Figure 237. An ultra-high-speed turning operation undertaken on a vertical machining centre. [Source: Smith, Littlefair, Wyatt & Berry, 2003] . Machining and Monitoring Strategies holding: chucks and face-plates; in combination with a reduction in gripping-force at high rotational speed is a potential safety hazard. In order to achieve the desired UHSM rotational speeds of >1,500 m min –1 , which can be considered as the ‘threshold’ for such a machining strategy, then the workpiece must be held in a machining centre spindle – with the tool station- ary, in a similar manner to vertical turning (i.e. see Fig. 237). Relatively recently in pioneering work by Youse and Ichida (2000), they utilised rotational speeds up to 15,000 m min –1 by this technique, showing that the turned surface texture parameter ‘Ra’ values (i.e see Section 7.5.1 in Chapter 7) steadily reduced with in- creases in cutting speed. Moreover, this Japanese ma- chining study found that the cutting forces remained relatively constant throughout the test range of: 1,200 to 15,000 m min –1 . is trend of reasonably constant turning forces with increased speed is contrary to that normally found for UHSM by milling operations (Ko- manduri, 1995, et al.), where a signicant reduction in milling forces results from high cutter rotations, allowing large length-to-diameter cutter ratios to be used (Gough et al., 1991). In any HSM operations, it is essential that the ro- tational mass of either the workpiece, or cutter – de- pending upon which is the rotating item, is dynami- cally balanced, in order to minimise out-of-balance eects which would otherwise impede both the cutting process and aect machined surface texture. Ideally, workpieces, or a cutter should be rigidly held in-situ during machining and at the very least, be single-plane balanced. UHSM: Turning Strategy Prior to undertaking the UHSM turning operations (Fig. 237), the machining centre was checked for di- agnostic errors by a ‘Telescoping Ballbar’ 32 assessment 32 ‘Telescoping Ballbar’ (Fig. 242a), is a powerful instrument for machine tool error diagnostics. e Ballbar as its name im- plies, is a ball-ended length transducer (i.e. an LVDT- measur- ing element, is positioned between the xed and telescoping balls). is LVDT has a range of ± 0.75 mm with a resolution of 0.1 µm and accuracy of 1 µm. It is held in kinematic (mag- netic) seatings between the machine spindle and its base. Extension bars can increase the radial length up to 300 mm, covering a large volumetric sweep for the two axes being diag- nostically monitored. Any kinematic plane can be rotationally swept by the Ballbar. In operation – from the soware pro- gram, the machine’s CNC will move the Ballbar to the start position (i.e. radially oset the required distance for the orien- set at the radial turning distance in the plane of the cut (i.e. see the Ballbar conguration in Fig. 242a). A range of rotational Ballbar speeds were utilised, albeit at considerably lower peripheral speeds than those which were employed for the UHSM turning trials. For this current UHSM work, a machining strategy was adopted utilising a ‘Variable quasi-pilgrim stepped arithmetic progression’ (i.e the progression is schemati- cally depicted in Fig. 238), for the selection of ‘turned testpiece’ rotational speeds, being based upon the fol- lowing general progression case criteria: Sn 1 → Sn 4 = 2000 + {n 1/ 2 [2a 1 + (n 1 – 1) d 1 ]} – 1000 + { n 2/ 2 [2a 2 + (n 2 – 1) d 2 ]} – 500 + {n 3/ 2 [2a 3 + (n 3 – 1) d 3 ]} – 250 + {n 4 /2 [2a 4 + (n 4 – 1) d 4 ]} – 125 + … Sn n Where: a 1 = 2,000, a 2 = 4,000, a 3 = 5,000, a 4 = 5,500; d 1 = 1,000, d 2 = 500, d 3 = 250, d 4 = 125; n 1 = 4,000, n 2 = 2,000, n 3 = 1,000, n 4 = 500. Such an unusual ‘progression’ mathematically de- scribed above (i.e also being shown schematically in Fig. 238), enables signicant discrimination of rota- tional results coupled to data analyses toward the up- per limit of the UHSM turning process, while giving ‘traceability’ to rotational speeds within the conven- tional range of the rotational turning process at the beginning of the turning process. A special-purpose workholding device – being dual-plane balanced to G2.5 @ 10,000 rev min –1 , was tted into the spindle of a vertical machining centre (Fig. 237). is BT40 tapered workpiece holder was constructed from one-piece of EN24T steel hardened by nitriding to >50 HR C , weighing ≈2.5 kg with the aerospace-grade aluminium disk-shaped testpiece in- tated planes), then it slowly rotates CW for 180° – to pick up uniform rotational velocity, where it rotates through a further 360° – for polar measurement, nally rotating another 180° – to slow down. is complete cycle is then repeated CCW. en polar plots are generated with a diagnostic printout, which ‘ranks’ these errors, so that they can then be eliminated, or signicantly reduced, accordingly. is is a speedy, ecient diagnostic ‘health-check’ of the machine tool errors in the two measured planes, providing signicant information, which can be utilised to improve the machine tool’s overall perfor- mance. (Source: Renishaw Ballbar Training Manual) NB Typical ‘polar plots’ are shown in Appendix 16, together with a diagnostic print-out of the results. Chapter Figure 238. A ‘variable quasi-pilgrim stepped arithmetic progression’ – being utilised for UHSM (turning). [Source: Smith, Littlefair, Wyatt & Berry, 2003] . Machining and Monitoring Strategies situ. ese pre-shaped testpiece disks were: φ300 mm by 6 mm thick, made from aluminium 2017F. e tool with various cemented carbide tooling insert grades, was held on a platform dynamometer (Kistler model: 9257B) – having complemntary charge ampliers coupled to suitable data analysis soware. A range of D OC ’s were utilised: 0.5, 1.0 and 1.5 mm, with a con- stant and rapid feedrate of 30 m min –1 . Apart from cut- ting force analysis, turned surface texture, harmonic roundness and micro-hardness results were obtained, together with metallographical inspection of the sub- surface regions. Hence, from this testpiece setup and utilising the ‘progression’ for peripheral workpiece speed strategy described above, the speeds ranged from the conventional, through to UHSM. UHSM: Turning Trends Unlike the previous ndings of Youse and Ichida (2000), where they suggested that the cutting forces remained relatively constant across a broad spectrum of UHSM – for turning operations. is UHSM turn- ing work indicated that there was a decrease in mean cutting forces between 2,000 to 6,000 m min –1 , with a corresponding improvement in turned surface texture (i.e. ‘Ra’) across this range. e harmonic departures- from-roundness were inuenced by the sinusoidal ef- fect of the uctuating tangential force as it progressed around and along the turned surface’s periphery. is harmonic behaviour was evident in the cutting force data, where the analysis soware showed both a rising and falling relationship, as the turning insert passed over the rotating workpiece’s surface at great peripheral speed. Such cutting force traces occur in high-speed interpolation by the milling process, where there is a general undulating increase/decrease in force genera- tion, this being related to the axis transition cross-over during cutter interpolation around the workpiece (i.e. see Fig. 159). is cutting force undulation is the result of the machine tool’s servo-motors reversing direc- tion at these transitions, albeit, at signicantly slower speeds than utilised for this UHSM turning work. At such high turning speeds, chip-streaming was apparent at peripheral speeds >4,000 m min –1 . Chip- streaming in UHSM by turning is the preferred chip- form, as it exhausts the work-hardened swarf away from the cutting vicinity, thereby minimising entan- glement around the newly-formed turned workpiece surface. At such high turning peripheral velocities, the chip-streamed swarf is directed radially-away from the work surface. Conversely, at lower rotational workpiece-to-insert velocities, there was a marked tendency for ‘chip-curling’. As a result of the inuence of the insert’s geometry: nose radius and D OC relation- ship, the so-called ‘theta-eect’ in conjunction with the feed-per-revolution occurs (i.e see Figs. 34c and d). ere is a direct and predictable relationship to chip- curl tendency when certain conditions arise at the lower peripheral turning speeds, which is not apparent at the UHSM turning range. is UHSM turning applied research work, has shown that it is feasible to employ ultra-fast turning practices to the relevant components if the correct tooling, workpiece and machine tool relationships can be met. .. Ultra-High Speed: Trepanning Operations Intoduction Trepanning has been a well-recognised production process for many years, it is principally utilised to pro- duce large hole diameters, since this technique does not require as much spindle power as solid drilling. Moreover, in the ‘conventional’ approach to trepan- ning, it is undertaken in one operation, but instead of all the workpiece material being removed in the form of a large volume of swarf, a cylindrically-shaped core is le behind at the centre of the hole. us, this method must be utilised for through-hole applications, assum- ing that the internal feature – hole manufactured is the scrap material. Conversely, in the UHSM trepanning work shortly to be discussed, the two cutting edges are externally set against the workpiece’s periphery, mak- ing the ‘slug’ the product of the machining operations (Fig. 239c). UHSM – Trepanning Fixture Design As in the case of vertical turning, UHSM by trepan- ning was undertaken on a vertical machining centre utilising the same workholding arrangement (Fig. 239). Here, a special-purpose trepanning xture - 600 mm in overall length, was designed and manufac - tured with twin-opposing tools (Fig. 239a). e tool- ing was conventional TiN-coated cemented carbide turning inserts – having straight toolholders these being positioned on their sides in opposing directions Chapter Figure 239. An ultra-high-speed trepanning xture and dual-plane balanced workpiece holder: utilised for an UHSM research programme of work. [Source: Smith, Hills & Littlefair, 2005] . Machining and Monitoring Strategies (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 diam- eter. is simultaneous tooling action was achieved, by the singular rotating action of both the φ20mm by 4 mm pitch le- and right-hand (i.e. M 20 × 4) square- threaded leadscrews (Fig. 239c). One of the major advantages of an UHSM trepan- ning 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 D OC , but allowing the ‘trailing tool edge’ to eectively act as a ‘nishing tool’. is tooling positioning strategy pro- duced an improved trepanned surface texture, while it signicantly reduced the harmonic departures-from- roundness, as metrologically assessed later on the roundness testing machine. Moreover, by eectively ‘halving’ the D OC , this allowed for an improvement in the chip-streaming behaviour to be attained. In a later modication to the trepanning xture (i.e. not shown), a large micrometer drum with its inte- grated vernier scale was tted 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 xture – 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 is preliminary work on UHSM by trepanning, has shown that with a suitably robust tooling xturing and allowing a large (indirect) range of tooling diameter adjustment – via the twin leadscrews, then not only is the process feasible, but it oers considerably improved machining performance and an inherent improvement in trepanned surface and roundness characteristics, over vertical turning processes. Possibly in a later modication to a heavily-revised tooling adjustment system, it might be possible to employ twin coaxial ballscrews, with CNC servo-control, allowing auto- matic control for machining tapers and proling 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 xture. is problem can be mitigated against, by increasing the relative stand-o height of the twin-tooling from the top of the xture 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 oers signicant advantages over ‘conventional’ vertical turning, in that, in this cur- rent work, if was found that the trepanned workpiece surface and roundness were signicantly improved from the previously discussed UHSM by vertical turn- ing, described in Section 9.6.2. .. Artefact Stereometry: for Dynamic Machine Tool Comparative Assessments Introduction e 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 Stan- dard 979: 1969; ISO Standard: 10791-7: 1997; Knapp, 1997), being developed just for this purpose. Both the NAS and ISO Standard testpieces incorporated nota- ble prismatic and rotational characteristics, manu- factured to specic geometric and dimensional toler- ances, 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 be- ing situated 6 mm below the diagonal shape. Taken in cross-section, the geometry of the machinable arte- facts resembles a stepped component, having an over- all height of 50 mm. In fact, this type of artefact has long been employed by industry to establish the over- all machining performance capabilities of a particu- lar 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 signicant limita- tions, 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, Chapter Figure 240. Artefact stereometry, illustrating its integrated volume geometries, for a: 1. (right) conic frustum, 2. (right) cylinder, 3. rectangular volume of machine tool’s axes. [Source: Smith, Sims, Hope & Gull, 2001] . Machining and Monitoring Strategies • Circular feature cannot be directly compared to that of diagnostic instrumentation – such as the Ballbar, as the diameter of this rotational feature diers from that of the standard Ballbar sizes, • Weight of the artefact does not realistically com- pare to any workspaces normally placed on the ma- chine 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 calibra- tion 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 oen been over-looked, but it deals with the volumetric content of a range of geometric shapes. However, if this ‘volumet- ric 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 aero- space 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, 2 and 3) and aer machining, the disks were exactly φ300 mm (i.e see Fig. 241). e conic frustum included angle was 22.5°, this being the result of producing 4 equi- spaced holes in each disk. Starting on the bottom (disk 1), then stopping the machine and tting the middle disk (disk 2) and drilling the 4 holes and likewise up- ward to the top disk (disk 3), while simultaneously producing a 3-dimensional Isosceles triangle 33 (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). ese 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 ese side lengths and associated angles can be varied, so long as they both (i.e lengths, or angles) remain of identical proportions. ric relationships were intrinsically set and datumed to a centrally-machined slot in the base of the precision mandrel. is fact, meant that the exact angular and volumetric relationships remained in-situ, when the stereometric artefact was then taken o the machine tool for subsequent analyses. Stereometric Artefact – Machining Trials Prior to the stereometric artefact having its machin- able 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 duty- cycles in both a loaded and unloaded condition; to- gether 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 de- sired orientation, with respect to each other and the machine tool’s axes. is mandrel body was produced from a eutectic steel 34 (0.83% carbon), which aer through-hardening to 54 HR C , 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 com- position 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 ne alternate layers of: Fe 3 C 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 diusion (i.e undertaken at greater than the ‘critical cooling velocity’), with the carbon atoms now being eectively ‘xed’ – though not intrinsically part – of the atomic lattice structure. is carbon entrapment, creates intense local strains that block dislocation movement. Hence, the resulting structure is both hard and ex- tremely strong, but also very brittle. Microscopically, the hard- ened structure appears as an array of random needles, being completely dierent from the original pearlitic structure. is needle-like structure formed by trapped carbon atoms in an iron crystal lattice is termed, ‘martensite’. us, the degree of hardness – aer quenching, being proportional to its lattice strain. Aer 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. Chapter 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, dow- elling datums (i.e. φ6 mm) were drilled and reamed, then 3 equi-spaced tapped clamping holes were pro- duced 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: • ese aerospace-grade aluminium disks were milled to φ300 mm diameter, which directly cor- responded to the radial path of the Ballbar (i.e see Fig. 242a) – used previously for diagnostic machine tool assessment, ensuring that some degree of cor- relation occurred between them, • e three Z-plane disk heights of: 70, 170 and 270 mm (i.e. modied from the original design Fig. 241), coincided with both the X-Y plane table po- sition and vertical heights utilised for the Ballbar plots, creating a reasonably large cylindrical volu- metric envelope (Fig. 242b). Moreover, the stereo- metric artefact was both designed and orientated to coincide with the start and nish positions of the Ballbar’s polar traces, • e 4 circular interpolated holes (φ10 mm) on each disk (i.e see Fig. 241), were geometrically posi- tioned to form a three-dimensional Isosceles tri- angle at the three Z-axis heights for each quadrant of these disks – with the 1 st an 3 rd holes relating to the axes transition points in the X-Y planes. us, each of the interpolated milled holes in the face of separate disk’s, produced the geometric stereom- etry of a conic frustum, having an included angle of 22.5° – when the angular orientation of the middle disk is ‘soware-realigned’ to produce a straight- line relationship (i.e see Fig. 242b), NB e temperature at which tempering is undertaken is critical, thus between 200–300°C, atomic diusion 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 diusion form- ing cementite, with a corresponding drop in the component’s bulk hardness. * A eutectic structure is a two-phase microstructure resulting from the solidication of a liquid having the eutectic compo- sition: the phases exist as ne lamellae that alternate with one another. (Sources: elning, 1981, Alexander et al., 1985; Cal- lister, Jr. et al., 2003) • Overall weight of the mandrel and three disk as- sembly 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 ma- chinable disks, HSM was employed using a spindle- mounted ‘Speed-increaser’ 35 (Fig. 243a) equipped with a φ6 mm slot drill. e HSM speed-increaser was oper- ated 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 ma - chinable 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 assem- bly was nearing completion, having previously had its φ10 mm quadrant-positioned holes for each disk ma- chined 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 Aer HSM by milled interpolation on the vertical machining centre, the complete artefact with its ma- chinable 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). is CMM having previously been thermally error-mapped, then checked with a ‘Machine Check- ing Gauge’ 36 (MCG) – prior to artefact inspection. e 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 xed relationship geared head. Here, this actual speed-increaser had a 3:1 gear- ing 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 po- tential ‘lobing-type errors’ from the ‘triggering-positioning’ mechanism of the touch-trigger probe, these being invariably used on such machines. Machining and Monitoring Strategies 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] . Chapter . operation undertaken on a vertical machining centre. [Source: Smith, Littlefair, Wyatt & Berry, 2003] . Machining and Monitoring Strategies holding: chucks and face-plates; in combination. invariably used on such machines. Machining and Monitoring Strategies Figure 241. Artefact stereometry was designed for the volumetric and positional uncertainites on machining centres, by: HSM. trepanning xture and dual-plane balanced workpiece holder: utilised for an UHSM research programme of work. [Source: Smith, Hills & Littlefair, 2005] . Machining and Monitoring Strategies (Fig.