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9 Machining and Monitoring Strategies ‘Inque brevi spatio mutander saecla animantum Et quasi cursores vitai lampada tradunt.’ TRANSLATION: ‘e generations of living things pass in a short time and like runners hand on the torch of life.’ (94 – 55 BC) [In: On the Nature of the Universe, II] 9.1 High Speed Machining (HSM) Background to HSM Possibly the rst work of note on HSM was that of Sa- lomon who ran a series of applied experiments from the period 1924 to 1931, when a German Patent was granted for this work. e Patent was founded upon a series of cutting speed curves plotted against machin- ing temperatures for a range of materials (Fig. 214). In these tests Salomon achieved peripheral cutter speeds of 16,500 m min –1 , utilising either y-cutters – for chip morphology data, or helical milling cutters – notably when cutting aluminium. Salomon contended that the cutting temperature ‘peaked’ at a specic cutting speed, which he termed the ‘Supercritical speed’ 1 , fur- ther, when the speed was increased still faster the tem- perature he noted, decreased. Moreover, either side of this ‘supercritical speed’ zone, it was suggested that ‘unworkable regimes’ occurred where the HSS cut- ters could not withstand the severe forces and tem- peratures generated. As mentioned, when the cutting speed increased beyond this ‘zone’ the temperature reduced to those expected by ‘normal data’ cutting conditions, permitting practical cutting operations to be carried out. e problem being with this early HSM work is that any theoretical rationale is not available and the experimental procedures are somewhat un- clear, but Salomon can be considered the founder of ultra-high speed machining (UHSM) – taking cutting 1 ‘Supercritical speed’ mentioned by Salomon when HSM, has never been truly substantiated. e practical data was based upon chip morphology* experiments with ‘y-cut‘ climb-mill- ing of: non-ferrous alloys; ‘so’ aluminium; red cast brass; the latter being ‘un-machinable’ with HSS cutters between 60 to 330 m min –1 (i.e. see Fig. 214). As the machining parameters and experimental details only exist as a partial fragment of the original German work. is fragmented machining informa- tion, is because during the Second World War, unfortunately, the vital details were lost, moreover, none of the participants in this work also surviving to explain how this applied re- search data was collated. *Chip morphology was achieved by situating a heavily wax- coated board, this being strategically positioned to allow the y-cut chips to stick to the board – during the peripheral climb-milling experimental operation, ready for future analy- ses. data beyond that considered in the so-called ‘Taylor- equations’. Eectively it can be established that there were four distinct periods of advancement in the eld of UHSM, with the rst one being from the early 1920’s to the late 1950’s, with each period during this time and there- aer, being separated by a signicant event. Obviously during the rst period, the work instigated by Salomon (i.e. in the 1920’s – alluded to above), was followed by the originally-commissioned United States Air Force (USAF) major research contract, being from 1958 to 1961. Previous to this contract award, little in the way of UHSM work in the preceding decades had oc- curred, apart from that in the States by Vaughn (1958). Vaughn, shortly became aware of the Salomon Patent, acquiring limited information on this work through the United States Consul in Berlin. Vaughn’s (Lock- heed) group were also familiar with the many tech- nical references concerning the ‘art’ of oil well tube perforation utilising perforated cutters, these ‘cutting actions’ being employed to perforate oil well casings at explosive speeds. Such background work, now meant that Vaughn had ‘set the scene’ for the second period of UHSM development. e USAF Materials Laboratory commissioned study – mentioned above, which was awarded to Lock- heed (i.e. Vaughn’s group), with the objective of evalu- ating the ‘machining response’ for a selected range of high-strength materials to that of (surface) cutting speeds of up to 152,400 m min –1 . e ‘principal aim’ of this ‘USAF-commissioned work’ was to increase producibility, while improving both the quality and eciency of the fabrication of aircra/missile com- ponents. Vaughn’s experimental apparatus included the use of modern-day ex-military cannons, which were positioned on ‘railed-sleds’ to minimise subse- quent recoil upon ring, while obtaining the desired ballistic exit velocity of the ‘material-slug’ for the cut- ting speeds 2 . Unfortunately, these ‘machining results’ 2 ‘Ballistic cutting speeds’ , were obtained as the cannon red the projectile (i.e. a specic material-slug) at ultra-high speed out of the cannon. At the projectile’s exit from the cannon’s barrel, a very robust cutting tool arrangement was situated and held in an accurate position to take a linear cut along the exiting slug’s periphery. e evidence of this ballistic cutting action was then recorded by very high-speed photographic equipment – this being both electronically-timed and stra- tegically positioned, for visual dynamic recording and future reference and analysis. Chapter Figure 214. Graphical relationship of high speed machining of metals – according to Salomon’s machining trials. [Source: Salomon, circa 1920’s–30’s] . Machining and Monitoring Strategies did not indicate how such ballistic speeds might be in- corporated into a production application, also, an ana- lytical model of this high-rate cutting phenomenon was not developed. In the 1960’s and early 1970’s some consolidation of important machining data occurred, with notable work on UHSM cutting mechanisms, etc., that oc- curred in various countries being led principally by the USA in work from: Coldwell and Quackenbush (1962), plus Recht (1964); Okushima et al. (1966) and Tanaka et al. (1967) in Japan; Fenton and Oxley (1967) in the United Kingdom; and Arndt (1972) in Australia. However, although there was a general increase global research activity during this period, it was somewhat disparate and mainly of a fundamental, rather than ap- plied nature. e third period of HSM development was insti- gated in the mid-to-late 1970’s by the United States Navy, in conjunction with the Lockheed Missiles and Space Company, Inc., who contracted a series of ma- chinability studies on marine propellers 3 . Here, the Lockheed group headed-up by King, were mainly concerned with the feasibility of utilising HSM in a production environment, primarily for aerospace alu- minium alloys, then later, working on nickel-alumin- ium-bronze. King’s team at Lockheed, demonstrated that it was economically feasible to introduce ‘high- speed-machining’ procedures into the production en- vironment, thereby realising the signicant improve- ments in productivity with this HSM application. Such applied research work, promoted signicant activity and interest in this HSM eld and, it soon became clear that attention needed to be focussed for all of the subsequent small and diverse research programmes. 3 ‘Marine propeller manufacture’ , whether from a wrought- solid material, or nishing-o a casting, is probably the most dicult and complex multi-axes milling operation that can be undertaken – due to the fact that the propeller surfaces to be machined are continuously changing their geometry as they are essentially parabolically-curved. Invariably, the part geometry is typied by say, the NACA/NASA Standard 16-021 ‘aerofoil cross-sectional prole’ , which for high-performance propellers are exacerbated by normally having both consider- able rake and skew to each blade – creating an exceedingly complex geometry and llet where the boss and blades inter- sect*. * See: Smith and Booth (1993) paper – in the references, which goes some way to explain the propeller manufacturing subject, and for more detailed multi-axes machining and for subsequent machined propeller measurement information. Finally, in these formative years of experimental work into HSM, the fourth period of development be- gan in 1979, when the USAF awarded a contract to the General Electric Company, in this instance, to provide a scientic basis for faster metal removal via HSM and Laser-assisted machining 4 . A further contract by the USAF in 1980, was also awarded to ‘General Electric’– the group also being headed by Flom (1980). With this new HSM contract, also being granted to Flom’s group, with the objective to evaluate the production implica- tions of the previous contract. Both of these contracts being supported by a consortium of industrial com- panies and selected universities in the USA – initiat- ing the fourth HSM period of development. At around this time (i.e. in the late 1970’s), the introduction of computer numerical control (CNC) systems occurred and as a result, they were immediately being tted to a new range of machine tools, signicantly enhanc- ing both their usability and programming capabilities, acting as a catalyst in the development of HSM strate- gies. As these CNC controllers became more sophis- ticated and processing speeds substantially increased, this meant that the potential for HSM could now be fully realised. In recent years, HSM machine tools are just about everywhere in machine shops around the world, where ever there is a need for highly produc- tive part production with very fast cycle times. Obvi- ously, on HSM machines as the spindle speeds have increased in association with both tool and their re- spective workpiece materials (i.e. see Fig. 215), this has meant that there are now considerable design implica- tions on these machine tools, these topics will now be succinctly discussed. .. HSM Machine Tool Design Considerations Prior to a discussion concerning machine tool design factors that must be addressed, before to fully-imple- menting this HSM technology, one might ask the question: ‘Why do we need to rotate cutters at such high spindle speeds?’ ere are a number of advantages that can accrue from adopting such a progressive produc- 4 USAF contract to the General Electric Company in 1979, was: F 33615-79-C-5119 – for a feasibility study into the fast metal removal operations by HSM and Laser-assisted machining. Chapter tion machining strategy (i.e. see Fig. 216) and, they can be succinctly summarised as follows: • Direct benets – improved machining eciency, – reduction in cutting tool varieties, – reduction in distortion of workpiece, – eectiveness of swarf removal. • Indirect benets – quality of nish improved, – increased cutter life, – reduced changes in material properties, – capability of machining thin walls/sections. ese production improvements are by no means all that occur, as invariably, due to the superior machined surface texture, the nal part surface may not need to be deburred – a signicant real saving on complex component geometries. Moreover, by employing an HSM strategy, more simple xturing can be utilised, as the actual tool forces are signicantly lessened. It is an established fact that with higher rotational cutter speeds the resulting cutting forces and tool push-o are considerably reduced. In order to benet from these improved cutting practices, the machine tool’s axes must have both faster acceleration and de- celeration – see Fig. 221, more will be mentioned con- cerning these important dynamic aspects of a machine tool’s operational performance shortly. Many of today’s conventional and HSM machine tools, are based upon a modular design concepts (Fig. 217a). is modular design philosophy, allows the machine tool builder the opportunity to standardise certain features over a range of machine tools. Such practice benets the manufacturer and consumer alike, by reducing design Figure 215. The chronological development of cutting tool material introductions, which had an inuence on high-speed cut- ting trends. [Courtesy of Yamazaki Mazak Corporation] . Machining and Monitoring Strategies and development costs, while minimising the custom- er’s purchasing costs, yet still allowing more attention to be given to the detailed design of each ‘module’ in the machine. So, an identical column, or table may be common to a variety of machines and this trend can oen be seen across a whole product range of ma- chine tools. Not only are the major castings, or large fabrications manufactured by employing modular concepts, but this design philosophy also allows any other smaller components to be standardised and t- ted accordingly. Such as: the recirculating ballscrews; servo-motors; linear scales – if tted; etc.; plus other standardised items to be built into the constructed machine tool ranges. In order to minimise ‘stick-slip’ 5 in the slideway mo- tions on heavy moving cast and fabricated members – 5 ‘Stick-slip’ or ‘Stiction’ , is the jerky-motion between sliding members due to the formation and destruction of junctions [due to localised pressure-eects]. (Kalpakjian, 1984) Figure 216. Diagram illustrating the main benets to be gained from adopting a high-speed ma- chining strategy. [Source: Smith, CNC Machining Technology, 1993] . Chapter thereby giving faster response to CNC commands, ‘Ty- choways’ (i.e see Fig. 217b), or similar mechanical aids are usually strategically situated at set positions along each hardened way of the machine tool’s orthogonal axes (i.e normally at the ‘Airy-points’ 6 ). 6 ‘Airy-points’ , are associated with any ‘elastic body’ that is subject to load, where will undergo elastic deformation. e magnitude of this deformation depends upon the extent of the: load; contact area; plus the mechanical properties of the contacting materials. Hence, if the prospective body is not correctly supported, it will be subject to elastic deformation – under its own weight. is problem was considered in 1856 by Sir G.B. Airy (Astronomer Royal) – who was interested in minimising the elastic deections that arose when attempt- ing to support and reduce the sagging in long focal length refracting telescopes. Airy showed that by positioning these two supports at prescribed distances, they could minimise any potential error – when equated to the overall length of the ‘elastic member’ (i.e. in the length of the telescope). He found that two conditions occur, one being for ‘line-standards’ – to bring the ends square for measurement and calibration, and secondly, relating to machine tools and many metrological applications, where the ‘Points of minimum deection’ were more appropriate, as follows: Distance between each support = 0.554L Using conventional recirculating ballsrews (Fig. 217c) for HSM applications is possible up to 100 m min –1 – in certain applications. Aer these linear velo- cities have been reached and it is necessary to reverse the axis direction, this can create ‘ballscrew wind-up’ problems. is ballscrew twisting, is despite the fact that the ballscrew is very sti of the order >2,000 N µm –1 , so if greater kinematic and dynamic perfor- mance is required, then it might be necessary to utilise linear-motor drives. A tabulated table for suitable comparisons of the various types of motional drive systems available today, is presented in Table 14. It has been widely-reported that either a high-qual- ity lead-, or ball-screw having rotary encoders, will have a unidirectional repeatability to within 6-to- Where: L = overall length of the ‘elastic member’. (Galyer and Shotbolt et al., 1990) Example: If a machine tool’s structural moving member is 1,000 mm in length and it is situated on a much longer ‘bed- way’ , then the ‘Tychoways’ should (ideally) be symmerically- positioned (i.e. xed to the moving member) at a distance of 554 mm apart – in order to obtain the minimum of elastic dis- tortion as it moves backward and forward along the hardened bedway. Table 14. Comparison of dierent drive systems for machine tools CONTRIBUTIONS: Leadscrew: Ballscrew: Belt-drives: Linear motor: Noise Quiet Noisy Quiet Moderate Back-driving Self-locking Easyback-drive Backlash Increases with wear Constant Increases with belt wear Negligible Repeatability ±0.005 mm ±0.005 mm ±0.004 mm Best (<2 µm) Duty Cycle Max. 60% Max. 100% Eciency Bronze bushing 40% 90% 90% 90 to 95% Life Short: high friction Longer Longer Longest Shock-loads Higher Lower Low Highest Smoothness Smooth: low speeds Smooth at all speeds Smoothest Speeds Low High Higher Highest Cost ££-Lowest £££-Moderate ££££-Highest [Source: Johnson, 2001] . Machining and Monitoring Strategies Figure 217. Typical ‘modular design’ and construction of machining centres, with ‘ballscrews’ and ‘tychoways’. [Courtesy of Cin- cinnati Machines] . Chapter 7 µm. However, if linear encoders are tted this mini- mises any potential ‘ballscrew wind-up’ , although the problem of an ‘Abbé -error’ 7 is still present. Yet another source of machine tool-induced prob- lems are more specically termed ‘uncertainties’ 8 , while ‘hysteresis’ 9 can also contribute to the overall ‘er- ror budget’. us, hysteresis may result when the same position has been commanded by the CNC controller, 7 ‘Abbés Principle’ – was derived by Professor Ernest Abbé in 1890 (i.e. having studied and graduating from the University of Jena) and is still valid today. e Abbé principle simply states that: ‘e line of measurement and the measuring plane should be coincident’. An example of this ‘Abbé Law’ well- known to engineers the world over, shows that a conventional micrometer calliper ‘virtually-obeys’ the ‘Abbé principle (i.e. there is a small ‘cosine error’ present – which can usually be ignored), whereas, the Vernier calliper has a much larger o- set between the xed and moving jaws – where the compo- nent being measured is situated, to that of the beam – where the scale’s reading for this measurement is taken. Ideally, both measurement and reading should be lined-up, without an o- set. [Sources: Busch, 1989; Whitehouse et al., 2002] 8 ‘Uncertainty of machine tool positioning’ – the question oen asked in calibration-related tasks, is: ‘What is measurement uncertainty?’ Uncertainty of measurement refers to the doubt that exists about any measurement; there occurs a margin of doubt for every measurement. is expression of measure- ment uncertainty raises other questions: ‘How large is the mar- gin?’ and ‘ How bad the doubt?’ Hence, in order to quantify uncertainty two numbers are required: (i) being the width of the margin – its interval, (ii) plusits condence level. NB is latter value states how sure one is that the actual value occurs within this margin.For example:On a CNC ma- chine tool, the command may be to move the X-axis slide- way 1000 mm plus, or minus 0.05 mm at the 95% condence level. is uncertainty could be expressed, as follows: X-axis slideway motion = 1000 mm ± 0.05 mm, at a level of con- dence of 95%. In reality, what this statement is implying to either the programmer, or calibration engineer is that they are 95% sure that the actual X-axis position now will lie between: 999.95 mm and 1000.05 mm. Many factors can contribute to the overall ‘error budget’ as it is sometimes known, but this is beyond the scope of the present discussion – see references for further information. (Smith, 2002) 9 ‘Hysteresis’ , can be dened as: ‘e dierence in the indicated value for any particular input when that input is approached in an increasing input direction, versus when approached in a de- creasing input direction.’ (Figliola and Beasley, et al. 2000)For example:Hysteresis usually arises because of strain energy stored in the system [i.e. in this case, within the machine tool], by slack bearings, gears, ballscrews, etc. (Collett and Hope, 1979) but from opposite directions, causing the motion sys- tem to creep by an amount larger than the backlash alone (i.e. the hysteresis). is eect is the result of un- seen working clearances and compounded by the ma- chine’s elastic deformations, although by pre-loading the ballscrews it will minimise some of these eects. In HSM machining applications, all ballscrew and indeed any screw-driven systems have some additional limitations, such as its ‘critical speed’ of rotation. At the critical speed, a ballscrew starts to resonate 10 at its rst natural frequency (i.e termed ‘whipping’). Hence, the critical speed is proportional to the ballscrew’s diam- eter and is inversely proportional to the distance be- tween the screw’s supports – squared. For a very long and slender screw-driven machine tool application with wide supports, here, most recirculating ballscrews would have a critical speed of approximately 2,500 rev min –1 . It should also be said, that for many ballscrews assemblies they can be rotated at higher rotational speeds than the 2,500 rev min –1 previously mentioned, before they reach their critical speed, but for very fast accelerations and decelerations, then they become in- creasing challenged. In fact, on some HSM machine tool congurations, multi-start ballscrews have been employed to increase linear response, but here the ‘critical speed’ will probably be lessened – due to re- duced inherent ballscrew stiness. Even ‘matched’ twin ballscrews have been tted to HSM machine tools – to minimise any potential ‘yawing motions’ at high lin- ear speed by the moving member along the machine’s bedway. ese ballscrew limitations are probably why linear-motional drives are becoming a realistic alter- native, but they are only tted at present, to high capi- tal cost HSM machine tools. One of the bi-products of HSM’s greater stock re- moval rates, is the excess volume of hot swarf which must be speedily and eciently removed from the vi- cinity of the machine – which is more readily achieved for horizontal machine tool congurations. er- mal eects in general on any machine tool become a problem, particularly as many milling spindles utilise direct-drives, with the motor being mounted in-situ with the spindle. Here, the spindle motor creates heat, the thermal eects of which can be analysed by either 10 ‘Resonant frequency’ , can be dened as: ‘e frequency at which the magnitude ratio reaches a maximum value greater than unity.’ (Figliola and Beasley et al., 2000) Machining and Monitoring Strategies a three-, or ve-probe spindle analyser, as depicted in Fig. 218 along with a typical case-study for a spindle’s thermal dri graph. Not only can such spindle analysis equipment determine a spindle’s thermal growth, it can also detect: thermal distortion; spindle error; machine resonance; vibration; plus repeatability. In the case- study graphically-depicted in Fig. 218b, the variation in the machine tool’s temperature is the major cause of positioning error. is thermal dri graph oers-up a number of signicant questions that need to be ad- dressed, such as: ‘How long does it take for the machine tool to stabilise?’; ‘How much Z-axis growth does the machine produce at full spindle speed?’; ‘How far has the machine’s displacement become, because of distor- tion in the machine tool structure?’. Spindle analysers are able to operate at rotational speeds varying from 0 to 120,000 rev min –1 , making them an ideal tool for condition monitoring diagnostics on machine tool’s equipped with HSM spindles for subsequent analyses. In Appendix 15, a trouble-shooting guide has been produced to high-light: problems; causes; tests; etc.; that can be obtained from analyses by employing ma- chine tool spindle analysers. In Fig. 219, the polar plots illustrated show how the spindle analyser has the ability to evaluate both a new and rebuilt machine tool spindle’s condition. In the spindle error indicated on the polar plot depicted in Fig. 219a, it illustrates here, that a very badly rebuilt spindle is simply not acceptable in this state. is poor spindle performance, is in the main, largely the result of signicant radial variability (i.e. total error: 12 µm @ 4,005 rev min –1 ), which would severely compro- mise any cutting tool’s machining performance. Con- versely, in Fig. 219b, a well-worn spindle assembly is shown prior to rebuild, having a total error of 4.6 µm @ 1,702 rev min –1 , aer its ‘correct’ rebuilding (Fig. 219c), the total error has been drastically reduced to a total error of 1.9 µm @ 1,700 rev min –1 . Visually, the dierences between these two polar plots is quite as- tounding, in that both the asynchronous and average errors present have virtually disappeared in the latter case, making it ‘as-new’ and, ready to perform signi- cant machining service. It is well-known fact by ma- chinists familiar with their older and heavily-utilised machine tools, that certain machine spindles will run smoother and produce better machined roundness on workpieces if they are run at the so-called ‘sweet-spot’ , equally, the quality of the parts produced will be af- fected if the spindle is run at its ‘sour-spot’. By utilis- ing a spindle analyser, signicant information can be gleaned from such rotational analyses, allowing speed ranges to be selected which would currently optimise the present status of the spindle’s condition, prior to its potential rebuild – when called for at a due date in the future. For most machine tools today that are involved in HSM activities, in general the spindle cartridges are of three distinct design congurations – which inci- dentally do not normally include ball bearing spindle types 11 , such as: • Magnetic ‘active’ spindles (Fig. 220a) – typically might have a cartridge with a speed range from: 4,000 to 40,000 rev min –1 , delivering 40 kW con- tinuous power, peaking at over 50 kW . Such ‘active’ magnetic spindles can maintain 1 µm maximum run-out, by digital control of the series of speci- cally-orientated magnetic currents – being initi- ated by radial and axial sensors, that continuously monitor the spindle’s rotational axis position 10,000 times second –1 , NB Further renements to the spindle occur, with these temperature-controlled milling spindle car- tridges maintaining dynamic balance, regardless of the milling cutting loads imposed, this latter fac- tor being an important criterion when attempting to reduce cutting tool vibrational eects. However, these ‘active’ spindles are not cheap to purchase and run, with another negative eect being that they are normally rated for only several thousand hours op- erational running time. Such cartridges come with a variety of rotational speed ranges and spindle power outputs. • Pneumatic spindles – have been available for many years, with aerostatic bearings equalising the forces exerted while cutting and remaining centralised within the spindle’s housing, yet still achieving dy- namic rotational balance, 11 ‘Ball bearing spindle designs’ , are not normally specied for HSM operations, because at around 20,000 rev min –1 – this be- ing ‘mechanically-set’ as the upper rotational velocity of such spindles. So, due to these high rotational speed eects and of the combination of centrifugal forces, it means that at approx- imately 80 m sec-1 rotational speed, the balls will lose contact with the journal walls. As a result of this loss-of-contact the hardened balls and raceways will rapidly wear out (i.e. the re- sults of so-called: ‘Brinelling’ in the raceways, creating both poor circumferential wear patterns, delaminations and associ- ated frictional eects – leading to major debilitating spindle roundness modications.) (Smith et al., 1992) Chapter . from: Coldwell and Quackenbush (19 62), plus Recht (19 64); Okushima et al. (19 66) and Tanaka et al. (19 67) in Japan; Fenton and Oxley (19 67) in the United Kingdom; and Arndt (19 72) in Australia ££££-Highest [Source: Johnson, 20 01] . Machining and Monitoring Strategies Figure 217 . Typical ‘modular design’ and construction of machining centres, with ‘ballscrews’ and ‘tychoways’. [Courtesy. in 19 79, was: F 33 615 -79-C- 511 9 – for a feasibility study into the fast metal removal operations by HSM and Laser-assisted machining. Chapter tion machining strategy (i.e. see Fig. 216 )
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