Turning and Chip-breaking Technology Machines are the produce of the mind of Man pdf

Turning and Chip-breaking Technology ‘Machines are the produce of the mind of Man; and their existence distinguishes the civilized man from the savage.’ WILLIAM COBBETT (1762–1835) [Letter to the Luddites of Nottingham] 34 Chapter 2.1 Cutting Tool Technology In the following sections a review of a range of Turning-related technologies and the importance of chipbreaking technology will be discussed 2.1.1 Turning – Basic Operations Turning can be broken-down into a number of basic cutting operations and in effect, there are basically four such operations, these are: Longitudinal turning (Fig 16a), Facing (Fig 16b), Taper turning – not shown, Profiling – not shown NB  These turning operations will now be very briefly reviewed In its most simple form, turning generates cylindrical forms using a single-point tool (Fig 1.16a) Here, a tool is fed along the Z-axis slideway of the lathe (CNC), or a turning centre, while the headstock rotates the workpiece (i.e the part is held in either: a chuck, on a mandrel, face-plate, or between centres – when overhang is too long), machining the component and thereby generating a circular and cylindrical form of consistent diameter to the turned part Facing is another basic machining operation that is undertaken (Fig 16b) and in this case, the tool is fed across the X-axis slideway while the part rotates, again, generating a flat face to the part, or a sharp corner at a shoulder, alternatively it can be cutting the partial, or finished part to length (i.e facing-off) Taper turning can be utilised to produce short, or long tapers having either a fast taper (i.e with a large included angle), or slow taper (i.e having a small included angle – often a ‘self-holding taper’ , such as a Morse taper) There are many different operations that can be achieved on a CNC lathe/turning centre,  Facing operations can also be used to produce either curved convex, or concave surface features to the machined part – here the surface is both generated and formed, requiring simultaneous programmed feeding motions to the Z- and Xaxes 2.1.2 Turning – Rake and Clearance Angles on Single-point Tools In order for a turning tool to effectively cut and produce satisfactory chips, it must have both a rake and clearance angle to the tool point (Fig 17) Today’s single-point cutting tools and inserts are based upon decades of: past experience, research and development, looking into all aspects of the tool’s micro-geometry at the cutting edge Other important aspects are an efficient chip-breaking technology, in certain instances critical control of the flexure (i.e elastic behaviour) of the actual tool insert/toolholder combination for the latest multi-functional tooling is essential – more will be said on some of these topics later in the chapter The rake angle is the inclination of the top face of the cutting edge and can vary according to the work- The range of turning operations is vast, feedrates can be varied, as can rotational speeds  including: forming, while others such as drilling, boring, screw-cutting, of internal features, and forming and screw-cutting of external features, to name just a few of the traditional operations undertaken With the advent of mill/turn centres, by having CNC control of the headstock and rotational, or ‘driven-/live-tooling’ to the machine’s turret, this allows prismatic features to be produced (i.e flats, slots, splines, keyways, etc.), as well as drilled and tapped holes across and at angles to the major axis of the workpiece, or off-axis Even this explanation of mill/turn centres is far from complete, with regard to today’s sophisticated machine tools As machine tool builders today, can offer a vast array of machine configurations, including: co-axial spindles (ie twin synchronised inline headstocks), fitted with twin turrets with X- and Y-axes simultaneous, but separate control, having programmable steadies (i.e for supporting long slender workpieces), plus part-catchers , or overhead gantries for either component load/unload capacity, to multiaxes robots feeding the machine tool This type of machine tool exists and has multi-axes CNC controllers to enable the machine’s down-time to be drastically reduced and in this manner achieving high productive output virtually continuously  Forming can be achieved in a number of ways, ranging from complex free-form features (externally/internally) on the machined part, to simply plunging a form tool to the required depth Turning and Chip-breaking Technology 35 Figure 16.  Typical turning operations with the workpiece orientation shown in relation to the cutting insert, for either: (a) cylindrical turning, (b) facing [Source: Boothroyd 1975] 36 Chapter piece material being machined In general, for ductile materials, the rake inclination is a positive angle, as the shearing characteristics of these materials tends to be low, so a weaker wedge angle (i.e the angle between the top face and the clearance angle) will suffice For less ductile, or brittle workpiece materials, the top rake inclination will tend toward neutral geometry, whereas for high-strength materials the inclination will be negative (see Fig 17), thereby increasing the wedge angle and creating a stronger cutting edge This stronger cutting edge has the disadvantage of requiring greater power consumption and needing a robust tool-workpiece set-up Machining high-strength materials requires considerable power to separate the chip from the workpiece, with a direct relationship existing between the power required for the cutting operation and the cutting forces involved Cutting forces can be calculated theoretically, or measured with a dynamometer – more will be said on this subject later in the text Both side and front clearances are provided to the cutting edge, to ensure that it does not rub on the workpiece surface (see Fig 17) If the tool’s clearance is too large it will weaken the wedge angle of the tool, whereas if too small, it will tend to rub on the machined surface Most tools, or inserts have a nose radius incorporated between the major and minor cutting edges to create strength here, while reducing the height of machined cusps, with some inserts having a ‘wiper’ designed-in to improve the machined surface finish still further – more will be mentioned on these insert integrated features later 2.1.3 Cutting Insert Edge Preparations Often, a minute edge preparation (see Figs 17 and 18b, c and d) is created onto the sharp cutting edge of the insert, this imparts additional strength to the outermost corners of the cutting edge, where the rake and clearance faces coincide There are four basic manners in which the honed edge preparation is fashioned, these are:  Machined cusps result from a combination of the feedrate and the nosed radius of the tool If a large feedrate occurs with a small nose radius then the resultant cusp height will be high and well-defined, conversely, if a small feedrate is utilised in conjunction with a large nose radius, then cusp height is minimised, hence the surface texture is improved Chamfer – which simply breaks the corner – not illustrated, Land – stretching back negatively from the clearance side to various lengths on the rake face (see Fig 18b), Radius – around the actual corner (see Fig 18c), Parabolic – has unequal levels of honing on two faces (see Fig 18d) Even here, more often than not, certain combinations of these four edge preparations are utilised, so that the cutting forces are redirected onto the body of the rake’s face, rather than directed down against the more fragile cross-section of the edge The T-lands and hones are often actually incorporated into the insert geometry of the contoured surface Typical T-lands range in size from 0.07 to 0.50 mm, having angles varying from to 25° off of the rake face (Fig 18b) Honing which is the ‘rounding’ of the cutting edge, can be performed in one of several ways Probably the oldest technique for honing, utilises mechanical means, which employs a vibrating tub filled with an abrasive media, such as aluminium oxide – to ‘break’ the corner on these inserts A variation in this design, uses an identical abrasive, except here the inserts are held by centrifugal force to the inside of a rotating tank While yet another method of honing using an abrasive media, involves spraying the inserts with fine abrasive particles – to hone the edges of the inserts Probably the most popular method for obtaining cutting insert honed edges, uses brushes made from extruded nylon impregnated with diamond (see Fig 18a) The inserts to be honed pass by these brushes in individual carriers and rotate as they all revolve under the brushes, thereby applying equal hones to all insert edges Depending upon the amount of desired honing, these brushes can be either raised, or lowered, or alternatively, the inserts can make multiple passes through the machine All of the above honing techniques produce a hone that is roughly equal on both the flank and rake faces – what is termed a ‘round hone’ (Fig 18c) Yet another honing profile termed the parabolic hone (i.e sometimes this honed edge is known as:  The radius is sometimes termed ‘edge rounding’ (i.e denoted by the letters ‘ER’) – often applied to most edge preparations, enabling the cutting forces to be directed on to the stronger part of the insert Turning and Chip-breaking Technology 37 Figure 17.  Typical turning ‘finishing’ insert/toolholder geometry and the insert’s edge chamfering, in relation to the workpiece 38 Chapter P-hone, oval, or waterfall), is produced by a machine with a soft, diamond-charged rotating rubber wheel Therefore, as the abrasive material rubs across the inserts, it tends to extend slightly over the inserts sides, producing a hone of uneven proportions between the two insert faces (Fig 18d) As in the case of the T-land cutting insert edge preparation, the P-hone directs the cutting forces into the body of the insert Honing can be specified in a number of sizes, usually being determined by the amount of time these insert spend in the honing device The original Standard for honing was established in the United States by Figure 18.  A honing machine (i.e brush-style) and several types of honing edge preparations [Courtesy of Ingersoll] Turning and Chip-breaking Technology the American National Standards Institute (ANSI) in 1981, which included dimensions and expected tolerances for these three basic hones Today, many cutting tool manufacturers have expanded upon this Standard, or adopted their own – specifying hone manufacturing and identification methods Hones must be applied prior to the application of coatings Inserts that are destined to receive a CVD coating, must have a minimum hone to strengthen the edge, in order to counteract the effects of this high temperature coating process Conversely, PVD coatings, can be equally applied either over fully-honed insert edges, or on an unhoned cutting edge In recent years, the cutting tool manufacturers have an emphasis toward providing honed edges of greater consistency and repeatability 39 Table 2.  Typical in-cut shear strengths of various materials Material: Shear yield strength in cutting –2 (N mm ) The cutting forces are largely the result of chip separation, its removal and chip-breaking actions, with the immense pressure and friction in this process producing forces acting in various directions Stresses at the rake face tend to be mainly compressive in nature, although some shear stress will be present (see Table 2, by way of illustration of the machining shear stresses for various materials), this is due to the fact that the rake is rarely ‘normal’ to the main cutting direction This compressive stress tends to be at its greatest closest to the cutting edge, with the area of contact between the chip and rake face being directly related to the geometry here, hence the need for tooling manufacturers to optimise the geometry in this region There are two distinct types of forces present in machining operations concerning single-point cutting tools/inserts (see Fig 19), these are: Orthogonal cutting forces – two forces (ie tangential and axial – see Fig 19b), Oblique cutting forces – three forces (i.e tangential, axial and radial – see Fig 19a)  As well as the tool/chip interface temperatures being up to 1,000°C, the interface pressures can reach a maximum of 3,000 MPa, these being sterile smooth surfaces makes them ‘ideal’ conditions for the occurrence of ‘pressure-welding’/seizure 370 0.13% C steel 480 Ni-Cr-V steel 690 Austenitic stainless steel 630 Nickel 420 Copper (annealed) 250 Copper (cold-worked) 270 Cartridge brass (70/30) 2.1.4 Tool Forces – Orthogonal and Oblique Iron 370 Aluminium (99.9% pure) Magnesium Lead 97 125 36 [Source: Trent ( 1984)] NB  Both of these cutting force models are heavily influenced by the: cutting tool/insert orientation to workpiece, tool’s direction of cut and its applied feedrate Oblique Cutting Forces Fig.1.19a, can be seen a model of the three-dimensional cutting force components in an oblique turning operation, when the principal cutting edge is at an angle to the main workpiece axis (i.e Z-axis) These component forces can be separated into the: • Tangential force (FT) – which is greatly influenced by the contact and friction between both the workpiece and tool, as well as the contact conditions between the chip and the rake face of the cutting edge The magnitude of the tangential cutting force is the greatest of these three component forces and contributes to the torque, which in turn, influences  Feedrates play a major role in determining the axial force in single-point cutting operations, in association with the tool’s orientation to the part being machined 40 Chapter Figure 19.  The two- and three-force models of orthogonal and oblique cutting actions, with the component forces approximately scaled to give an indication of their respective magnitudes Turning and Chip-breaking Technology • • the power requirement for cutting Fundamentally, the product of the tangential force and the cutting speed represent the power required for machining The specific cutting force is a unit expression for the tangential cutting force, being closely related to the material’s undeformed chip thickness and selected feedrate, Axial force (FA) – the magnitude of this force will vary depending on the selected feedrate and the chosen tool geometry and in particular, the ‘plan approach angle’ , or ‘entering angle’ , – more will be said on this topic later Its direction is from the feeding of the tool, along the direction of workpiece machining, Radial force (FR) – is directed at right angles to the tangential force from the cutting point The ‘plan approach angle’ and the size of the nose radius, will influence this force NB  These three component forces are significantly influenced by the rake angle, with positive rakes producing in general, lower cutting forces The resultant force, its magnitude and angle, will be affected by all three component forces, in conjunction with the tool’s geometry and the workpiece material to be cut Orthogonal Cutting Forces In Fig 19b the two-dimensional model for orthogonal cutting is depicted, once again, for comparison to the oblique cutting model, in a single-point turning operation For simplicity, if one assumes that the point of the tool is infinitely sharp and that the tool is at right angles to the workpiece axis having no deflection present, then the two component forces are the tangential force and axial force (i.e previously mentioned above) In this case, this tool geometry-workpiece configuration, allows long slender bars to be turned, as there is less likelihood of tool ‘push-off ’ (i.e as the radial force  In reality, the specific cutting force is a better indication of the power requirement, as it is the force needed to actually deform the material prior to any chip formation It will vary and is influenced by the: undeformed chip thickness, feedrate, and yield strength of the workpiece material For example, if the cutting conditions are kept the same and only the material changed, then if a nickel-based alloy is machined, the initial chip forming force (i.e specific cutting force) will be more than ten times greater than when cutting a pure aluminium workpiece 41 has been neutralised – as indicated by the fact that the resultant force shows no X-axis offset) If any radial force was present, this would create either a ‘candlestick effect’ , or ‘barrelling’ to the overall turned length In reality, there will always be some form of nose radius, or chamfer to the tool point, which will have some degree of ‘push-off ’ , depending upon the size of this incorporated nose feature – creating a ‘certain degree’ of radial component force affect 2.1.5 Plan Approach Angles The manner in which the cutting edge contacts the workpiece is termed the ‘plan approach angle’ (Fig 20a), being composed of the entering and lead angles for the selected tool geometry In effect for singlepoint turning operations, the tool’s orientation of its plan approach, is the angle between the cutting edge and feeding direction When selecting a tool geometry for turning specific workpiece feature – such as a 90° shoulder – it is important as it will not only affect the machined part geometry, but has an influence on consequent chip formation and the direction and magnitude of the component cutting forces, together with the length of engagement of the cutting edge (see Fig 20b) In single-point turning (Fig 20b), the depth of cut (DOC), or ‘cutting depth’ is the difference between an un-cut and cut surface, this being half the difference in the un-cut and cut diameter (i.e the diameter is reduced by twice the DOC in one pass along the workpiece) This DOC is always measured at 90° to the tool’s feed direction, not the cutting edge The manner in which the cutting edge approaches the workpiece is termed the ‘entering angle’ (i.e plan approach angle), this being the angle between the cutting edge and feed direction (Fig 20a – shown here in a cylindrical turning operation) Moreover, the plan approach angle not only influences the workpiece features that can be produced with this cutting geometry, it also affects the formation of chips and the magnitude of the component forces (Fig 20b) The ‘entering angle’ affects the length of the cutting edge engaged in-cut, normally varying from 45° to 90°, as illustrated in the four cases of differing plan approach angles shown in Fig 20b Here, in ‘case I’ an  In single-point turning operations, the depth of cut (DOC) is sometimes referred to by the term: ‘undeformed chip thickness’ 42 Chapter Figure 20.  Insert approach angle geometry for turning operations 72 Chapter Figure 35.  The chip-breaking envelopes related to cutting data and chip-curling behaviour [Courtesy of Sandvik Coromant] Turning and Chip-breaking Technology wrap itself around either the tool, or workpiece, but such a geometry is perfect for machining aluminium, or non-ferrous materials • Radial top rake (illustrated in Fig middle and to the left – three grooving insert sizes illustrated) This radial top rake is designed to thin the chip Such chip thinning, eliminates the need to undertake finishing passes on the groove’s side walls Furthermore, this type of grooving insert geometry being on-centre, enables axial turning of diameters for wide shallow grooves33, or recesses • Raised bumps on top rake (see Fig 27a – left) This sophisticated grooving geometry is utilised for materials where chip control is difficult, as it provides an ‘aggressive barrier’ to the curling chip The 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 obtain full advantage of a grooving insert’s chip-forming abilities, the chip must be allowed to flow into the chip-former This chip-flow can be achieved by either decreasing the workpiece’s surface speed, or increasing the feed – more will be said on this shortly The former technique of decreasing the surface speed, allows the material to move slower across the top rake of the cutting edge and as a result, has greater contact time to engage the chip-former This slower workpiece speed, has the benefit of increasing tool life, through lower 33 A groove, or recess, can normally be considered as a straightwalled recessed feature in a workpiece, as illustrated in Fig 40 Typical applications for grooves are to provide thread relief – 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 The chip formed with insert geometries having a flat 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 geometry, reduces the chip width and provides an efficient cutting action, this results in decreasing the cutting edge pressure somewhat Chip-formers offer longer tool life and improved sidewall finishes with better chip control, than those top-rakes that have not incorporated such insert chip-forming geometric features 73 tool/chip interface temperatures The 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 duration, more heat will be conducted into the tool, than into the chip, which could have a negative impact of inconsistent workpiece size control, • Due to the lower workpiece surface speed, the benefits of the insert’s coating will be reduced, as such coating technology tends to operate more effectively at higher interface temperatures (iii) Increasing the feedrate – by increasing the feed allows it to engage the chip-former more effectively – 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 insert’s geometry with higher force, creating a greater tendency to break Hence, by holding a constant workpiece surface speed, allows the faster feedrate to reduce cycle times Transversal, or Face Grooving Transversal grooving geometry has a curved tearshaped blade onto which, the insert is accurately located and positioned The transversal insert follows the 90° plunged feed into the rotating face of a workpiece These tools are categorised as either right-, or left-hand, with the style adopted depending upon whether the machine tool’s chuck rotates anti-clockwise (i.e using a right-hand tool), or clockwise (i.e left-hand) The minimum radius of curvature for such transversal grooving tooling is normally about 12mm, with no limit necessary on the maximum radial curvature that can be machined For shallow face grooves, off-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 success here, is establishing where in the face to make the first plunge This initial face plunge should be made within the range of the tool’s diameter, otherwise the tool will not have sufficient clearance and will ultimately 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 The rotational speed for face grooving is usually about 80% of the speed used for parting-off – soon to be 74 Chapter mentioned Feedrates are normally around 50% of parting-off 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 This results in the chip which no longer flows in a straight line across the insert’s edge, instead it moves at an angle Such a naturally curved chip is difficult to exhaust from the face groove, particularly if it is broken 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 chatter34, any excessive overhang of the tool should be minimised The 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 premature tool failure Typically, chatter is initiated by a disturbance in the cutting zone, for several reasons, such as: – Lack of homogeneity – in the workpiece material (i.e typically 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 DOC), – Workpiece geometry (i.e if the component shape produces either a variation in the DOC – for example, because of uneven depth of casting material being machined, or light cuts on interrupted shapes, such as hexagon, square, or rectangular bar stock), – Frictional conditions (i.e tool/chip interface frictional variations, whilst machining) Regenerative chatter is a type of self-excited vibration, resulting from the tool cutting a workpiece surface that has either significant roughness, or more likely the result of surface disturbances from the previous cut These disturbances in the workpiece surface, create fluctuations 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 increasing the dynamic stiffness of the system, or by increasing the damping NB  Dynamic stiffness can be defined as the ratio of the amplitude 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 flushing from this groove Parting-off The parting-off 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-off ’ to a previously formed internal diameter (shown in Fig 40 for left-hand side operations) Essentially in a parting-off 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 allows 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-off tool from getting ‘pinched’ between the stock and the soonto-be-released component Often, ‘Part-catchers’ are utilised to reduce any surface damage to the falling component, once it has been parted-off NB  If the component to be parted-off is held in a coaxial/sub-spindle, at component release, the additional spindle supports the workpiece and under these conditions, the parting-off operation is virtually identical to that of found in a grooving cycle (ii) Surface speed reduction – this effectively occurs when the machine’s spindle attains its maximum speed For example, on a machine tool having a maximum 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 fixed 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 significantly increasing the chip loading as the tool approaches the workpiece’s centreline In order to alleviate the increasing tool loading, lowering the feedrate by about 50% until separation is just about to occur, then finally 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-off operations, Turning and Chip-breaking Technology to utilise the ‘canned cycle’ such as the ‘constant surface speed’  35 function NB  A more serious parting-off 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 deflect This parting-off tool deflection, 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-off inserts normally consist of two main types with top rakes that are either of, negative, or positive cutting edge chip-forming geometries The negative-style of chip-formers are possibly the most commonly utilised These inserts have a small negative land at the front edge which increases the insert’s strength, giving protection in adverse cutting conditions, such as when interrupted cutting is necessary during a parting-off operation The land width – often termed a ‘T-land’ , is relative to the breadth of the parting-off tool This width of the insert’s land has a direct correlation to the feedrate and its accompanying chip formation The feedrate must be adequate to force the workpiece material over the land and into the chipformer36 Notwithstanding the widespread usage of negative parting-off tooling, positive-style insert geometries have some distinct advantages The chief one being the ability to narrow the chip at light feedrates, with mini- 75 mal tool pressure If excessive tool pressure occurs, this can promote work-hardening of the ‘transient surface’ 37 of the workpiece These abilities are important points when machining relatively low mechanical strength components, which might otherwise buckle if machined with negative-style inserts when subsequently parted-off Positive cutting edge parting-off tooling having chip-formers, are ideal for applications on machine tools when either low fixed feedrates are utilised, or if the workpiece material necessitates lower cutting speeds This positive-style of parting-off tooling, operates efficiently when machining softer workpiece materials, such as: aluminium-or, cooper-based alloys and many non-metallic materials, typically plastics Feed­ rates can be very low with these positive-type parting tools, down to 0.0254 mm rev–1 with exceptional chip control and consistent tool life One major disadvantage of using these positive tooling geometries for parting-off, is that the tool is much weaker than its equivalent negative geometry type The concept of insert self-grip in its respective toolholder, was developed by the cutting tool manufacturer Iscar tools in the early 1970’s and has now been adopted by many other tooling manufacturers (Fig 40 top left-hand side) These ‘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, doubleended inserts termed ‘dogbones’ , were often used but were limited to low DOC’s – due to the length of the secondary cutting edge, so have been somewhat overshadowed by the ‘self-grip’ varieties of parting-off tooling 2.5.5 Chip Morphology 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 The main problem with using this ‘canned cycle’ , is that as the maximum spindle speed is reached, the chip load will also increase This is not a problem, so long as the maximum speed has not occurred, such as when parting-off a component with a large hole at its centre 36 Parting-off 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 multiplying the width of the insert by a constant of 0.04 For example, 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 This will give a ‘start-point’ for any parting-off operations, although it might be prudent to check this feedrate is valid, from the tooling manufacturer’s recommendations The Characterisation of Chip Forms (Appendix 2) In the now withdrawn ISO 3685 Standard on Machinability 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.) 76 Chapter chip forms under eight headings, with several variations appearing in each groups (i.e see Appendix for an extract showing these chip form classifications) Although in the main, the chip forms were related to turning, some of these chip morphologies could be extrapolated to other manufacturing processes The chip type that will be formed when any machining operation is undertaken is the product of many interrelated factors, such as: • Workpiece material characteristics – will the material that forms the chip significantly work-harden?, • Cutting tool geometry – changing, or modifying the cutting insert geometries38 and its plan approach angles will have a major influence 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 affect the type and formation of chips produced, • Cutting data utilised – by modifying the cutting data: feeds and speeds and DOC’s, with the insert geometry maintained, this can play a significant role in the chip formed during machining operations NB  Chip formation has become a technology in its own right, which has shown significant development over the last few decades of machining applications 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 resultant chip morphology shown in Fig 35a Uncontrolled chip-steaming (i.e long continuous workpiece strands), must be avoided, being a significant risk-factor 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 DOC on two axes, with their relative size and position within the graphical area being significantly affected 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 recommended application area – termed its ‘chip-breaking envelope’ (i.e see footnote 38 below) – with regard to its range of feedrates and DOC’s Within this ‘envelope’ , chips of acceptable form are produced by the cutting insert’s geometry Conversely, any chips that are formed outside this ‘envelope’ are not acceptable, because they are either formed as unbroken strands, or are too thick and over-compressed When component profiling operations are necessary (Fig 31a), this normally involves several machining-related parameters: variations in DOC’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 These factors are important as they can modify the chip morphology when profiling operations include: recessed/undercut shoulders, tapers and partial arcs, facing and sliding operations with the same tool, together with many other combined profiled features All of these operations make significant demands on the adaptability of the cutting insert’s geometry to efficiently break the chip In general, the cutting insert’s chip formation principles 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 specific machining 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 workhardened swarf and possibly degrading this machined 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 affect specific part tolerances for the individual part, or could lead to modifications in the statistical variability39 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 Turning and Chip-breaking Technology 77 • Increased built-up edge (BUE) formation – which having a small angle when compared to the cutting through ‘attrition wear’ 40 may cause the risk of premature cutting edge failure When the chip curling is too open, this may result in the following negative tendencies: • Poor chip control – creating an inefficient chipbreaking 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 – affecting both the machined surface texture and shortening tool life Chip formation and its resultant morphology, is not only affected by the cutting data selected, but will be influenced 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 insert orientation, so the affect is for the entering angle to modify the chip formation process The insert’s entering angle affects 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 softer’ in operation as the plan approach angle tends toward say, 10° to 60°, furthermore, the chip flow direction will also advantageously change with the spiral pitch increasing As the nose radius is changed with different cutting inserts, this has the effect of changing both the direction and shape of the chips produced This 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 difference being the DOC’s, with a shallow DOC in Fig 35b (left), giving rise to a slow chip helix, whereas in Fig 35b (right) the DOC is somewhat deeper, creating a tighter chip helix which is beneficial 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 affinity, creating ‘ideal’ conditions for a pressure welding situation Hence, the BUE develops, which builds-up rapidly and is the ‘swept away’ by the chip flow streaming over the top rake’s surface, taking with it minute atomic surface layers from the tool’s face This continuous renewal and destruction of the BUE, enhances crater wear formation, eventually leading to premature cutting edge failure edge Equally, a larger depth means that the nose radius has somewhat less affect from its radius and greater influence by the entering angle of the cutting edge, producing an outward directed spiral Feedrate also affects the width of the chip’s cross-section and its ensuing chip flow41 Chip formation begins by the chip curving, this being significantly affected by combinations of the cutting data employed, most notably: feedrate, DOC, rake angle, nose radius dimensions and workpiece condition A relatively ‘square’ cross-sectional chip normally indicates that an excessively hard chip compression 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 effect on any machining process By careful use of CAD techniques coupled to FEA to construct the insert’s cutting edge, commashaped chips are the likely product of any machining, providing that the appropriate cutting data has been selected In some machining operations, chip formation can be superior using a slightly negative insert rake angle, thereby introducing harder chip compression and self-breaking of the chip, particularly if utilising 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-flow is the result of a compound angle between the chip’s side- and back-flow The chip’s side-flow being a measure of the flow over the tool face (i.e for a flat-faced tool), whilst backflow establishes the amount of chip-streaming into the chipbreaker groove Detailed analysis of chip side-flow (i.e via high-speed photography), has indicated that it is influenced by a combination of groove dimensions and cutting data If the feedrate is increased, this results in a higher chip backflow angle, promoting chip-streaming into the chip-breaker groove The ratio of feed-to-length of restricted contact has been shown to be an important parameter in the determination of chip- back-flow Typically with low feedrates the corresponding chip back-flow 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-flow does not fully engage the chip-breaker and will as a result curve upward, with minimal ‘automatic’ chip-breaking effect 78 Chapter ally, for larger feedrates, a positive insert rake angle might optimise the chip-curving tendency, by not producing and excessively tight chip helix Chip curve, its resultant chip flow direction, the chip helix and its accompanying shape are designed into each cutting edge by the tooling manufacturers Tool companies ensure that a controlled chip formation should result if they are exploited within the recommended cutting data ranges specified In Fig 36a (left), effective chip-breaking decisionmaking recommendations are shown on a flow-chart, indicating how to obtain the desired chip-breaking control In the chart shown in Fig 36a (right), the DOC’s indicate on the associated visual table the expected chip type showing that here types ‘C and D’ offer ‘good’ broken chips Such chip morphology charts as these from tooling manufacturers, attempt to inform the user of the anticipated chip-breaking if their recommendations are followed Whereas the flow-diagram illustrated in Fig 36b, indicates that ‘good chip control’ improved productivity will result, if a manufacturing company adopts the machining Figure 36.  Chip-breaking control and chip morphology and its affect on productivity [Courtesy of Mitsubishi Carbide] Turning and Chip-breaking Technology strategy high-lighted to the left-hand side On the contrary, ‘poor chip control’ with an attendant decrease in productivity will occur, if the problems shown to the right-hand side transpire Chip morphology can indicate important aspects of the overall cutting process, from the cutting edge’s geometry and its design, through to work-hardening ability of the workpiece Many other factors concerning cutting edge’s mechanical/physical properties can be high-lighted, these being important aids in determining a material’s machinability – which will be discussed in more depth later in the text 2.5.6 Chip-Breaker Wear Any form of tool failure will depend upon a combination of different wear criteria, usually with one, or more wear mechanisms playing a dominant role Previously, it was found that the workpiece surface texture and the crater index act as appropriate tool failure criteria, particularly for rough turning operations Moreover, tool life based upon these two factors, approximated the failure curve more exactly than either the flank, or crater wear criterion In cutting tool research activities, it has been found that when machining with chip-breaker inserts, flank wear (i.e notably VB) is not the most dominant factor in tool failure In most cases, the ‘end-point’ of useful tool life occurs through an alteration of the chipgroove parameters, well before high values of flank wear have been reached The two principal causes of wear failure for chip-breaker inserts are: • For recommended cutting data with a specific insert, the design and positioning of chip-breakers/ grooves may promote ‘unfavourable’ chip-flow, resulting in wear in the chip-breaker wall – causing consequent tool failure, • Alterations in the cutting data, particularly feedrate, affects chip-flow, which in turn, generates various wear patterns at the chip-breaker’s heel and edge (see Fig 37) In the schematic diagrams shown in Fig 37, are illustrated the concentrated wear zones on the: back wall (i.e heel), cutting edge, or on both positions for a typical chip-breaker insert Under the machining conditions for Fig 37a, the chip-groove utilisation is very low, with the chip striking the heel directly Thus, as machining continues, this results in abrasive 79 wear of the heel and ultimately this heel becomes flattened and chip-breaking is severely compromised Conversely, when the cutting data produces a wear zone concentrated at the insert’s edge (Fig 37b), then chip side-flow occurs and poor chip-breaking results, together with low tool life This accelerated tool wear, resulting from an extended tool/chip contact region over the primary rake face, promotes a rough surface texture to the machined part In the case of Fig 37c, these are ideal conditions for optimum chipbreaking action and a correspondingly excellent and predictable tool life, because the wear zones at both the heel and edge are relatively uniform in nature, illustrating virtually a perfect chip-forming/-breaking action Higher tool/chip interface temperatures can result as the heel wears, forming a crater at the bottom of the chip-breaker groove Combination wear – as shown in Fig 37c – generally results in significantly improved tool wear, in conjunction with more predictable tool life In the photographs of chip-breaker grooves shown for an uncoated and coated Cermet cutting insert material in Figs 38a and b respectively, the relative wear patterns can clearly be discerned In the case of Fig 38a – the uncoated insert – the predominant wear concentration is primarily at the edge, indicating that the cutting data had not been optimised While in the case of the coated Cermet insert of identical geometry (Fig 38b), the wear is uniform across the: edge, groove and heel This would seem to suggest that ideal cutting data had been utilised in its machining operation In both of these cases some flank wear has occurred, but this would not render the chip-breaking ability when subsequent machining invalid NB  A complex matrix occurs (i.e Fig 38c) with Cermets, this ‘metallurgy’ can be ‘tailored’ to meet the needs of specific workpiece and machining requirements 2.6 Multi-Functional Tooling The concept of multi-functional tooling was developed from the mid-1980’s, when multi-directional tooling emerged This tooling allowed a series of operations to be performed by a single tool, rather than many, typically allowing: side-turning, profiling and 80 Chapter Figure 37.  Schematic representations of differing chip-breaking insert tool wear mechanisms – due to alterations in the cutting data [Source: Jawahir et al., 1995] Turning and Chip-breaking Technology Figure 38.  Improved wear resistance obtained with an uncoated and coated cermet, when turning ovako 825B steel, having the following cutting data: Cutting speed 250 m min–1, feed 0.2 mm rev–1, DoC 1.0 mm and cut dry [Courtesy of Sandvik Coromant] 81 82 Chapter grooving, enabling the non-productive elements42 in the machining cycle to be minimised In the original multi-directional tooling concept, the top rake geometry might include a three-dimensional chip-former, comprising of an elevated central rib, with negative Klands on the edges Such a top rake profile geometry could be utilised for efficient chip-forming/-breaking of the resultant chips This tooling when utilised for say, grooving operations, employed a chip-forming geometry – this being extended to the cutting edge, which both narrowed and curled the emerging chip to the desired shape, thereby facilitating easy swarf evacuation A feature of this cutting insert concept, was a form of effective chip management, extending the insert’s life significantly, thus equally ensuring that adequate chip-flow and rapid swarf evacuation would have taken place When one of these multi-directional tools was required to commence a side-turning operation, the axial force component43 acting on the insert caused it to elastically deflect at the front region of the toolholder This tool deflection enabled an efficient feed motion along the workpiece to take place, because of the elastic behaviour of the toolholder created a positive plan approach angle in combination with a front clearance angle – see Fig 39a and b (i.e illustrating in this one of the latest ‘twisted geometry’ insert multi-functional tooling geometries) Any of today’s multi-functional tooling designs (Figs 39 and 40), allow a ‘some degree’ of elastic behaviour in the toolholder, enabling satisfactory tool vectoring to occur, either to the right-, or left-hand of the part feature being machined These multi-func- 42 Non-productive elements are any activity in the machining cycle that is not ‘adding value’ to the operation, such as: toolchanging either by the tool turret’s rotation, or by manually changing tools, adjusting tool-offsets (i.e for either: tool wear compensation, or for inputting new tool offsets – into the machine tool’s CNC controller), for component loading/unloading operations, measuring critical dimensional features – by either touch-trigger probes, non-contact measurement, or manual inspection with metrology equipment (i.e micrometers, vernier calipers, etc.), plus any other additional ‘idle-time’ activities 43 An Axial force component is the result of engaging the desired feedrate, to produce features, such as: a diameter, taper, profile, wide groove, chamfer, undercut, etc – either positioned externally/internally for the necessary production of the machined part tional tools are critically-designed so that for a specific feedrate, the rate of elastic deflection is both known and is relatively small, being directly related to the applied axial force, in association with the selected DOC’s At the tool-setting stage of the overall machining cycle, compensation(s) are undertaken to allow for minute changes in the machined diameter, due to the dynamic elastic behaviour of one of these tools in-cut For a specific multi-functional tool supplied by the tooling manufacturer, its actual tool compensation factor(s) will be available from the manufacturer’s user-manual for the product In-action these multi-functional tools (Fig 39b), can significantly reduce the normal tooling inventory, for example, on average such tools can replace three conventional ones, with the twin benefit of a major cycle-time reduction (i.e for the reasons previously mentioned) of between 30 to 60% – depending upon the complexity of features on the component being machined Some other important benefits of using a multi-functional tooling strategy are: • Surface quality and accuracy improvements – due to the profile of the insert’s geometry, any ‘machined cusps’ 44, or feedmarks are reduced, providing excellent machined surface texture and predictable dimensional control, • Turret utilisation improved – because fewer tools are need in the turret pockets, hence ‘sister tooling’ can be adopted, thereby further improving any untended operational performance, • Superior chip control – breaks the chips into manageable swarf, thus minimising ‘birds nests’ 45 and entanglements around components and lessens automatic part loading problems, • Improved insert strength – allows machining at significantly greater DOC’s to that of conventional in- 44 ‘Machined cusps’ the consequence of the insert’s nose geometry coupled to the feedrate, these being superimposed onto the machined surface, once the tool has passed over this surface 45 ‘Birds nests’ are the rotational entanglement and pile-up of continuous chips at the bottom of both trough and blind holes, this work-hardened swarf can cause avoidable damage in the machined hole, furthermore, it can present problems in coolant delivery for additional machining operations that may be required Turning and Chip-breaking Technology 83 Figure 39.  Multi-functional cutting insert geometry for efficient stock removal and increased part productivity [Courtesy of Iscar Tools] 84 Chapter Figure 40.  Multi-functional tooling for the machining of rotational features such as: turning, grooving and profiling operations – having excellent chip control [Courtesy of Sandvik Coromant] Turning and Chip-breaking Technology 85 Figure 41.  By employing twin turrets on a mill/turn centre, ‘balanced turning’ of the component can remove large volumes of stock at one pass [Courtesy of DMG (UK) Ltd.] serts, with improved insert security its toolholder location, • CNC programming simplified – many tooling manufacturer’s have specially-prepared software to significantly reduce CNC programming input times NB  This latter point utilises CNC ‘canned cycles’ to reduce program lengths In Appendix 3, a guide for ‘Trouble-shooting for turning operations’ are listed, with possible causes and remedies to potential production problems In the following chapters, many other important chip-forming production processes will be discussed, with hole-making techniques such as drilling being around 25% of all manufacturing techniques undertaken by machining-related companies – these and associated hole-production methods will be reviewed next References Journal and Conference Papers Boston, O.W A Research into the Elements of Metal Cutting Trans ASME 48, 749–848, 1926 Cocquilhat, M Experiences sur la Résistance utile Produite dans le Forage Ann Trav Publ.en Belgique 10, 199, 1851 Doi, S and Kato, S Chatter Vibration of Lathe Tools, Trans of ASME, Vol 78 (5), 1127–1134, 1956 Fabry, D The Tool Channel Cutting Tool Eng’g, 58–64, Sept 2003 Gadzinski, M Parting Know-how Cutting Tool Eng’g, 52–57, March 2001 Galloway, D.F Some Experiemnts on the Influence of Various Factors on Drill Performance Trans of ASME, 191–231, Feb., 1957 Humphries, J.R Energing Technologies and Recent Advances in Multi-functional Groove/Turn Systems Int 86 Chapter Conf on Industrial Tooling, Shirley Press Ltd, 65–85, Sept 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