Cutting Tool Technology Episode 2 pps

2 Turning and Chip-breaking Technology ‘Machines are the produce of the mind of Man; and their existence distinguishes the civilized man from the savage.’   (1762–1835) [Letter to the Luddites of Nottingham] 2.1 Cutting Tool Technology In the following sections a review of a range of Turn- ing-related technologies and the importance of chip- breaking technology will be discussed. .. Turning – Basic Operations Turning can be broken-down into a number of basic cutting operations and in eect, there are basically four such operations, these are: 1. Longitudinal turning (Fig. 16a), 2. Facing (Fig. 16b), 3. Taper turning – not shown, 4. Proling – not shown. NB ese turning operations will now be very briey 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 man- drel, 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 1 . 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 at face to the part, or a sharp corner at a shoulder, alternatively it can be cutting the partial, or nished part to length (i.e facing-o) 2 . 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 – oen a ‘self-holding taper’ , such as a Morse taper). ere are many dierent operations that can be achieved on a CNC lathe/turning centre, 1 e range of turning operations is vast, feedrates can be var- ied, as can rotational speeds. 2 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 X- axes. including: forming 3 , while others such as drilling, bor- ing, 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. ats, slots, splines, keyways, etc.), as well as drilled and tapped holes across and at angles to the major axis of the workpiece, or o-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 oer a vast array of machine congurations, including: co-axial spindles (ie twin synchronised in- line headstocks), tted with twin turrets with X- and Y-axes simultaneous, but separate control, having pro- grammable steadies (i.e. for supporting long slender workpieces), plus part-catchers , or overhead gantries for either component load/unload capacity, to multi- axes robots feeding the machine tool. is 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. .. Turning – Rake and Clearance Angles on Single-point Tools In order for a turning tool to eectively 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 de- cades 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 ef- cient chip-breaking technology, in certain instances critical control of the exure (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. e rake angle is the inclination of the top face of the cutting edge and can vary according to the work- 3 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.  Chapter  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] . Turning and Chip-breaking Technology  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 suce. 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. is 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 dyna- mometer – 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 4 , with some inserts having a ‘wiper’ designed-in to improve the machined surface nish still further – more will be mentioned on these insert integrated features later. .. Cutting Insert Edge Preparations Oen, 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 out- ermost corners of the cutting edge, where the rake and clearance faces coincide. ere are four basic manners in which the honed edge preparation is fashioned, these are: 4 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-dened, conversely, if a small feedrate is utilised in conjunction with a large nose radius, then cusp height is mini- mised, hence the surface texture is improved. 1. Chamfer – which simply breaks the corner – not illustrated, 2. Land – stretching back negatively from the clearance side to various lengths on the rake face (see Fig. 18b), 3. Radius – around the actual corner (see Fig. 18c) 5 , 4. Parabolic – has unequal levels of honing on two faces (see Fig. 18d). Even here, more oen 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 frag- ile cross-section of the edge. e T-lands and hones are oen 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 5 to 25° o 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 lled 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 ne 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). e 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 ank and rake faces – what is termed a ‘round hone’ (Fig. 18c). Yet another honing prole termed the parabolic hone (i.e. sometimes this honed edge is known as: 5 e radius is sometimes termed ‘edge rounding’ (i.e. denoted by the letters ‘ER’) – oen applied to most edge preparations, enabling the cutting forces to be directed on to the stronger part of the insert.  Chapter  Figure 17. Typical turning ‘nishing’ insert/toolholder geometry and the insert’s edge chamfering, in relation to the workpiece. Turning and Chip-breaking Technology  P-hone, oval, or waterfall), is produced by a machine with a so, diamond-charged rotating rubber wheel. erefore, 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 specied in a number of sizes, usu- ally being determined by the amount of time these insert spend in the honing device. e original Stan- dard 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].  Chapter  the American National Standards Institute (ANSI) in 1981, which included dimensions and expected toler- ances for these three basic hones. Today, many cutting tool manufacturers have expanded upon this Stan- dard, or adopted their own – specifying hone manu- facturing and identication 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 eects of this high temperature coating process. Conversely, PVD coatings, can be equally applied either over fully-honed insert edges, or on an un- honed cutting edge. In recent years, the cutting tool manufacturers have an emphasis toward providing honed edges of greater consistency and repeatability. .. Tool Forces – Orthogonal and Oblique e cutting forces are largely the result of chip separa- tion, 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, al- though 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. is compressive stress tends to be at its greatest clos- est to the cutting edge, with the area of contact between the chip and rake face 6 being directly related to the geometry here, hence the need for tooling manufacturers to optimise the geometry in this region. ere are two distinct types of forces present in machining operations concerning single-point cutting tools/inserts (see Fig. 19), these are: 1. Orthogonal cutting forces – two forces (ie tangential and axial – see Fig. 19b), 2. Oblique cutting forces – three forces (i.e. tangential, axial and radial – see Fig. 19a). 6 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’/sei- zure. NB Both of these cutting force models are heav- ily inuenced by the: cutting tool/insert orientation to workpiece, tool’s direction of cut and its applied feedrate 7 . 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). ese component forces can be separated into the: • Tangential force (F T ) – which is greatly inuenced 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. e magnitude of the tangential cutting force is the greatest of these three component forces and contributes to the torque, which in turn, inuences 7 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. Table 2. Typical in-cut shear strengths of various materials Material: Shear yield strength in cutting (N mm –2 ) Iron 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) 370 Aluminium (99.9% pure) 97 Magnesium 125 Lead 36 [Source: Trent ( 1984)] . Turning and Chip-breaking Technology  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 .  Chapter  the power requirement for cutting. Fundamentally, the product of the tangential force and the cutting speed represent the power required for machining. e specic cutting force 8 is a unit expression for the tangential cutting force, being closely related to the material’s undeformed chip thickness and selected feedrate, • Axial force (F A ) – 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 (F R ) – is directed at right angles to the tangential force from the cutting point. e ‘plan approach angle’ and the size of the nose radius, will inuence this force. NB ese three component forces are signicantly inuenced by the rake angle, with positive rakes producing in general, lower cutting forces. e resultant force , its magnitude and angle, will be af- fected by all three component forces, in conjunction with the tool’s geometry and the workpiece material to be cut. Or thogonal 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 innitely sharp and that the tool is at right angles to the workpiece axis having no deection 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 congura- tion, allows long slender bars to be turned, as there is less likelihood of tool ‘push-o ’ (i.e. as the radial force 8 In reality, the specic 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 inuenced 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. specic cutting force) will be more than ten times greater than when cutting a pure aluminium workpiece. has been neutralised – as indicated by the fact that the resultant force shows no X-axis oset). If any radial force was present, this would create either a ‘candle- stick eect’ , 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-o ’ , depending upon the size of this incorporated nose feature – creating a ‘certain degree’ of radial component force aect. .. Plan Approach Angles e 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 eect for single- point 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 specic workpiece feature – such as a 90° shoulder – it is important as it will not only aect the machined part geometry, but has an inuence on con- sequent 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 (D OC ) 9 , or ‘cutting depth’ is the dierence between an un-cut and cut surface, this being half the dierence in the un-cut and cut diameter (i.e. the diameter is reduced by twice the D OC in one pass along the workpiece). is D OC is always measured at 90° to the tool’s feed direction, not the cutting edge. e 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 inuences the workpiece features that can be produced with this cutting geometry, it also aects the formation of chips and the magnitude of the component forces (Fig. 20b). e ‘entering angle’ aects the length of the cutting edge engaged in-cut, normally varying from 45° to 90°, as illustrated in the four cases of diering plan approach angles shown in Fig. 20b. Here, in ‘case I’ an 9 In single-point turning operations, the depth of cut (D OC ) is sometimes referred to by the term: ‘undeformed chip thickness’. Turning and Chip-breaking Technology  Figure 20. Insert approach angle geometry for turning operations.  Chapter  [...]... alloys Turning and Chip-breaking Technology Figure 27 . Chip-breaking inserts and chip control whilst turning – in action [Courtesy of Iscar Tools] 57 58 Chapter 2 Table 4. Strength and hardness of chips when turning mild steel Rake angle (γ°): 45 35 27 10 10 10 0.30 0.30 0 .20 0 .20 0 .20 0 .20 50 50 168 168 168 76 None None None None 75 84 91 92 93 95 27 2 28 9 3 02 320 314 325 HV/UTS 3.6 3.4 3.3 3.5 3.4... IV Machine tool – type, power, its condition and specifications, V Stability – loop stiffness/rigidity (i.e from the cutting edge to its foundations), VI Set-up – tool accessibility, workpiece clamping and toolholding, tool changing, VII Tool programme – the correct/specified tool and its tool offsets, etc., VIII Performance – cutting data, anticipated tool- life and economics, IX Quality – tool delivery... system, ↓ Toolholder size and type, ↓ Insert shape, ↓ Insert size, ↓ Nose radius, ↓ Insert type, ↓ Tool material, ↓ Cutting data → 45 part access, as a toolholder is defined by its: effective entering and point angles 12, together with the insert’s shape (see Fig 22 ) Toolholders should be the largest possible size for the turning centre’s tool turret, this requirement is vital, as it reduces the tool overhang... wiper geometries with the cutting edge and nose radii to improve machined surface finish, while increasing tool life, can be considered as outstanding tool design 2. 5 Chip-Breaking Technology 2. 5.1 Introduction to Chip-Breaking The technology of both chip-forming and chip-breaking has been one of the major areas of advancement in recent years A whole host of novel toolholders and cutting inserts has been... process than was apparent with the 5° tool (i.e illustrated in Fig 30a) This more efficient cutting process, results in smaller variations in the cutting forces (1 and 2) The chip is seen to flow over the rake face in a more Turning and Chip-breaking Technology Figure 29 . Discontinuous chip formation [Courtesy of Sumitomo Electric Hardmetal Ltd.] 61 62 Chapter 2 consistent manner (3) It was found... relationship: Rmax = fn2 × 25 0/rε (µm) Where: Rmax = maximum peak-to-valley height within the sampling length fn = feedrate (m min–1) rε = tool nose radius (mm) Turning and Chip-breaking Technology Figure 32. The application of wiper insert geometry on the resulting surface texture when fine turning [Courtesy of Iscar Tools] 65 66 Chapter 2 tions With recent advancement in wiper28 geometry, this has... selecting tooling for the optimum production of either part batch sizes, or for continuous production runs, then the following section has been incorporated 2. 1.6 Cutting Toolholder/Insert Selection When deciding upon the correct selection of a toolholder /cutting insert for a given application, a range of diverse factors must be considered, as indicated in Fig 21 As can be seen by the diagram (Fig 21 ) and... geometry will allow 46 Chapter 2 Figure 22 . Tool paths in finish turning operations [Courtesy of Sandvik Coromant] Turning and Chip-breaking Technology 47 Figure 23 . Selecting indexable inserts for turning operations [Courtesy of Stellram] NB Appendix 1d shows the ISO ‘Code Key’ – for Indexable Inserts Insert Size An indexable insert size is directly related to the toolholder selected for the operation,... shape and nose radius Tool Material The penultimate evaluation to be made concerning tooling decision-making is the choice of insert mate- Table 3. Typical working areas for external turning operations Type of machining operation: Feedrate (f ): Extreme finishing 0.05 to 0.15 0 .25 to 2. 0 Finishing 0.1 to 0.3 0.5 to 2. 0 Light roughing 0 .2 to 0.5 Roughing 0.4 to 1.0 >1.0 >0.7 8 to 20 (mm) (mm) Once all... workpiece material in an experimental cutting procedure, that the tangential cutting force component, was closer to the actual cutting edge than when similar machining was undertaken on unalloyed steel specimens NB The cutting data for machining the stainless steel specimens in Figs 30 a and b, were: 180 m min–1 cutting speed, 0.3 mm rev–1 feedrate, 3 mm DOC 2. 4 Tool Nose Radius The insert’s nose radius . toolholding, tool changing, VII. Tool programme – the correct/specied tool and its tool osets, etc., VIII. Performance – cutting data, anticipated tool- life and economics, IX. Quality – tool. 0.15 0 .25 to 2. 0 Finishing 0.1 to 0.3 0.5 to 2. 0 Light roughing 0 .2 to 0.5 2. 0 to 4.0 Roughing 0.4 to 1.0 4.0 to 10.0 Heavy roughing >1.0 6.0 to 20 Extremely heavy roughing >0.7 8 to 20 (mm). Both of these cutting force models are heav- ily inuenced by the: cutting tool/ insert orientation to workpiece, tool s direction of cut and its applied feedrate 7 . Oblique Cutting Forces

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