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508 Chapter 9 having the necessary surface texture. In the case of hard-part turning – to be discussed in the next section, hard-turned surface texture of better than 2.5 Ra, is achievable, across a range of workpiece materials. According to Hanson (2005), hard materials can be classified into two distinct groups: 1. Single-component materials – might typically include hardened tool steels, glasses such as Pyrex™ and borosilicate, ceramics including silicon carbide (SiC) and aluminium oxide (Al2O3), 2. Composite materials – could be either metal/ceramics such as metal-matrix composites (MMC’s) and tungsten-carbide /cobalt, glass/ceramics such as Zerodur™ and Cervit™, as well as ceramic/ceramic composites such as silicon-carbide/silicon. NB These groupings only list a few of the hard workpiece materials currently available today, many more exist, but they can still be classified within these groupings at present. The selection of the cutting tool composition and its associated geometry when hard-part machining, is influenced by the severe demands made by these hardened workpieces. The problems encountered can range from very rapid tool wear rates, cracks and chipping of the cutting edge(s), to an unacceptable machined surface condition. Some multi-coated cemented carbides and aluminium oxide ceramics (Fig. 10) can cope with some operations on hardened workpiece materials, but it is more usual to utilise ultra-hard cutting tool materials, or at the very least, specialised-coatings on cemented carbide tooling. Some technical difficulties can be encountered when hard-part machining, these might range from: • Elevated temperatures in the cutting zone, • Greater and more variable cutting force magnitudes, • Intense pressure on a relatively small cross-section of the chip – near the edge, • Rapid cutting edge wear, or catastrophic breakdown, • Workpiece stresses being released during the machining operation, • Poor homogeneity in part material – creating vibrational effects on tool’s edge, • Insufficient stability and rigidity – created in the ‘machine-tool-workpiece’ loop stiffness. The extreme thermal and mechanical conditions, will dictate the manufacturing circumstances, concerning: tool material and its geometry; machining methods utilised; together with the cutting data selected. So, the properties demanded from a cutting tool when one is about to embark on hard-part machining exercise, are that it has: • Superior abrasive wear resistance, • Chemical stability at the high temperatures encountered, • Ability to retain its cutting edge at high temperatures – ‘hot-hardness’ , • High compression and bending strength, • Good cutting edge strength and toughness, • Cutting edge inertness and resistance to diffusion wear. The hardness of a part, should not be confused with its ‘modulus’ , or the material’s toughness. As it is the combination of elastic and plastic properties that determine a material’s resistance to yield, namely, its permanent deformation54. In the following sections, particular approaches to that of hard-part machining by various production processes will be concisely reviewed. 9.9.1 Hard-Part Turning For some years now the application of hard-part turning has increased in popularity, as the time needed to finish these hardened components – with their hardnesses ranging between 45 to ≈68 HRC – has significantly reduced. The major time-saving is from the virtual elimination of finish-grinding operations, when ‘fine-turned’ surface texture of <2.5 Ra is now possible, with matching dimensional tolerances. Complex contouring of a component’s profile using hard-part turning operations is achievable (i.e. see Fig. 154 – top), which overcomes the previously expensive and timeconsuming profiling operation of cylindrical grinding with custom-formed grinding wheels. However, successful hard-part turning is more than simply ‘chucking’ a hardened component (Fig. 250a), 54 ‘Yield, or permanent deformation’ , differs here, from that of either: elastic deformation; or dislocation, slip, etc.; that may result during machining. (Source: Hanson, 2005) Machining and Monitoring Strategies then utilising for example, either a CBN, or ceramic cutting insert to machine them. A variety of important factors require consideration if the turning operation is to be successful, including the fact that higher cutting forces involved and their affect on the machine tool’s structural rigidity and stiffness (Fig. 250d). These hard-part cutting forces can be ≈200% higher than the forces generated when similar operations are undertaken on ‘softer parts’. Due to the fact that either the CBN, or ceramic tool materials tend to be relatively brittle, cutting tool companies will apply chamfers to the cutting edges – to strengthen them. A problem with a ‘supporting chamfer’ on the insert’s edges is that it reduces its shearing capability, this causes the cutting forces to increase – due to the so-called ‘ploughing-effect’. The application of higher cutting forces here, makes both the insert’s edge and the part move relative to one another – resulting in chatter. The onset of chatter causes surface texture degradation – at best, or destroys the cutting edge/tool and even the part itself – at worst. Moreover, the size and the shape of the workpiece when hardpart turning is important, as if too long a length-todiameter ratio – without support, this will prove to be exceedingly difficult to successfully machine. With some hard-part machining companies stating that L/D ratios of >3:1 can be a problem – with respect to chatter generation, resulting from the higher imposed cutting forces now in operation. Workholding plays a crucial role in successful hard-part turning operations, with a firm all-round grip – from say a collet positioned and rigidly held well inside machine’s headstock, is much preferred to that of a three-jaw chuck – as this latter device may not necessarily provide either the ‘clamping forces’ 55, nor inherent rigidity. The location of the turning insert’s 55 ‘Three-jaw chucks – clamping forces’ (Fig. 250 – bottom right), due to the fact that the ‘conventional’ chuck’s self-centring mechanism is a face-scroll – this being provided by a spiral groove cut onto the face of a flat disk, then having the equispaced jaws offset by the scroll’s pitch and numbered for replacement as either internal, or external jaws. When the chuck key turns the ‘scroll-pins’ which then rotates the scroll plate and thus, they simultaneously radially move the jaws – inward/outward. So, depending upon the scroll plate’s rotation, the torque supplied by the chuck key creates only one third of the total force supplied at each individual chuck jaw. Therefore, the force may not be adequate to provide sufficient force to grip and rigidly hold the part as it is hard-turned. 509 positional location, relative to the machine’s spindle bearings is significant. The further the distance away from the front spindle bearing’s location relative to that of the cutting action, the greater any potential for the part to flex and chatter – acting like a ‘lever mechanism’ (i.e. its force times distance). So, machine tools that are designed so that their collets are well-seated into the headstock and close to the front bearings and, when hard-part turning, the minimum of workpiece overhangs should be utilised. Turning centres that are designed from the outset to cope with the demands of hard-part turning, are usually of greater size and weight, plus being extremely rigid structures. Even here, concerning a machine’s rigidity, there are practical limits to the amount of ‘static stiffness’ (i.e. the ratio of applied force to that of an associated displacement) that can be built into the turning centre. The design objective is to increase the machine tool’s ‘dynamic stiffness’ 56, which involves dampening the frequency of vibrations via specific applied technologies, such as ‘composite-filling’ 57 the machine bases. While a different approach to dampening vibration, involves the use of hydro-static linear ways, which ride on non-contact pressurised fluid bearing surfaces. In a similar manner to that of ‘conventional’ linear-ball guide ways, they exhibit low friction and resist omni-directional loadings (i.e see Fig. 250d – for a performance comparison of these two linear bearing types). Moreover, hydro-static guide ways provide dampening as-and-when vibrations occur during hard-part machining process. Furthermore, hydro- NB In Fig. 250e on the other hand, the force exerted by this type of self-centring mechanism, should prove to be able to cope with the demands of hard-part turning , particularly as the ‘soft-machinable jaws’ have been bored-out, to provide more circumferential location and support for the part. 56 ‘Dynamic stiffness’ , can be defined as: The ratio of the applied force to the displacement, occurring at the frequency of the exciting force’. (Source: Hardinge Inc., USA) 57 ‘Composite-filling’ – machine tool bases. This dampening techinque is traditionally achieved by employing such materials such as Granitan™ (i.e. typically, being a crushed granite and epoxy resin), or Harcrete™ – this latter product is a polymer composite, having an 800% better damping capacity to that of the equivalent grey cast iron structure. Although these composite-filled structures can be expensive, in order to alleviate some of this cost, ‘traditional’ castings have strategically-reinforced composite-filled cavities. (Source: Kennedy, 2004) 510 Chapter 9 . Figure 250. Hard-part turning operations are replacing some grinding operations: assuming dimensional size and machined surface texture are acceptable. [Courtesy of Sandvik Coromant] Machining and Monitoring Strategies static bearings58 offer the indirect benefits of: improved hard-part machined surface texture; greater component accuracy and precision; coupled with increased tool life. While, due to the fact that hydraulic fuild is virtually incompressible, if an interrupted cut occurs for any reason, the tool can later pick-up exactly where the interruption occurred thereby eliminating the ostensibly termed ‘witness marks’ on the machined surface. Great demands for increased spindle power are not necessary, as typical DOC’s are ≈0.75 mm, coupled to small feedrates. With for example, the cutting data for a φ12 mm, hard-turned part, might be by utilising a spindle speed of 4,500 rev min–1, which equates to a surface speed of ≈170 m min–1. This is well within the capabilities of some turning centres today, that have spindle speeds of 10,000 rev min–1. As one can gather from this discussion, hard-part turning and threading (Fig. 251a) is principally concerned with saving money, by removing additional and now, superfluous operations from the overall product’s cycle time. It has been reported by industrial-users, that by utilising a hard-part turning strategy, then the total time to complete the component has been reduced by up to 75%, when compared to the traditional approach, of: roughturning; then heat-treating; and finally cylindrically grinding. This latter process of cylindrical grinding, can still be valid where extremely tight tolerances are to be held, coupled to when high-quality surface texture demands are to be met. Often, both for large-batches, or if continuous production runs are necessary, for either hard-part finished turning, or when cylindrical grinding, they require critical dimensional tolerances to be held across certain diameters, or indeed, for their lengths. In this metrological situation, a ‘receiver gaug- 58 ‘Hydro-static bearing performance’ , is characterised by three factors, these are its: load-carrying capacity; oil flow-rate; and pumping power. The magnitudes of the hydro-static coefficients depend very much on the pad design, such that: W = af Ap pr Q = qf W/Ap × h3 / µ P = pr Q = Hf (W/Ap)2 × h3 / µ Where: W = Load on the bearing (N); Q = Volume flow-rate of oil (m3 s–1); P = Pumping power (N × m s–1 ≡ W);af = Pad load coefficient (dimensionless); qf = Pad flow coefficient (dimensionless); Hf = Pad power coeficient (dimensionless, because Hf = qf/af); Ap = Pad area (m3 );pr = Oil pressure in the recess of the pad (Pa); h = Film thickness (m); µ = Dynamic viscosity of the oil (Pa.s). (Source: Mott, 1985) 511 ing’ unit specially-configured to measure such part dimensional features can be built-up from modular units and its associated instrumentation (Fig. 252). Not only can this custom-built receiver gauge assembly (Fig. 252b), simultaneously measure many component features quickly, but through the instrumentation unit (Fig. 252a), ‘Statistical Process Control’ (SPC) can be employed to up-date the whole process through ‘closed-lop’ feedback to the machine’s controller. This up-dating will automatically modify the tool offsets, thereby minutely adjusting the process and as such, reducing variability in the process to a minimum. Parts can be loaded into the adjacent ‘receiver gauge unit’ , via a gantry-robot (Fig. 250e), or alternatively by a ‘dedicated robotic device’. Once held in the robot’s gripper, the finished hard-turned/-ground part, is manipulated into the desired orientation by its axes, then steadily lowers the completed workpiece onto the component support plates within the receiver unit. In the case of Fig. 252b, the centres will automatically engage with the centre-drilled holes, slightly lifting the component to its measuring height, prior to the measuring heads progressively moving forward to contact each machined component’s surface feature – for automatic measurement and control. 9.9.2 Hard-Part Milling Introduction Until relatively recently, the application of HSM by milling of hardened die and mould steels was considered something of a ‘black-art’. This is not the case, as by adhering to some basic machining principles and guidelines the whole process becomes a somewhat: straightforward; predictable; and a profitable activity. In effect, there are three primary machining methods utilised to produce hardened dies and moulds, although it should be stated that the die/mould configuration, along with its respective hardness will determine which technique, or combination of processes produces the optimum manufacturing route. However, notwithstanding these circumstances, the primary methods can be classified as follows: 1. ‘Soft machining’ – milling – this is where the part is ‘roughed-out’ prior to its hardening heat treatment. The technique of ‘soft-machining’ is normally considered when milling large workpieces, or com­ ponents requiring deep-cuts, or wide features – such 512 Chapter 9 . Figure 251. Hard-part machining operations undertaken on many components by through-hardening, hard-facing or by surface-hardening heat-treatment; are acceptable. [Courtesy of Sandvik Coromant] Machining and Monitoring Strategies . Figure 252. In-process gauging, used for up-dating the cutting process voiding ‘tool-drifting’ during a production run. [Courtesy of Mahr/Feinpruf ] 513 514 Chapter 9 as that depicted in Fig. 251b. After rough-milling, any semi-finishing and finishing operations can be undertaken in the hardened condition, 2. Hard-part milling – this is mainly where small-dimensional parts, or components requiring the production of shallow-cut features that can be readily milled (e.g. threads – Fig. 251a; gears and hobs – Fig. 251c) – in the hardened state, 3. Electrical discharge machining (EDM) – is usually utilised when the part incorporates thin features, requiring deep cuts, thus the EDM process may be the only practical solution to this problem. Hard-Milling – Tool Selection and Replacement For most die and mould operations, selecting the appropriate cutter geometry is important, with many operators choosing ball-nosed end mills (Fig. 249 – bottom right) for such hard-part milling work. Such geometry is chosen for rouging and finishing oper­ ations, because the tool’s large radius dissipates both the heat, while ‘spreading’ the cutting forces across its longer cutting edges. Additionally, the ball-nose end mill enables the user to cut closer to the net shape of the part’s geometry at high speeds and feeds. When a part incorporates wide and flat areas across its base – needing to be milled, a corner-radiused tool should ideally be utilised after the surface has been roughedout with the ball-nosed tooling. The logic behind employing the corner-radiused tool for finishing, is that with its smaller radius it cannot dissipate the heat and forces as readily as the ‘ball-nose’ , this is why it is usually used for semi-finishing/finishing operations when hard-part milling. If a square shouldered part feature is needed, then a ‘square-ended tool’ is only used after the ‘ball-nose’ has roughed-out the component’s feature – leaving the minimum of stock to be removed. This ‘square-ended’ cutter – due to its sharp corner, has a tendency to chip/fracture, since it acts as a ‘stressraising source’ for the heat and cutting forces. Tool rigidity is important, with the tool’s shank being much larger in diameter to that of the cutting diameter. With ball-nosed cutters, a small draft angle of about ½° is employed for additional strength, while the tool’s neck is usually slightly-relieved when HSM milling straight walls. In both of these cases, the tool’s projection from its holder should be kept to a minimum – to improve its intrinsic rigidity. Returning briefly to the former case of the cutter having a draft angle. Another reason for the ½° draft angle when machining dies and moulds, is that if for example, when the hardened die has a draft angle of 5°, the cutters modified relief should be ½° clearance, producing an included angle of cutter body relief of 4½°. During machining a die cavity (Figs 246 – bottom right and 249b), the excessive heat that is generated modifies the part’s surface topography, which in turn, reduces component accuracy. One technique to ­minimise such heat generation and retention while milling, is by controlling the radial step-over (i.e. pickfeed) distance for adjacent tool paths – when taking ‘parallel cuts’ 59 (Fig. 84c). This radial step-over is the distance between the centrelines of successive parallel cuts ‘ae (p)’ – shown in Fig. 247b. Therefore, for ballnosed roughing-out operations, this radial step-over should ideally be between 25 to 40% of the cutter’s diameter. Conversely, for finish-milling – for a given cusp height on a flat surface, the radial step-over can be calculated, as follows (Fig. 247b): Radial step-over (mm) = √4(ap × De) – 4 (ap2) Where: cusp height is the chordal deviation – finish tolerance, thus, ap = cusp height (mm), De = tool diameter @ a set DOC (mm). Cusp height (mm) = De/2 – √ (De2 – ae (p)2 )/4 Where: ae (p) = radial step-over (mm). (Sources: adapted from Sandvik Coromant, 1994; Macarthur, 2001) Since, the radial step-over determines the length of time each cutting edge spends in the actual cut, in conjunction with the amount of time it has to cool, prior to re-entering the following cutting-pass. This in effect, simplistically determines the quantity of heat that will accumulate in both the tool and the machined workpiece. So, when the radial step-over is too wide, the heat builds-up in the cutter’s edge, due to the fact that there is insufficient time for it to conduct the heat away, before it re-enters the following cut. While, smaller step-overs can facilitate ‘almost’ a continuous cooling action, which limit’s the heat generation and its 59 ‘Parallel cuts’ – when milling, are sometimes referred to as ‘lace cuts’. If they are not parallel – such as when pocket-milling a triangular feature, where parallel cuts would be ineffective, then the technique here is to utilise variable step-over cuts, termed: ‘non-lace cuts’.(Source: Smith et al., 1993) Machining and Monitoring Strategies 515 retention, allowing a slightly higher cutter rotational speed to be programmed. A suitable coating selection for example, on a cemented carbide cutter, will also enable higher speeds to be realised. In hard-part milling operations, coatings, such as: titanium carbonitride (TiCN) can withstand temperatures up to 400°C, comparing this to titanium aluminium nitride (TiAlN), which can withstand cutting temperatures of up to 800°C, indicates for many hardened alloy steel dies and moulds, the latter coating makes for a wiser choice in these production circumstances. The selection of speeds and feeds will also aid in controlling heat build-up. With large chip thicknesses helping to remove heat build-up in the tool and workpiece. When chip loads are too light, the heat quickly builds-up – as edges tend to rub the surface being milled, which in turn, affects tool life and may create ‘white-layering’ in the sub-surface in steel-based products. So using the largest possible chip loads improves through-put of parts. By way of illustration, if the chip load per tooth should be 0.2 mm, but instead it is only 0.05 mm, then a workpiece that normally takes around 18 minutes to machine, will now actually take 72 minutes. This increased time means the tool’s edge will now spend 400% longer in-cut. Flood coolant should not be used when adopting an HSM milling strategy for hardened metals (>40 HRC). In the USA some industrial trials were conducted into milling such hardened workpieces and, it was reported that by not using flood coolant then tool life was increased by 500% – on average. These trials including various methods of coolant delivery, via: throughthe-tool coolant holes; coolant grooves; coolant hoses; and for normal- and high-pressure coolant applications; in all cases the tool life was reduced. The main problem with the various forms of coolant delivery it would seem, is the result of the cemented carbide tooling suffering from ‘thermal-shock’ , creating by the high tool/chip interface temperatures and the immediate ‘quenching-effect’ of the coolant application – this ‘thermalcycling behaviour’ occurring at very fast rates. Nonetheless, work-hardened chips in the cutting vicinity must still be evacuated from deep recesses and pockets to avoid the ‘recutting effect’. By using an air-and-mist application – close to the tool’s edge, this will provide a means of swarf removal, while producing some ‘token’ cutting edge lubrication – assuming that ‘coolant-effect’ permissible exposure levels (PEL’s) can be safely dealt, thereby with minimising potential health hazards. Any decisions concerning tool replacement will depend on the users machining needs, with the tool failure generally being apparent by the naked eye, or under low optical magnification – simply observing the cutting edges to determine the ‘wear-patterns’. In-cut, a worn tool’s edges will tend to emit a dull ‘red glow’60, this indicates that excessive forces and heat are being generated in the cutting zone, shortly leading toward a rapid and catastrophic tool failure condition. This ‘vis­ ual glowing effect’ is initially usually confined to the 60 ‘Tooling – glowing red’ , this temperature-induced machining condition has been widely reported. Trent, 1984, stated: ‘Under very exceptional conditions, when cutting fully hardened steel, or certain nickel alloys at high speed, chips have been seen to leave the tool red hot* – i.e. a temperature of over 650°C’. *This term ‘red hot’ – relating to temperature is somewhat vague, as shown in the chart, for: Variation of colours with temperatures – tempering, stress relief and hardening: Colour: °Fahrenheit: °Celsius: Colour: °Fahrenheit: °Celsius: Colour: °Fahrenheit: °Celsius: Straw yellow 430 220 Light blue 590 310 Faint red 950 510 Light brown 465 240 Grey 615 325 Dark red 1150 620 Brown 520 270 Grey-purple 660 350 Dark cherry 1175 635 Purple 545 285 Grey-blue 705 375 Cherry red 1300 705 Dark blue 565 295 Dull Grey 750 400 Bright cherry 1470 800 NB Temperatures above are either slightly rounded-up, or -down. Conversion: °Celsius = 5/9 (°F - 32) (Sources: Avner, 1974; Bofors, 1981) 516 Chapter 9 cutting edge corners – where high stresses and temperatures are generated, which can be precisely temperature-monitored by thermographic equipment61, or simply rather crudely, by naked eye observation – with the machine tool’s lights turned out! So, by applying the correct tooling in a consistent and repeatable manner, becomes a vital factor for any form of predictability with all hard-metal machining applications. This is also particularly true for any form of hard-part: drilling; reaming; and tapping operations; where these production processes offer serious challenges to the cutting edges, as the bulk hardness of the workpieces increase to >40 HRC. 9.10 Ultra-Precision Machining Introduction In the last few years, there has been a momentous drive toward producing components and indeed assemblies, significantly more minute than was previously the case. The demand might be to locate and align ­mechanical parts together in much closer proximity, or perhaps, offering improved functionality and providing enhancement of power-to-weight ratios necessary for electronic micro-circuitry. In fact, the adjacent circuit dimensions for nanometric electronic devices can have a proximity to each other of: 0.000006 mm (i.e 6 nm ≡ 6 × 10–9 m). 61 ‘Thermography’ , utilises the infra-red radiation emitted by a temperature-induced body. These thermographic cameras, offer considerable benefits to any form of actual temperaturemonitoring applications. Thermal gradients, hot-spots and heat losses can be observed – by ‘line-of-sight’ measurements – during actual machining operations. They can also be used to assess temperatures in electrical cabinets, servo-motors, ballscrews, etc., for actual condition monitoring of the machine tools. (Source: Smith et al., 1996) NB Typical temperature ranges for thermographic cameras are: –40°C to >2,000°C with a thermal sensitivity of 0.08°C, making then ideal for some forms of tool temperature monitoring – assuming that the chip-stream is away from the camera’s lens. (Source: Flir Systems™) The challenges for the whole of the ultra-precision manufacturing industries, are to be able to make and supply miniscule devices that will meet these latest design objectives. Before, discussing the tooling requirements and machining techniques necessary to produce these diminutive parts, often with a high volume demand. It is worth trying to comprehend the ‘true dimensional size and scale’ of these miniature components and assemblies. Previously, the term ‘hair’s breadth’ was often quoted as a very minute dimensional size, but if one looks at Fig. 253a, here, the large circle is supposed to represent the diameter of an actual hair – for comparison. Although even here, a hair is not of uniform diameter. In some very simple comparison tests undertaken about 6 years ago (Smith, 2002). He plucked one of his own hairs from his head – that he could not really afford to miss!, plus four more from several other people in the vicinity! Then, he located this group of hairs within an scanning electron microscope (SEM) chamber and simply measured them. The surprise was that they varied quite considerably, ranging from the smallest hair: at φ30 µm to that of the largest hair at: φ100 µm. So, the diagram (Fig. 253a) indicating that the hair’s size was ≈φ89 µm is somewhat misleading as a form of measurement criteria, as we will begin to appreciate, that the difference of a few ‘microns’ can be excessively out-of-tolerance in some ultra-precision components and assemblies. Even the previous high-accuracy value of a ‘micrometre’ – often simply termed the ‘micron’ this dimensionally-being 10–6 m (i.e. illustrated against the hair for comparison in Fig. 253a), which is not considered and exceptional dimensional size to ‘hold’ in today’s ultra-precision machining world. In fact, the technical challenge now and into the future, is not one of the actual manufacture of these parts (Fig. 253b), but measuring them, as the old statement, that: ‘We make it, then measure it, at ten times this accuracy’ , does not hold true anymore. We are ‘almost routinely’ of late, making ultra-precision components at the ‘atomic levels of resolution’ , so how can one measure sub-atomic sized components at the ‘absolute limits’ of today’s metrological instrumentation? This form of ‘infinitesimal measurement’ , is where the term ‘uncertainty of measurement’ really does become and important factor. There are so many variables in the actual manufacturing process that can influence the overall dimensional sizes of critical features with these miniscule components. Machining and Monitoring Strategies . Figure 253. Micro- and nano-machining of parts is a big challenge for both today and tomorrow 517