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 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 . 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. 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. 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