, resulting from machining stainless steel and the work-hardening zone – which is aected by the sharpness of the insert’s edge. [Courtesy of Sandvik Coromant] . Chapter . mm –2 ) 75 84 91 92 93 95 Vickers hardness number (Hv) 272 289 30 2 32 0 31 4 32 5 H V /UTS 3. 6 3. 4 3. 3 3. 5 3. 4 3. 4 Shear strain (Pa-s) 1.1 1.7 2.1 2.9 3. 1 4.0 [Source: Nakayama and Kalpakjian 1997] . . titanium and titanium-based alloys. Chapter Figure 27. Chip-breaking inserts and chip control whilst turning – in action. [Courtesy of Iscar Tools]. Turning and Chip-breaking Technology. the formation of a BUE, Table 4. Strength and hardness of chips when turning mild steel. Rake angle (γ°): 45 35 27 10 10 10 Feed (f - mm rev –1 ) 0 .30 0 .30 0.20 0.20 0.20 0.20 Cutting speed ( V C
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