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ponent’s strength and the mechanical strength of the chip, illustrating that a complex metallurgical and cut- ting tool geometric relationship exists whilst machin- ing occurs. In particular for turning operations, a convention- ally-turned chip is a rather frail product of serrated appearance (see Figs 25 and 34a and b). In order to promote good chip-breaking tendencies, thus enabling short elements to be formed, it is necessary to encour- age this basic character by causing these serrations to be as deep as possible and the chip sections in between to be rigid. is chip occurrence causes the chip to be inexible, which can then subsequently be broken. ere are several distinct ways in which chips can then be broken, these include: • Self-breaking – this is when the chip’s mechanical strength is not great enough to hold the chip seg- ments together and they consequently break upon exiting the machining region (Fig. 31a), • Chip collision with the workpiece – as the chip is steered towards an obstacle such as the workpiece’s surface this provides the breaking force (see Figs. 33 and 34b), • Chip is stopped by the tool – here the chip-curling behaviour comes into play, this being a function of the: tool’s nose radius geometry, depth of cut and feedrate employed (see Fig. 34 bottom le-hand photograph), the latter two functions aecting the chip cross-section, or chip thickness 31 . 31 Chip thickness is inuenced by the plan approach angle utilised and the D OC , in association with the selected feedrate. e chip thickness is measured across the cutting edge, per- pendicular to the cut (i.e. along the main cutting edge). e chip width and thickness are the dimensions that dene the theoretical cut of the edge into the workpiece material. Hence, the chip thickness will vary with the size of the plan approach angle according to the relationships involving: feedrate, D OC and the eective cutting depth. e chip thickness is related to the plan approach angle and this aects the amount of pressure bearing upon the cutting edge. Hence, the thinner the chip, the smaller the distributed pressure along the edge and the less power consumed, conversely, the thicker the chip, the greater will be the machine tool’s power consumption. A thicker chip is generally advantageous for an increased tool life, because of the improved contact between the chip and its cutting edge. Furthermore, if the plan approach angle is too small and chip thickness is thin, this will reduce tool life, however, this can be compensated for by increasing the feedrate, to produce a thicker chip. NB e helical formation of this chip-curling behav- iour will shortly be mentioned, but prior to this, chip- breakers/formers will be discussed. .. Chip-Breakers and Chip-Formers Chip-breakers have been utilised on turning tools for many years, initially introduced in the 1940’s in the form of an abutment, or step, situated behind the rake face of the tool. Hence, with this type of early chip- breaker, as the continuous chip moves across the rake face it collides with this step and breaks. is origi- nal form of chip-breaker geomtery was relatively in- ecient as the resultant force direction changed with the programmed tool path, this meant that the step would be approached by the chip from diering di- rections making chip-breaking less controlled. Such chip-breakers were superseded in the 1970’s by in- built ‘wavy-shaped’ chip-breakers sintered into the in- sert’s top face (Fig 34 bottom le-hand photograph). Recent developments in designing chip-breaker geom- etries by computer-generated (i.e. CAD) techniques, has shown a signicant step-forward in both chip- former design enabling chip control and reduction in frictional forces across the rake face at a range of cut- ting data to be achieved. Such ‘automatic’ chip breaker geometry forces the chip to deect at a narrower angle, causing it to break o, either immediately, or just aer the free end of the chip has hit either the tool’s ank or, the workpiece before the rst coil has formed. If such a collision does not take place, the result would be a smaller diameter spiral chip and, it can be anticipated that the chip would still break, but only when it be- came slightly longer – this later chip breakage is due to the increasing chip mass and the eect of gravity upon it, with, or without any further collision. Chip ow direction will depend upon several fac- tors, such as the: chip-breaker prole, back rake and setting angles, nose radius, D OC and feedrate – these latter three factors require further discussion. e relationship between the nose radius, D OC and feedrate will oen change during vectored tool paths in any machining operation. Even though the insert’s nose radius is preset, its inuence on the chip direction diers for dierent D OC ’s, depending on how much corner rounding is represented by the total engaged edge length (Fig. 34c). Further, the feedrate also af- fects the chip thickness: at dierent D OC ’s and with a constant feedrate, the form of chip cross-section (i.e. Turning and Chip-breaking Technology Figure 34. The principles of chip-breaking and chip-breaking envelopes for ‘coma-shaped swarf’ control and insert edge preparations . Chapter the ratio of chip width-to-thickness), will change and this has a deleterious eect on the insert’s chip-break- ing ability. .. Helical Chip Formation Conventional Turning For the general turning operations, such as sliding (i.e. Z-axis tool feeding) and facing (i.e. X-axis tool path motions), the chip is rolled into a helix, simply because the chip edges are formed from dierent rotation radii (Fig. 34d). Here, the two edges of the chip consume dierent quantities of workpiece material, creating dif- fering edge lengths, coupled to the fact that a varia- tion in cutting speed is present, these relationships will result in a helical chip formation. e appearance of the chip’s helix depends upon the workpiece’s diameter and its metallurgical specication/condition, which means the chip helices are extremely dicult to quan- tify. Most common types of helical chip diameters are determined either directly by the initial curvature from its origin, or are the result of additional bending, introduced by the chip-breaker. For example, the heli- cal chip type shown in Fig. 34c (le), has its chip seg- ments turned inwards, this being a desirable chip form when not fully developed, that is prior to the rst coil being completed. Whether, or not the chip is of this form will already be determined even before it meets the chip-breaker, this being the result of its cross-sec- tion and the natural tendency to bend according to the ‘line of least resistance’. If the chips width is no larger than its thickness, for example, the resistance to bend- ing in the segment-stiened thickness direction is larger than in the width direction. In this case, unless this kind of chip is broken early, by colliding with ei- ther part of the tool, or the workpiece whilst it is still sti and short – called ‘self-breaking’ – a helical chip will be formed. In this case, the barbed, or serrated edge is turned outwards causing additional bending, this being introduced by the chip-breaker. For exam- ple, the helical chip type shown in Fig. 34c (right), becomes dicult and awkward to control. is out- ward-curving helical chip also has weakened sections in the serrations between the chip segments, but ap- plied loads on it are readily absorbed by the spring ac- tion of the chip. is type of chip will break as it hits the insert’s ank face (see Fig. 27b) 32 . Only today’s very complex chip-breaker designs can reduce these out- ward-curling helical chips. Although such chip helices produced by combinations of the feeds and D OC ’s that result in the chip width being too small in relation to its thickness must be avoided. Grooving and Recessing In conventional turning operations, it is signicantly easier to form a manageable chip, than for features re- quiring either grooving, or recessing. e chip formed during plunge grooving counter-rotates in relation to the workpiece, whereby it does not experience the same twisting force as chips produced by either Z-, or X-axes turning operations. When grooving, ideally the chip resembles a ‘watch spring’ , indicating that the chip is curling back onto itself and will ultimately break in several distinct ways: such as at the completion of the grooving cycle, or due to friction between the chip and its groove side walls – as the chip diameter becomes greater. In grooving operations, three signicant fac- tors aect chip control, these are: (i) Insert geometry – applied to the rake face, can be classied into distinct groupings: • Radial-ground top rake (not shown), producing the desired ‘watch-spring’ chip formation. is grooving insert geometry will not thin the chip, therefore surface nish passes are necessary on both groove side walls. NB For long-chipping materials the chip-former does not provide enough resistance to produce chip curling, hence, a straight at chip occurs, that may 32 One of the problems with this type of chip-breaking, is the potential for secondary wear on the insert’s non-cutting zone on the face, caused by the chip helix breaking locally against this face. Such an occurrence happens when the chip helix at- tains such a diameter and pitch that its free-end continually strikes the non-cutting portion of the insert’s edge – termed ‘chip-hammering’ – causing the edge to be locally weakened and to subsequently crumble. NB Chip-hammering can be alleviated by slightly increasing the helix diameter (i.e. by somewhat modifying the cutting data) causing the chip to break against the tool’s ank – be- low the insert’s cutting edge, this being one of the previously employed and favoured chip-breaking mechanisms, as shown in Fig. 27b. Turning and Chip-breaking Technology Figure 35. The chip-breaking envelopes related to cutting data and chip-curling behaviour. [Courtesy of Sandvik Coromant] . Chapter wrap itself around either the tool, or workpiece, but such a geometry is perfect for machining alumin- ium, or non-ferrous materials. • Radial top rake (illustrated in Fig. 4 middle and to the le – three grooving insert sizes illustrated). is radial top rake is designed to thin the chip. Such chip thinning, eliminates the need to under- take nishing passes on the groove’s side walls. Fur- thermore, this type of grooving insert geometry be- ing on-centre, enables axial turning of diameters for wide shallow grooves 33 , or recesses. • Raised bumps on top rake (see Fig. 27a – le). is sophisticated grooving geometry is utilised for materials where chip control is dicult, as it pro- vides an ‘aggressive barrier’ to the curling chip. e raised bumps force the chip back onto itself, either producing a tightly curled watch-spring chip, or causes the chip to break. (ii) Surface speed of the workpiece – in order to ob- tain full advantage of a grooving insert’s chip-form- ing abilities, the chip must be allowed to ow into the chip-former. is chip-ow can be achieved by either decreasing the workpiece’s surface speed, or increasing the feed – more will be said on this shortly. e former technique of decreasing the surface speed, allows the material to move slower across the top rake of the cut- ting edge and as a result, has greater contact time to engage the chip-former. is slower workpiece speed, has the benet of increasing tool life, through lower 33 A groove, or recess, can normally be considered as a straight- walled recessed feature in a workpiece, as illustrated in Fig. 40. Typical applications for grooves are to provide thread re- lief – usually up to a shoulder – so that a mating nut and its washer can be accurately seated , or for retaining O-rings. As the groove is produced in the workpiece, the tool shears away the material in a radial manner, via X-axis tool motion. e chip formed with insert geometries having a at top rake, will have an identical width as the tool and can be employed to ‘size’ the component’s width feature. However, this chip action – using such a tool geometry, creates high levels of pressure at the cutting edge as a result of the chip wall friction, which tends to produce a poor machined surface texture on these sidewalls. Grooving with an advanced chip-former insert ge- ometry, reduces the chip width and provides an ecient cut- ting action, this results in decreasing the cutting edge pressure somewhat. Chip-formers oer longer tool life and improved sidewall nishes with better chip control, than those top-rakes that have not incorporated such insert chip-forming geomet- ric features. tool/chip interface temperatures. e negative factors of such a machining strategy, are that the: • Part cycle times are increased and as a result, any batch throughput will be lessened, • As the cutting edge is in contact for a longer du- ration, more heat will be conducted into the tool, than into the chip, which could have a negative im- pact of inconsistent workpiece size control, • Due to the lower workpiece surface speed, the ben- ets of the insert’s coating will be reduced, as such coating technology tends to operate more eec- tively at higher interface temperatures. (iii) Increasing the feedrate – by increasing the feed allows it to engage the chip-former more eectively – this being the preferred technique for chip control. A heavier applied feedrate, produces a chip with a thicker cross-section. Further, a thicker chip engages the in- sert’s geometry with higher force, creating a greater tendency to break. Hence, by holding a constant work- piece surface speed, allows the faster feedrate to reduce cycle times. Transversal, or Face Grooving Transversal grooving geometry has a curved tear- shaped blade onto which, the insert is accurately lo- cated and positioned. e transversal insert follows the 90° plunged feed into the rotating face of a work- piece. ese tools are categorised as either right-, or le-hand, with the style adopted depending upon whether the machine tool’s chuck rotates anti-clock- wise (i.e. using a right-hand tool), or clockwise (i.e. le-hand). e minimum radius of curvature for such transversal grooving tooling is normally about 12mm, with no limit necessary on the maximum radial curva- ture that can be machined. For shallow face grooves, o-the-shelf tooling is available, but for deep angular face grooves they require specialised tools from the tooling manufacturers. If a relatively wide face groove requires machining with respect to the insert’s width, then the key to suc- cess here, is establishing where in the face to make the rst plunge. is initial face plunge should be made within the range of the tool’s diameter, otherwise the tool will not have sucient clearance and will ulti- mately break. Successive plunges to enlarge the face groove should be made by radially moving the insert 0.9 times the insert’s width, for each additional plunge. e rotational speed for face grooving is usually about 80% of the speed used for parting-o – soon to be Turning and Chip-breaking Technology mentioned. Feedrates are normally around 50% of parting-o values, with the proviso that for material which is subject to work-hardening, minimum feeds are necessary. In transversal grooving operations, a unique chip form occurs, because the chip is longer the further away it is from the workpiece’s centre line of rotation. is results in the chip which no longer ows in a straight line across the insert’s edge, instead it moves at an angle. Such a naturally curved chip is dicult to exhaust from the face groove, particularly if it is bro- ken. Hence, no attempt should be made to break the chip. For deep and narrow grooves, the best solution is to retract the tool at short intervals, to check that the blade shows no signs of rubbing, this is to guard against any likely breakage that might occur when machining outside the blade’s range. Due to the fact that transversal grooving tooling is susceptible to chat- ter 34 , any excessive overhang of the tool should be mi- nimised. e chip should never be allowed to become entangled within the transversal groove and should be ejected speedily, otherwise the tool is likely to break. 34 Chatter is a form of self-excited vibration and such vibrations are due to the interaction of the dynamics of the chip-removal process, together with the structural dynamics of the machine tool. Such chatter, tends to be at very high amplitude, which can result in either damage to the machine tool, or lead to pre- mature tool failure. Typically, chatter is initiated by a distur- bance in the cutting zone, for several reasons, such as: Lack of homogeneity – in the workpiece material (i.e. typi- cally a porous component, such as is found in a Powder Metallurgy compact), Workpiece surface condition (i.e. typically a hard oxide scale on a hot-rolled steel component, utilsing a shallow D OC ), Workpiece geometry (i.e. if the component shape produces either a variation in the D OC – for example, because of un- even depth of casting material being machined, or light cuts on interrupted shapes, such as hexagon, square, or rectan- gular bar stock), Frictional conditions (i.e. tool/chip interface frictional variations, whilst machining). Regenerative chatter is a type of self-excited vibration, result- ing from the tool cutting a workpiece surface that has either signicant roughness, or more likely the result of surface dis- turbances from the previous cut. ese disturbances in the workpiece surface, create uctuations in the cutting forces, with the tool being subjected to vibrations with this process continuously repeating, hence the term ‘regenerative chatter’. Self-excited vibrations can be alleviated by either increas- ing the dynamic stiness of the system, or by increasing the damping. NB Dynamic stiness can be dened as the ratio of the am- plitude of the force to the vibrational amplitude. – – – – For any face grooving of workpiece material that is subject to a continuous chip formation, always use copious amounts of coolant and at high-pressure – if possible, to not only lubricate the cutting zone, but to aid in chip ushing from this groove. Parting-off e parting-o process is normally considered to be a separate machining operation, but it simply consists of cutting a groove to centre of rotation of the workpiece, to release it from the bar stock, or to ‘part-o ’ to a pre- viously formed internal diameter (shown in Fig. 40 for le-hand side operations). Essentially in a parting-o operation, two time-periods are worthy of mention, these are: (i) At separation from the bar stock – a lower spindle speed than was previously used on the workpiece, will prevent the ‘released part’ from hitting the machine and potentially damaging its surface. Moreover, it al- lows an operator – if present – to hear the change in the lower spindle speed tone, as it is about to separate from the bar stock, avoiding the parting-o tool from getting ‘pinched’ between the stock and the soon- to-be-released component. Oen, ‘Part-catchers’ are utilised to reduce any surface damage to the falling component, once it has been parted-o. NB If the component to be parted-o is held in a co- axial/sub-spindle, at component release, the additional spindle supports the workpiece and under these con- ditions, the parting-o operation is virtually identical to that of found in a grooving cycle. (ii) Surface speed reduction – this eectively oc- curs when the machine’s spindle attains its maximum speed. For example, on a machine tool having a maxi- mum speed of 3,000 rpm, 90 m min –1 would only be achievable until the parting diameter has reached about 8.6 mm. When parting to a smaller diameter than 8.6 mm, the surface speed would decrease at a xed spindle speed. As the parting diameter reaches 5.8 mm the surface speed would be 55 m min –1 , or 60% of the ideal, thus signicantly increasing the chip load- ing as the tool approaches the workpiece’s centreline. In order to alleviate the increasing tool loading, lower- ing the feedrate by about 50% until separation is just about to occur, then nally dropping the surface speed to almost zero at this point, reduces the tendency for a ‘pip’ to be present on the workpiece. On a CNC driven spindle, it is not advisable for parting-o operations, Chapter to utilise the ‘canned cycle’ such as the ‘constant surface speed’ 35 function. NB A more serious parting-o problem has been that in order to eliminate the pip formed at the centre of the ‘released component’ , some tools have been ground with the front edge angle of between 3° to 15°. Such a front edge geometry, can introduce an axial cutting force component, leading to poor chip control, which in turn, causes the tool to deect. is parting-o tool deection, can lead to the component’s face ‘dishing’ , creating a convex surface on one face and a concave surface on the other – so this tool grinding strategy should be avoided. Today, parting-o inserts normally consist of two main types with top rakes that are either of, negative, or positive cutting edge chip-forming geometries. e negative-style of chip-formers are possibly the most commonly utilised. ese inserts have a small nega- tive land at the front edge which increases the insert’s strength, giving protection in adverse cutting condi- tions, such as when interrupted cutting is necessary during a parting-o operation. e land width – oen termed a ‘T-land’ , is relative to the breadth of the part- ing-o tool. is width of the insert’s land has a direct correlation to the feedrate and its accompanying chip formation. e feedrate must be adequate to force the workpiece material over the land and into the chip- former 36 . Notwithstanding the widespread usage of negative parting-o tooling, positive-style insert geometries have some distinct advantages. e chief one being the ability to narrow the chip at light feedrates, with mini- 35 ‘Constant surface speed’ CNC capability as its name implies, allows the machine tool to maintain a constant surface speed as the diameter is reduced. e main problem with using this ‘canned cycle’ , is that as the maximum spindle speed is reached, the chip load will also increase. is is not a prob- lem, so long as the maximum speed has not occurred, such as when parting-o a component with a large hole at its centre. 36 Parting-o operations that employ a negative-style insert (i.e. with a land and accompanying chip-former), normally have the feedrate determined in the following manner: by multi- plying the width of the insert by a constant of 0.04. For ex- ample, for a 4 mm wide tool, it is necessary to multiply the insert’s width of 4 mm by 0.04 to obtain a feedrate of 0.16 mm rev –1 . is will give a ‘start-point’ for any parting-o opera- tions, although it might be prudent to check this feedrate is valid, from the tooling manufacturer’s recommendations. mal tool pressure. If excessive tool pressure occurs, this can promote work-hardening of the ‘transient surface’ 37 of the workpiece. ese abilities are impor- tant points when machining relatively low mechanical strength components, which might otherwise buckle if machined with negative-style inserts when subse- quently parted-o. Positive cutting edge parting-o tooling having chip-formers, are ideal for applications on machine tools when either low xed feedrates are utilised, or if the workpiece material necessitates lower cutting speeds. is positive-style of parting-o tooling, oper- ates eciently when machining soer workpiece mate- rials, such as: aluminium-or, cooper-based alloys and many non-metallic materials, typically plastics. Feed- rates can be very low with these positive-type part- ing tools, down to 0.0254 mm rev –1 with exceptional chip control and consistent tool life. One major dis- advantage of using these positive tooling geometries for parting-o, is that the tool is much weaker than its equivalent negative geometry type. e concept of insert self-grip in its respective tool- holder, was developed by the cutting tool manufac- turer Iscar tools in the early 1970’s and has now been adopted by many other tooling manufacturers (Fig. 40 top le-hand side). ese ‘self-grip’ tooling designs, rely on the rotation of the part and subsequent tool pressure to keep the ‘keyed and wedged’ insert seated in its respective toolholder pocket. Previously, double- ended inserts termed ‘dogbones’ , were oen used but were limited to low D OC ’s – due to the length of the secondary cutting edge, so have been somewhat over- shadowed by the ‘self-grip’ varieties of parting-o tooling. .. Chip Morphology The Characterisation of Chip Forms (Appendix 2) In the now withdrawn ISO 3685 Standard on Ma- chinability Testing Assessment, of some interest was the fact that this Standard had visually characterised 37 Transient surfaces are those machined surfaces that will be removed upon the next revolution of either the: Workpiece (i.e in rotating part operations), or Cutter (i.e. for rotating tooling – drilling, milling, reaming, etc.). – – Turning and Chip-breaking Technology chip forms under eight headings, with several varia- tions appearing in each groups (i.e. see Appendix 2 for an extract showing these chip form classications). Although in the main, the chip forms were related to turning, some of these chip morphologies could be ex- trapolated to other manufacturing processes. e chip type that will be formed when any machining opera- tion is undertaken is the product of many interrelated factors, such as: • Workpiece material characteristics – will the mate- rial that forms the chip signicantly work-harden?, • Cutting tool geometry – changing, or modifying the cutting insert geometries 38 and its plan approach angles will have a major inuence on the type of chip formed, • Temperatures within the cutting zone – if high, or low temperatures occur as the chip is formed, this will have an impact on the type of chip formed, • Machine tool/workpiece/cutting tool set-up – if this ‘loop’ is not too rigid, then vibrations are likely to be present, which will destabilise the cutting process and aect the type and formation of chips produced, • Cutting data utilised – by modifying the cutting data: feeds and speeds and D OC ’s, with the insert ge- ometry maintained, this can play a signicant role in the chip formed during machining operations. NB Chip formation has become a technology in its own right, which has shown signicant devel- opment over the last few decades of machining ap- plications. As has been previously mentioned, chip formation should always be controlled, with the resultant chips formed being broken into suitable shape formation, such as ‘spirals and commas’ , as indicated by the re - sultant chip morphology shown in Fig. 35a. Uncon- trolled chip-steaming (i.e. long continuous workpiece strands), must be avoided, being a signicant risk-fac- tor to both the: machine tool’s operation and its CNC setter/operator alike. 38 Chip-breaking envelopes (see Fig. 34 middle right), are the product of plotting both the feedrate and D OC on two axes, with their relative size and position within the graphical area being signicantly aected by the cutting insert’s geometry – as depicted by the three cutting insert geometric versions shown by types: A, B and C (Fig. 34). For every cutting insert geometry, there is a recom- mended application area – termed its ‘chip-breaking envelope’ (i.e. see footnote 38 below) – with regard to its range of feedrates and D OC ’s. Within this ‘envelope’ , chips of acceptable form are produced by the cut- ting insert’s geometry. Conversely, any chips that are formed outside this ‘envelope’ are not acceptable, be- cause they are either formed as unbroken strands, or are too thick and over-compressed. When component proling operations are necessary (Fig. 31a), this nor- mally involves several machining-related parameters: variations in D OC ’s, together with path vectoring of the feeds and as a result of this latter point, changes to the resultant chip’s path on the rake face. ese factors are important as they can modify the chip morphology when proling operations include: recessed/undercut shoulders, tapers and partial arcs, facing and sliding operations with the same tool, together with many other combined proled features. All of these opera- tions make signicant demands on the adaptability of the cutting insert’s geometry to eciently break the chip. In general, the cutting insert’s chip formation prin- ciples are concerned with the chip-breaker’s ability to create a chip form that is neither not too tight a curl, nor too open. If chip curling is too tight for the specic machin- ing application, the likely consequences are for a chip form creating: • ‘Chip-streaming’ – producing long chip strands that are undesirable, wrapping itself around the machined surface of the workpiece with work- hardened swarf and possibly degrading this ma- chined surface, or may become entangled around the various parts of the machine tool, which could impede its operation, • Excessive heat generation – this can decrease tool life, or be conducted into the machined part and consequently may aect specic part tolerances for the individual part, or could lead to modications in the statistical variability 39 of a batch of parts, 39 Statistical variability in component production can cause variations from one part to another, as the standard deviation and mean changes, these important factors will be mentioned later in the text. Chapter • Increased built-up edge (BUE) formation – which through ‘attrition wear’ 40 may cause the risk of pre- mature cutting edge failure. When the chip curling is too open, this may result in the following negative tendencies: • Poor chip control – creating an inecient chip- breaking ability by the cutting insert, • Chip hammering – breaking down the edge and causing it to crumble and as a result creating the likelihood of prematurely failing, • Vibrational tendencies – aecting both the ma- chined surface texture and shortening tool life. Chip formation and its resultant morphology, is not only aected by the cutting data selected, but will be inuenced by the plan approach (i.e. entering) angle of the insert. In most machining operations, they are usually not of the orthogonal, but oblique cutting in- sert orientation, so the aect is for the entering angle to modify the chip formation process. e insert’s en- tering angle aects the chip formation by reducing the chip thickness and having its width increased with a smaller angle. With oblique cutting geometry, the chip formation is both ‘smoother and soer’ in operation as the plan approach angle tends toward say, 10° to 60°, furthermore, the chip ow direction will also advanta- geously change with the spiral pitch increasing. As the nose radius is changed with dierent cutting inserts, this has the eect of changing both the direc- tion and shape of the chips produced. is nose radius geometry is a fundamental aspect in the development of chips during the machining process – as depicted by Fig. 35b. Here, an identical nose radius and feedrate is utilised, but the dierence being the D OC ’s, with a shallow D OC in Fig. 35b (le), giving rise to a slow chip helix, whereas in Fig. 35b (right) the D OC is somewhat deeper, creating a tighter chip helix which is bene- cial to enhanced chip-breaking ability. Shallow cutting depths produce ‘comma-shaped’ chip cross-sections, 40 Attrition wear is an unusual aspect of tool wear, in that it is the result of high cutting forces, sterile surfaces, together with chip/tool anity, creating ‘ideal’ conditions for a pressure welding situation. Hence, the BUE develops, which builds-up rapidly and is the ‘swept away’ by the chip ow streaming over the top rake’s surface, taking with it minute atomic surface lay- ers from the tool’s face. is continuous renewal and destruc- tion of the BUE, enhances crater wear formation, eventually leading to premature cutting edge failure. having a small angle when compared to the cutting edge. Equally, a larger depth means that the nose ra- dius has somewhat less aect from its radius and greater inuence by the entering angle of the cutting edge, producing an outward directed spiral. Feedrate also aects the width of the chip’s cross-section and its ensuing chip ow 41 . Chip formation begins by the chip curving, this be- ing signicantly aected by combinations of the cut- ting data employed, most notably: feedrate, D OC , rake angle, nose radius dimensions and workpiece condi- tion. A relatively ‘square’ cross-sectional chip nor- mally indicates that an excessively hard chip compres- sion has occurred, whilst a wide and thin band-like chip formation is usually indicative of long ribbon-like chips producing unmanageable swarf. If the chip curve is tight helix, coupled to a thick chip cross-section, this means that the length of the chip/tool contact has increased, creating higher pressure and deformation. It should be noted that excessive chip cross-sectional thickness, has a debilitating eect on any machining process. By careful use of CAD techniques coupled to FEA to construct the insert’s cutting edge, comma- shaped chips are the likely product of any machining, providing that the appropriate cutting data has been selected. In some machining operations, chip forma- tion can be superior using a slightly negative insert rake angle, thereby introducing harder chip compres- sion and self-breaking of the chip, particularly if utilis- ing small feeds. Conversely, positive rakes can be give other important machining advantages, depending which chip form and cutting data would be the most advantageous to the part’s ensuing manufacture. Usu- 41 Chip-ow is the result of a compound angle between the chip’s side- and back-ow. e chip’s side-ow being a measure of the ow over the tool face (i.e. for a at-faced tool), whilst back- ow establishes the amount of chip-streaming into the chip- breaker groove. Detailed analysis of chip side-ow (i.e. via high-speed photography), has indicated that it is inuenced by a combination of groove dimensions and cutting data. If the feedrate is increased, this results in a higher chip back- ow angle, promoting chip-streaming into the chip-breaker groove. e ratio of feed-to-length of restricted contact has been shown to be an important parameter in the determina- tion of chip- back-ow. Typically with low feedrates the cor- responding chip back-ow is going to be somewhat lessened, resulting in poor chip-breaker utilisation. When the restricted contact between the chip and the tool is small – due to low feed – the chip-ow does not fully engage the chip-breaker and will as a result curve upward, with minimal ‘automatic’ chip-breaking eect. Turning and Chip-breaking Technology . across the cutting edge, per- pendicular to the cut (i.e. along the main cutting edge). e chip width and thickness are the dimensions that dene the theoretical cut of the edge into the workpiece. are the product of plotting both the feedrate and D OC on two axes, with their relative size and position within the graphical area being signicantly aected by the cutting insert’s geometry. operations with the same tool, together with many other combined proled features. All of these opera- tions make signicant demands on the adaptability of the cutting insert’s geometry to eciently
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