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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 Figure 31. The cutting insert’s tool nose radius when either proling, or general turning, will modify both the prole and diameter as ank wear occurs. [Courtesy of Sandvik Coromant] . Turning and Chip-breaking Technology ously as the insert progresses (i.e circular interpolates with the X- and Z-axes of the machine tool) around the curved prole. If the geometry of the tool was not itself of round geometry, then the ‘point-contact’ could not be maintained, leading to signicant variations in chip formation. If this lack of tool-work contact were not to occur, then the machined prole would be compromised and due to insucient chip control, the actual cut surface prole would not have a consistent and accurate surface texture. e machined surface texture generated by the pas- sage of the cutting insert’s geometry, is to a large extent the product of the relationship, between the nose ra- dius and the feedrate and, to a lesser degree the cutting speed and its tool wear pattern. e size of the tool nose radius will have quite an eect on the surface tex- ture produced, if the feedrate is set, then a small nose radius will create a dierent workpiece surface texture to that of a larger one (see Fig. 31b). Moreover, if a large nose radius is selected for a lighter D OC , or if the feed is equal to the nose radius, then this larger nose geometry will be superior to that of a smaller tool nose radius. is is because the ‘larger nose’ oers a smaller plan approach angle, having the pressure of the cut distributed across a longer cut length, creating an en- hanced surface texture. ere are several disadvan- tages to utilising a larger tool nose radius geometry, these are that the: • Chip formed becomes more dicult to bend and eectively break, • Radial cutting forces are greater, • Power consumption increases, • Rigidity of the set-up is necessary – leading to pos- sible vibrational tendencies on either weaker, or unstable workpieces. Tool wear (i.e. denoted by ‘∆’ in Figs. 31ci and cii) and in particular ank wear 25 , can signicantly inuence the resulting machined component dimensional accu- racy (Fig. 31cii), which on a batch of components cut with the same insert, will result in some level of ‘tool 25 Flank wear is normally denoted by specic ‘zones’ – more will be said on this topic later – but, in this example, the tool’s in- sert wear ‘V B ’ is shown in both Figs. 31ci and cii. dri’ which could aect the process capability 26 of the overall parts produced. is ank wear ‘V B ’ can be cal- culated and utilised to determine the anticipated tool’s life (ie, in-cut), this important factor in production machining operational procedure, will be discussed in due course. Wiper blades (Fig. 32) are not a new insert geom- etry concept, they have been used for face milling op- erations for quiet a long time, but only in recent years are they being utilised for component nish turning. e principle underlying a wiper insert for turning op- erations, concerns the application of a modied ‘tool nose radius’ (see Fig. 32 – bottom le and right dia- grams). When a ‘standard’ tool nose geometry insert is used (i.e. Fig. 32 – bottom le), it creates a series of peaks and valleys (i.e. termed ‘cusps’ 27 ) aer the pas- sage of the ‘insert nose’ over the machined surface. Conversely, a cutting insert with wiper blade geom- etry (i.e. Fig. 32 – bottom right), has trailing radii that blends – beyond the tangency point – with the tool nose radius which remains in contact with the work- piece, allowing it to wipe (i.e. smooth) the peaks, leav- ing a superior machined surface texture. In the past, wiper insert geometries were only em- ployed for surface improvement in nishing opera- 26 Process capability denoted by ‘C P ’ , is a measure of the quality of the parts produced, which is normally found by the follow- ing simple relationship: *C P = Drawing specication tolerance/6 σ Where: σ = a statistical measure, termed the ‘standard devia- tion’ for the particular production process. *C P values of <1.0 denote low process capability, C P values of between 1.0 and 1.33 are moderate process capability, C P values of >1.33 are termed as high process capability. NB Today, process capabilities of 2.0 are oen demanded for high-quality machined parts for the automotive/aerospace sectors of industrial production, reducing likelihood of part scrappage. 27 Cusps are the product of the partial geometry of the tool nose radius geometry, positioned at regular intervals related to the selected feedrate. e cusp height (i.e. the dierence in height between the peak and valley), will inuence the machined surface texture of the component, in the following relation- ship: R max = f n 2 × 250/r ε (µm) Where: R max = maximum peak-to-valley height within the sam- pling length. f n = feedrate (m min –1 ) r ε = tool nose radius (mm). Chapter Figure 32. The application of wiper insert geometry on the resulting surface texture when ne turning. [Courtesy of Iscar Tools] . Turning and Chip-breaking Technology tions. With recent advancement in wiper 28 geometry, this has allowed them to be used at double the previ- ous feedrates for semi-nishing/roughing operations, without degrading the surface texture. e wiper ge- ometry being in contact with the workpiece’s surface for longer than equivalent standard insert nose radius tends to wipe – hence its name, or burnish the ma- chined surface, producing a smoother surface texture. Due to the fact that a ‘wiper’ has an extended edge, the cutting forces are distributed across a longer tool/chip contact region. e wiper portion of the insert, being somewhat protected, enables these wiper inserts to in- crease tool life by up to 20% more than when using conventional tool nose geometries. Wiper blades have their clearance lengths care- fully designed, if they are too long, the insert gener- ates too much heat, on the contrary, they need to be long enough to cope with relatively large feeds, while still smoothing over the surface cusps. Wipers with positive turning insert geometries, they can cope with feedrates of 0.6 mm rev –1 at D OC ’s of up to 4 mm. For example, with steel component hardnesses of 65HR c , this oen negates the need for any successive precision grinding operations. By designing wiper geometries with the cutting edge and nose radii to improve ma- chined surface nish, while increasing tool life, can be considered as outstanding tool design. 2.5 Chip-Breaking Technology .. Introduction to Chip-Breaking e technology of both chip-forming and chip-break- ing has been one of the major areas of advancement in recent years. A whole host of novel toolholders and cutting inserts has been developed to enable the cut- ting process to be under total chip control, allowing some toolholder/inserts combinations to machine multiple component features with just one tool, re- moving at a ‘stroke’ the non-productive aspects of 28 Some tooling manufacturers have re-named wiper inserts as high-feed inserts, as they have demonstrated in production conditions to promote higher component output, without the recourse to expensive capital outlay. tool-changing and setting, signicantly increasing ma- chine tool utilisation rates. Even when conventional turning inserts are employed, for heavy roughing cuts (Fig. 33a), where feedrates are high as are the large D OC ’s, ecient control of the chip must be achieved. To enable excellent control of chip-breaking with rough- ing cuts (Fig. 33b), a similar overall insert geometry is shown to that in the previous example, but here the rake face embossed dimples/chip-breakers dier sig- nicantly. Finally, for light nishing cuts (Fig. 33c), chips are broken in a totally dierent manner to that of the previous examples. Hence, with all of these dier- ing types of turning operations on workpieces, control of the chip is vital, as it can drastically impair the over- all production rates and aect part quality, if not given due consideration. Chip formation is chiey inuenced by the follow- ing factors: • Workpiece material composition – its heat treat- ment (i.e. if any), which aects the chip’s strength, • Insert’s cutting geometry – rake and clearances, as well as any chip-formers present, the geometry be- ing associated with the work piece material, • Plan approach angle – depending upon whether roughing, or nishing cuts are to be taken, • Nose radius – this being linked to the feedrate and here, to a lesser extent, the surface texture require- ments, • Undeformed chip thickness (i.e. D OC ) – this will af- fect the chip curling aspect of the chip’s formation – more will be said on this topic in the following sec- tion. Note: Another important factor that can also play a signicant role in chip formation, is the application of coolant and its supply velocity. e shear angle has some eect on the contact length between workpiece and the rake face and, it is in this vicinity that cutting forces and machining-induced temperatures predominantly aect the cutting insert. Moreover, the insert’s rake is signicant, in that as the rake angle increases the contact length decreases, the more positive the rake, the shorter the contact length. Actual chip formation is primarily dependent upon several factors: D OC , feedrate, rake angle, together with the workpiece’s mechanical strength, noting that the chip starts forming in the primary deformation zone (see Fig. 26). us, the chip is subsequently formed by the bending force of the cutting action, eectively ‘pivoting’ from the chip’s roughen ‘free top surface’ , Chapter Figure 33. Turning cuts and associated insert geometries for forming and shearing of a chip. [Courtesy of Sandvik Coromant] . Turning and Chip-breaking Technology this being a somewhat shorter length than that of the ‘shiny’ underside at the tool/chip interface. Many theories have been given for the actual ‘cause and eect’ of preliminary chip formation which is schematically illustrated Fig. 33d – ‘A’- one such, be- ing that any formation is related to the cutting speed. A large insert rake angle normally means that there is less tendency for chip curling through a larger radius, but it will have lower cutting forces. In Fig. 33d – ‘B’ , is depicted a somewhat ‘idealised’ view of the actual cutting process, which can be expressed via the simple relationship of ‘λ’ and ∆X/∆Y. NB: In this schematic representation: ‘h 1 ’ represents D OC and, ‘ϕ’ is the ‘shear plane angle’. When utilising CNC machine tools and in particu- lar turning centres, a major problem is the variety of continuous chip forms created and the large quantity and volume of swarf 29 produced. e manner to which swarf aects machining operations depends upon the operating conditions, but fundamentally there are sev- eral requirements in any form of swarf control, these are: • e swarf must ow freely away from the cutting zone, without impairing the cutting action’s e- ciency, • Swarf must be of convenient size and shape to fa- cilitate handling manually, or in swarf conveyors (i.e. if tted), together with any future large-volume storage, then transportation and subsequent dis- posal, • Any swarf should drop away into the machine’s swarf tray, without snarling around, the workpiece, tool, or interfering with other functions such as: automatic tool-changing magazine/turret, in-situ touch-trigger inspection probes, component load- 29 Individual chips when in any great volume are generally termed swarf. It is important to be able to manage this swarf volume and, satisfactory chip control can be determined by ‘Lang’s chip-packing ratio’ , this being denoted by the letter ‘R’ , in the following manner: R = Chip volume (mm 3 )/Equivalent volume of uncut work- piece material (mm 3 ) NB: ‘R’ ranges from values of 3-to-10, where an R-value of 4 gives satisfactory chip-breaking control, producing neatly curled ‘6 and 9-shaped’ chips. ing equipment, such as overhead gantries, or dedi- cated robotic loading devices. In terms of priority for these swarf control factors, pos- sibly the most important one is that the swarf should ow smoothly away from the cutting area, as with the latest chip-breakers tted to today’s cutting inserts, chips can be readily broken and controlled 30 , this will be theme of the following section. .. The Principles of Chip-Breaking In machining, the cutting edge’s primary function is to remove stock from the workpiece. Whether this is achieved by forming a continuous chip, or by the ow of elemental chips will depend upon several fac- tors, including the properties of the workpiece mate- rial, cutting data employed and coolant type and its delivery. e terms ‘long-chipping’ and ‘short-chipping’ are utilised when considering the materials to be ma- chined. Short-chipping materials such as most brasses and cast irons, do not present a chip-breaking problem for swarf disposal, so this section will concentrate on the long-chipping workpiece materials, with particu- lar focus on ‘steel family’ grades. Steels are produced in a wide variety of specications and this allows their properties to be ‘tailored’ to the specic indus- trial applications. In addition, these steels methods of primary processing, such as: casting, forging, rolling, forming and sintering, together with the type of subse- quent heat treatment, creates still further metallurgical variations that may have an even greater inuence on the workpiece’s chip-breaking ability. e workpiece’s strength and hardness values describe the individual material’s character to some extent, but it should be borne in mind that it is the chip’s mechanical strength that determines whether it can be broken with ease. No absolute correlation exists between a steel com- 30 Today, many high-volume manufacturing companies have re- alised the benet of the value of clean and briquetted swarf, as opposed to oily scrap swarf, which sells at just ‘fractions’ of this value. At present, briquetted and cleaned aluminium swarf can be sold for approaching £1,000/tonne, moreover, the coolant/oil can be reclaimed, further driving down the overall machining costs. For other non-ferrous ‘pure’ metals and others, such as copper alloys and brasses, the economic savings are even greater. Chapter . 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. interpolates with the X- and Z-axes of the machine tool) around the curved prole. If the geometry of the tool was not itself of round geometry, then the ‘point-contact’ could not be maintained,. 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
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