486 Chapter (Fig 239b) So, by the simple action of turning a hand wheel at its end, the tools could be simultaneously opened and closed – for the required trepanned diameter This simultaneous tooling action was achieved, by the singular rotating action of both the φ20mm by 4 mm pitch left- and right-hand (i.e M 20 × 4) squarethreaded leadscrews (Fig 239c) One of the major advantages of an UHSM trepanning operation over its equivalent turning counterpart, is that the cutting forces are virtually ‘cancelled-out’ , in a similar fashion to a conventional ‘balanced turning’ operation (Figs 41 and 238 – top right) Here in this instance, one tool is set and positioned slightly ahead of the other, thereby not only reducing the overall DOC, but allowing the ‘trailing tool edge’ to effectively act as a ‘finishing tool’ This tooling positioning strategy produced an improved trepanned surface texture, while it significantly reduced the harmonic departures-fromroundness, as metrologically assessed later on the roundness testing machine Moreover, by effectively ‘halving’ the DOC, this allowed for an improvement in the chip-streaming behaviour to be attained In a later modification to the trepanning fixture (i.e not shown), a large micrometer drum with its integrated vernier scale was fitted in place of the knurled adjustable hand-wheel (i.e see Fig 239a), allowing for some considerable discretion over the linear tooling’s diametral adjustment With such a large trepanning fixture – having the opposing tooling widely-spaced, it is vital that these tools are centralised directly beneath the machine’s spindle Otherwise, there is a possibility of both sine and cosine errors being present, creating ‘Abbé-type errors’ , when adjusting and setting these tools for their diametral in-feed UHSM – Trepanning Operation This preliminary work on UHSM by trepanning, has shown that with a suitably robust tooling fixturing and allowing a large (indirect) range of tooling diameter adjustment – via the twin leadscrews, then not only is the process feasible, but it offers considerably improved machining performance and an inherent improvement in trepanned surface and roundness characteristics, over vertical turning processes Possibly in a later modification to a heavily-revised tooling adjustment system, it might be possible to employ twin coaxial ballscrews, with CNC servo-control, allowing automatic control for machining tapers and profiling to the workpiece – by utilising the supplementary rotary axis control in the machine’s CNC controller Moreover, one limitation to this UHSM trepanning technique is the length of longitudinal cut that can be taken, prior to the Z-axis motion causing the rotating part to foul on the central portion of the trepanning fixture This problem can be mitigated against, by increasing the relative stand-off height of the twin-tooling from the top of the fixture by mounting each toolholder in an extended tool block, so allowing greater Z-axis feeding to be undertaken Moreover by rearranging the tools in relation to the workpiece, it would be possible to ‘turn’ shallow, depth internal trepanned features UHSM by trepanning offers significant advantages over ‘conventional’ vertical turning, in that, in this current work, if was found that the trepanned workpiece surface and roundness were significantly improved from the previously discussed UHSM by vertical turning, described in Section 9.6.2 9.6.4 Artefact Stereometry: for Dynamic Machine Tool Comparative Assessments Introduction The use of machinable artefacts for the assessment of machine tools such as machining centres, has been utilised for some of years (i.e typically: NAS Standard 979: 1969; ISO Standard: 10791-7: 1997; Knapp, 1997), being developed just for this purpose Both the NAS and ISO Standard testpieces incorporated notable prismatic and rotational characteristics, manufactured to specific geometric and dimensional tolerances, such as: at the top, an φ110 mm circular feature; 6 mm below this round shape, an 110 mm diagonal feature is cut; a central φ30 mm though-running hole is produced; with a series of counter-bored holes at four equi-spaced quadrants are generated these being situated 6 mm below the diagonal shape Taken in cross-section, the geometry of the machinable artefacts resembles a stepped component, having an overall height of 50 mm In fact, this type of artefact has long been employed by industry to establish the overall machining performance capabilities of a particular machine tool under test However, although this prismatic and rotational featured machinable artefact achieves some measure of conformance and indicates the likely operational performance of the machine tool, it does tend to have several significant limitations, such as the: • Overall dimensional size of the artefact is quite small – when compared to that of the volumetric envelope of typical industrial machining centres, Machining and Monitoring Strategies Figure 240.  Artefact stereometry, illustrating its integrated volume geometries, for a: (right) conic frustum, (right) cylinder, rectangular volume of machine tool’s axes [Source: Smith, Sims, Hope & Gull, 2001] 487 488 Chapter • Circular feature cannot be directly compared to ric relationships were intrinsically set and datumed to that of diagnostic instrumentation – such as the Ballbar, as the diameter of this rotational feature differs from that of the standard Ballbar sizes, • Weight of the artefact does not realistically compare to any workspaces normally placed on the machine tool in its ‘loaded-state’ , meaning that ‘true’ machine tool loading-conditions are not directly comparable With these machinable testpiece limitations in mind, it was thought worthwhile developing a new calibration strategy for such machine tools, but here, under more realistic ‘loaded conditions’ , also this new arte­ fact being more directly comparable to diagnostic instrumentation (i.e such as the Ballbar), but having considerably larger volumetric size and weight, with the capacity for reuse of the expensively-produced precision part of the machinable artefact’s assembly Stereometric Artefact – Conceptual Design Stereometry has been a concept that has often been over-looked, but it deals with the volumetric content of a range of geometric shapes However, if this ‘volumetric concept’ is carefully integrated into a single artefact, it could be employed for calibration work on machine tools such as machining centres (i.e see Fig 240) Here, the cylinder was represented by three machinable aerospace aluminium disks (grade: 2017F – produced from 6 mm sheet, to nominally slightly >φ300 mm) each one being set 100 mm apart in height (i.e disks: 1, and 3) and after machining, the disks were exactly φ300 mm (i.e see Fig 241) The conic frustum included angle was 22.5°, this being the result of producing equispaced holes in each disk Starting on the bottom (disk 1), then stopping the machine and fitting the middle disk (disk 2) and drilling the holes and likewise upward to the top disk (disk 3), while ­ simultaneously producing a 3-dimensional Isosceles triangle33 (Fig 241) Each disk had these individual holes being set at an angular relationship of 90° equi-spaced apart, so, when they are taken as a ‘volume’ , a conic frustum is produced (Fig 242b) These geometric and volumet- 33 ‘Isosceles triangle’ , has two sides with two angles being equal, but in this case, with the geometry of a right-angled triangle NB  These side lengths and associated angles can be varied, so long as they both (i.e lengths, or angles) remain of identical proportions a centrally-machined slot in the base of the precision mandrel This fact, meant that the exact angular and volumetric relationships remained in-situ, when the stereometric artefact was then taken off the machine tool for subsequent analyses Stereometric Artefact – Machining Trials Prior to the stereometric artefact having its machinable disks milled, the initial test machine tool (i.e in the initial trials on a Cincinnati Milacron Sabre 500 equipped with a Fanuc OM CNC controller) was fully diagnostically calibrated by: Laser interferometry; long-term dynamic thermal monitioring of its dutycycles in both a loaded and unloaded condition; together with Ballbar assessment Prior to discussing the actual machining of the disks, it is worth taking a few moments to consider the precision mandrel that accurately and precisely locates each disk in the desired orientation, with respect to each other and the machine tool’s axes This mandrel body was produced from a eutectic steel34 (0.83% carbon), which after through-hardening to 54 HRC, was precision cylindri- 34 ‘Eutectic steel’ or ‘Silver-steel’ as it is generally known, due to its almost ‘shiny appearance’ when compared to other grades of plain carbon steels In brief, this 0.83% carbon content steel is so-called a eutectic* steel as it relates to the eutectic composition derived from the iron-carbon thermal equilibrium diagram Producing an 100% pearlitic structure (i.e hence its ‘metallographic-brilliance’ , or its ‘irridescence’) when viewed under a microscope, exhibiting fine alternate layers of: Fe3C and Fe To harden eutectic steel, its temperature is raised slightly above the ‘arrest point’ (i.e arrest point here, equals 723°C, so hardening could be undertaken at ≈765°C) into ‘γ-solid solution’ (i.e austenitic region), then rapidly quenched and agitated in water to prevent carbon atomic diffusion (i.e undertaken at greater than the ‘critical cooling velocity’), with the carbon atoms now being effectively ‘fixed’ – though not intrinsically part – of the atomic lattice structure This carbon entrapment, creates intense local strains that block dislocation movement Hence, the resulting structure is both hard and extremely strong, but also very brittle Microscopically, the hardened structure appears as an array of random needles, being completely different from the original pearlitic structure This needle-like structure formed by trapped carbon atoms in an iron crystal lattice is termed, ‘martensite’ Thus, the degree of hardness – after quenching, being proportional to its lattice strain After hardening, the mandrel needed to be tempered Tempering is a controlled heat-treatment process to allow some of the trapped carbon to escape from the interstitial spaces between the iron atoms distorted lattice structure, where they eventually form particles of cementite Machining and Monitoring Strategies cally-ground on the three register diameters, with the top and bottom faces being surface ground Previous to this heat-treatment and the grinding processes, dowelling datums (i.e φ6 mm) were drilled and reamed, then equi-spaced tapped clamping holes were produced for each disk, along with a ground tenon groove in the base – all these features being orientated to the geometry of the machines axes (Fig 241) Several unique features are introduced within the machinable portions of the disks, such as: • These aerospace-grade aluminium disks were milled to φ300 mm diameter, which directly corresponded to the radial path of the Ballbar (i.e see Fig 242a) – used previously for diagnostic machine tool assessment, ensuring that some degree of correlation occurred between them, • The three Z-plane disk heights of: 70, 170 and 270 mm (i.e modified from the original design Fig 241), coincided with both the X-Y plane table position and vertical heights utilised for the Ballbar plots, creating a reasonably large cylindrical volumetric envelope (Fig 242b) Moreover, the stereometric artefact was both designed and orientated to coincide with the start and finish positions of the Ballbar’s polar traces, • The circular interpolated holes (φ10 mm) on each disk (i.e see Fig 241), were geometrically positioned to form a three-dimensional Isosceles triangle at the three Z-axis heights for each quadrant of these disks – with the 1st an 3rd holes relating to the axes transition points in the X-Y planes Thus, each of the interpolated milled holes in the face of separate disk’s, produced the geometric stereometry of a conic frustum, having an included angle of 22.5° – when the angular orientation of the middle disk is ‘software-realigned’ to produce a straightline relationship (i.e see Fig 242b), NB  The temperature at which tempering is undertaken is critical, thus between 200–300°C, atomic diffusion rates are slow with only a small amount of carbon being released, thereby the component retains most of the hardness So if higher ‘soaking-temperatures’ are employed (i.e between 300–500°C), then this creates greater carbon diffusion forming cementite, with a corresponding drop in the component’s bulk hardness * A eutectic structure is a two-phase microstructure resulting from the solidification of a liquid having the eutectic composition: the phases exist as fine lamellae that alternate with one another (Sources: Thelning, 1981, Alexander et al., 1985; Callister, Jr et al., 2003) 489 • Overall weight of the mandrel and three disk assembly was 38 kg, consequently, this could be considered as a realistic ‘loaded condition’ for the machine tool to operate under, from a practical sense In order to minimise the milling forces on the machinable disks, HSM was employed using a spindlemounted ‘Speed-increaser’35 (Fig 243a) equipped with a φ6 mm slot drill The HSM speed-increaser was operated under the following conditions: 18,000 rev min–1; at a circular interpolation feed of 750 mm min–1; with the disks having 1 mm of excess stock for each machinable disk – to be milled by circular interpolation In Fig 243a, the last machinable disk has been located and clamped and the whole mandrel-and-disk assembly was nearing completion, having previously had its φ10 mm quadrant-positioned holes for each disk machined by small circular interpolated motions by the slot drill (i.e see the sectional details of the φ10 mm hole geometry in each disk’s quadrant co-ordinates, as illustrated in Fig 241) Stereometric Artefact – HSM Results After HSM by milled interpolation on the vertical machining centre, the complete artefact with its machinable disks in-situ, was carefully removed from the machine tool, then automatically-inspected for its quadrant hole positions and disk diameters, on an Eastman bridge-type Co-ordinate Measuring Machine (CMM) This CMM having previously been thermally error-mapped, then checked with a ‘Machine Checking Gauge’ 36 (MCG) – prior to artefact inspection The CMM utilised a specially-made and calibrated 35 ‘Speed-increasers’ , are a means of multiplying the rotational speed of the machine’s spindle, by utilising a fixed relationship geared head Here, this actual speed-increaser had a 3:1 gearing ratio, equating to a top speed of 18,000 rev min–1, when it is operating at the top speed for this particular machine tool (i.e 6,000 rev min–1) NB  Normally, these HSM milling/drilling geared heads are limited to a certain proportion of running time per hour at its top speed, as they could over-heat and thereby damage the bearing/gearing mechanism 36 ‘Machine Checking Gauge’ (MCG), is utilised to check a CMM’s repeatability and accuracy and to detect for any potential ‘lobing-type errors’ from the ‘triggering-positioning’ mechanism of the touch-trigger probe, these being invariably used on such machines 490 Chapter Figure 241.  Artefact stereometry was designed for the volumetric and positional uncertainites on machining centres, by: HSM interpolation of machinable disks [Source: Smith, Sims, Hope & Gull, 2001] Machining and Monitoring Strategies 491 Figure 242.  HSM (milling) of three machinable disks in-situ on a stereometric artefact, on a vertical machining centre [Source: Smith, Sims, Hope & Gull, 2001] 492 Chapter cranked-probe – with its calibration obtained from the ‘reference measurement sphere’ being located on the CMM’s table, utilised to inspect the φ10 mm hole geometry and there respective co-ordinate positions This probe arrangement was swapped for a conventional ‘touch-trigger probe assembly’ to measure the machinable disk diameters – while holding the same cartesian co-ordinate relationships as when it was originally UHM Later – without ‘breaking-down’ , while still maintaining the same angular orientation, this stereometric artefact assembly was inspected on a roundness testing machine (Taylor Hobson: ‘Talyrond 265’) for individual disk parameters of roundness and cylindricity37 assessment – for the ‘three-disk relationship’ The results of all of these ‘averaged’ roundness measurements and Ballbar polar plots are graphically depicted as histograms in Fig 243c When a comparison is made of these results from three individual and completely differing inspection procedures, namely: Ballbar; CMM; and Talyrond, they show some degree of measurement consistency individually, but less so when each disk data is grouped For example, in the case of the Ballbar, it indicated a 1 µm variation (i.e range) from the top-tobottom disks, while having a mean value of 17.5 µm The Talyrond polar plots (i.e ‘Least Squares Reference Circle’ 38: departures from roundness) also produced consistent roundness results, ranging from