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Int J Adv Manuf Technol (2002) 20:639648Ownership and Copyright 2002 Springer-Verlag London LimitedAn Advanced Ultraprecision Face Grinding MachineJ. Corbett1, P. Morantz1, D. J. Stephenson1and R. F. Read21School of Industrial & Manufacturing Science, Cranfield University, Bedford, UK;2Cranfield Precision, Division of Landis Lund,Cranfield University, Cranfield, Bedford, UKCranfield Precision, Division of Landis Lund, has recentlydeveloped an ultraprecision face grinding machine which incor-porates several automatic supervision features. The companysupplied the machine to Cranfield Universitys Precision Engin-eering Group in order that the group can undertake research,particularly in the area of damage-free grinding with highsurface and subsurface integrity. The paper discusses thedesign of the machine, initial machining trials and potentialresearch projects. Such projects will benefit from the avail-ability of such an advanced machine system which incorporatesmany state-of-the-art features for the automatic supervisionand control of the machining process.Keywords: Automatic supervision; Grinding; Machine tooldesign; Precision machining1.IntroductionCranfield Precision, which is a UNOVA Company, specialisesin the design and manufacture of machines for cost-effectiveproduction of components in advanced materials includingceramics, glasses, intermetallics and hard alloy steels. TheSchool of Industrial and Manufacturing Science (SIMS), Cran-field University, places great importance on developing closecollaborative links with industry and is currently undertakinga range of ultraprecision and high-speed machining researchprojects including superabrasive machining, ductile machiningof brittle materials and precision machining for the automotiveindustry. The complementary research interests of the twoorganisations have resulted in Cranfield Precision developingand supplying an advanced ultraprecision face grinding machineto the Precision Engineering Research Group within SIMS.This will enable the group to undertake a wide range ofresearch programmes, particularly in the area of damage-freegrinding with high surface and sub surface integrity.Correspondence and offprint requests to: Prof. J. Corbett, School ofIndustrial and Manufacturing Science, Cranfield University, BedfordMK43 0AL, UK. E-mail: j.corbett?cranfield.ac.uk.Materials processing with nanometric resolution and controlis viewed as a mid- to long-term solution to the cost and timeproblems that plague the manufacturing of electro-optics andother precision components. For example, ductile grinding ofbrittle materials can provide surfaces, as ground, to nanometresmoothness and figure accuracy at higher production rates thanusually encountered 1. More significantly, a ductile groundsurface experiences little or no subsurface damage, therebyeliminating the need for the subsequent polishing step associa-ted with conventional grinding. The performance of many“microfeatured” products (e.g. semiconductor, optical communi-cations systems, computer peripherals, etc.), as well as largercomponents for aerospace and automotive applications, dependsincreasingly on higher geometric accuracies and micro- andnanostructured surfaces. Recently, the automotive industry hasindicated a future requirement for the surfaces of certain keytransmission components to be of “optical” quality, with targetsof 10 nm Ra surface finish to be economically produced onhardened steel by direct machining, without polishing.The conditions under which damage-free surfaces can beproduced on glasses and ceramics, and “optical” surfaces canbe produced on hardened steel, are exacting, requiring (a) theuse of a class of machine tool not normally found in even thebest production facilities, e.g. high accuracy, smoothness ofmotion, loop stiffness 2, (b) the incorporation of ancillaryfeatures specially developed to suit the particular application(e.g. grinding wheel truing and conditioning), and (c) the useof the correct grinding technology for the application (manyvariables wheel type, coolant, speeds, feeds, etc). All theconditions must be satisfied and the wafer face grindingmachine has been developed to meet them.2.ObjectivesIn order to meet the demands of surface integrity and pro-ductivity mentioned above, for a wide range of components,the principal objectives include the development of:1. A machining process capability for the manufacture ofsizeable components with high levels of surface/subsurfaceintegrity.640J. Corbett et al.2. Optimised “ductile mode” machining processes for brittlematerials (glasses and ceramics).3. A single process, with only one set-up, to replace the typicalthree-stage lapping, etching and polishing process, resultingin much higher productivity.3.The ProcessA prime requirement of the process is that it should be capableof machining extremely flat surfaces on workpieces up to 350mm diameter. Further, the surfaces should be smooth (50 nmRa) and have minimum subsurface damage. Ideally the surfaceshould be close to the quality obtained by polishing. In orderto meet these demanding requirements rotation grinding isutilised. A feature of rotation grinding is that unlike conven-tional surface grinding, it has a constant contact length andconstant cutting force. Figure 1 illustrates the grinding prin-ciple. Both the cup grinding wheel and workpiece rotate andthe axial feed of the grinding wheel removes stock from thesurfaceoftheworkpieceuntilitreachesitsfinalthickness/geometry.4.The MachineThe demanding requirements of the process and componentquality necessitate a machine of extremely high loop stiffness.The design targets for the face grinding machine (Fig. 2) are:1. Loop stiffness better than 200 N ?m?1with good dynamicdamping, required to achieve submicron subsurface damage.2. Control of pitch (wheel to component surface) to betterthan 0.333 arc seconds, required to achieve a total thicknessvariation (TTV) tolerance of 0.5 ?m.3. Control of cut-depth to better than 0.1 ?m, required toachieve submicron subsurface damage.4. Axial error motions of spindles better than 0.1 ?m, requiredto achieve submicron subsurface damage.5. Measurement and feedback of component thickness to 0.5?m, required to achieve micron thickness tolerance.Fig. 1. Face grinding operation.The geometry of the ground flat surface is determined by therelative position of the rotary axes of the grinding wheel andworkpiece. Figure 3 indicates the relative machine motions andaxes. There are 11 axes, plus three automatic robot loadingmotions (not shown), all driven under servo control. These are:S1Grinding spindleCWorkhead spindleZInfeedXCrossfeedS2Truing spindleWDressing axisATilt pitchBTilt yawS3Chuck wash brushPProbe thicknessWash armAs described below, the flatness accuracy can be achievedby the superimposed rotations of the rotary axes coupledwith an appropriate spindle alignment strategy. Further, thisprototype research machine benefits from the incorporation ofthe following state-of-the-art features for the automaticsupervision and control of the machining process.4.1Adjustment of the Workpiece and GrindingWheel PlanarityThe relative alignment of the two rotary spindles S1 and C(Fig. 3) is simplified because the geometry of the groundsurface can be described by geometrical equations. The grind-ing process requires a specific angle between the plane of thegrinding wheel and the plane of the workpiece to be maintainedas the Z-axis infeed is applied. This angle is typically muchless than a degree, so that the workpiece and wheel are nearlyparallel. This angle is monitored by three gauging LVDTsensors which measure the displacement between the grindingspindle housing, and a precision-machined surface on the work-spindle housing. The gauging sensors are placed around thegrinding spindle housing, roughly equidistant from the centreof the wheel spindle axis in the plane of the wheel, at aknown separation. The information from these sensors is fedback into the control system to amend the control for the A-(pitch), B- (yaw) and Z- (infeed) axes. This is a unique featureof the machine, to maintain workpiece flatness because, as theworkpiece subsurface damage reduces and the surface finishimproves the grinding forces increase significantly. This hasthe effect of distorting the grinding spindle to workhead align-ment, which then produces non-flat surfaces. On conventionalmachines this alignment is adjusted by mechanical trial-and-error adjustment, and relies on the force and deflection alwaysbeing uniform. However, on this machine if the process con-ditions are changed, the alignment is automatically compensatedfor. This can then be optimised to suit the material and wheelconditions by changes in the software of the control system.A functional block diagram for the servo control of the Z-axisis illustrated in Fig. 4.Ultraprecision Face Grinding Machine641Fig. 2. Face grinding machine.Fig. 3. Axes nomenclature.4.2Grinding WheelsThe roughing and finishing wheels are concentrically mountedon one spindle via a patented system which incorporates anadvance/retract mechanism for the roughing wheel, as shownin Fig. 5. In order to maximise component throughput, a coarse-grained wheel is first used to obtain a high material removalrate. The fine-grained finishing wheel is then used to obtainthe finished size and surface integrity.4.3Detecting Grinding Wheel ContactAcoustic emission (AE) sensors are used to establish the initialtouch between the grinding wheel and component. Because ofthe importance of establishing first touch to very fine limits,when finish grinding, ring sensors are used on the workheadand grinding spindles. These are extremely sensitive and arelocated at the front of the spindles, close to the signal source.An on-machine grinding wheel truing spindle is also fittedwith AE sensors which enables “touch dressing” of the grind-ing wheel.4.4Automatic Measurement of Grinding ForcesGrinding forces are measured via sensors placed within theforce loop away from external forces, such as lead screw nutsand their associated friction. Measurement of the grindingforces gives a good indication of grinding wheel wear.4.5Measurement of Grinding Wheel Wear andComponent ThicknessGrinding wheel wear is monitored together with componentthickness.AspeciallydesignedanvilandLVDTprobeassembly are used to measure component thickness. This isdone by initially datuming the anvil and probe on to the porousceramic vacuum chuck face to which the component is fixed.When measuring the component thickness, the anvil, which ison the same slideway as the probe, contacts the chuck datumand the LVDT probe makes contact with the face of thecomponent, thus giving a measurement of thickness. Grindingwheel wear can be found by reading directly the position ofthe Z-axis, and relating this to the chuck face datum position.Thermal growth is measured by pairs of eddy current probesmounted on the workhead and grinding spindles. Any growthis automatically compensated by adjusting the relative positionsof the two spindles.642J. Corbett et al.Fig. 4. Z-axis functional block diagram.Fig. 5. Single axis dual wheel system.4.6Ancillary FeaturesThe machine also has facilities for on machine component andchuck washing and also a robotic loading and unloading capa-bility to load and unload automatically components onto andfrom the workhead spindle.5.Machine CommissioningMachine services consist of an air supply, grinding coolantsupply and motor coolant supply, as well as three-phase electri-cal power. Air is provided by a high performance supply andconditioning system, which delivers clean dry air at 13 bar atover 5000 l min?1. The air consumption within the face grindermachine is around 2000 l min?1in the air bearings, the remain-der being for the various air purge and cleansing systems. Theair is filtered to 0.1 ?m and dried to a pressure dewpoint of?40C as required for the operation of ultraprecision air bear-ings.Coolant supply is by recirculating water. This is pumped at4.5 bar at a flow rate of up to 100 l min?1. Coolant isdistributed to various coolant nozzles, under individual control,as required by the machine process. Used coolant is returnedto the collection tank, and then fed to the main coolant tankby a scavenge pump. Some water-borne debris (workpiece andgrinding wheel residues) settles out here; the remainder isremoved by filtration on the machine in various stages downto 0.01 ?m.Services provision requires a multiplicity of controlled pro-cess fluid distribution points, together with appropriate safetyinterlock and performance monitoring systems.5.1The Control SystemThe control system is in two parts, based on an industrystandard Isagraph PLC, and a Cranfield Precision CNC system.Machine I/O is on a distributed Interbus S system and servocontrol is implemented by a Sercos fibre-optic ring.The PLC program required only minor modification duringcommissioning, most effort being concentrated in further devel-opment of the CNC program, particularly in grinding touchsensing, and in the truing and grinding operations.5.2Machine PreparationIn preparation for grinding operations, an assessment of severalareas was critical:1. Machine alignments.2. Balance of spindles.3. Condition of wheels.4. Application of coolant.5. Control of machine motions.Ultraprecision Face Grinding Machine643Fig. 6. Grinding spindle horizontal amplitude response to out-of-balance forces.These are the major determinants of grinding surface quality,and were tackled in order.5.3Machine AlignmentsThe machine tool builders had set most machine alignmentsaccurately; metrology checks confirmed these to be in order.However, the critical alignment datum (alignment betweenworkspindle axis and grinding spindle axis) had been lost,since the grinding spindle had to be removed, prior to relocatingthe machine in the Precision Engineering Laboratory.This alignment datum had to be re-established by using aminiature eddy current probe (with a measurement range ofaround 6 ?m) mounted on the grinding spindle faceplate. Aspecial purpose alignment jig was mounted on the workspindlefaceplate. Measuring the variation of distance of the probefrom features on the alignment jig, as the two spindles wereindependently rotated, allowed the angular alignment of thetwo spindle axes to be determined, using a multiparameterleast squares fit.Fig. 7. Grinding spindle horizontal phase response to out-of-balance forces.5.4Wheel BalanceThe machine is configured to enable automatic balancing onthe grinding spindle. This is included on the machine toaccommodate the automatic selection of grinding wheels. Thegrinding spindle carries two concentric segmented cup wheels,rough and fine grit; the rough wheel is of slightly largerdiameter. The roughing wheel can be selected automaticallyby sliding it parallel to the spindle axis, under air pistoncontrol, to engage in one of two face tooth couplings, so thatit either projects or is just below the face of the fine wheel.These two configurations, with rough or fine wheel selected,have marginally different out-of-balance moments, and theautomatic balancing is included to compensate for this.Figures 6 and 7 show the amplitude and phase response fora balance (displacement) sensor in a horizontal orientation,located at the grinding spindle nose, over a range of revolutionsper minute (rpm) shown along the x-axis of the figure. The y-axis represents a nominal peak-to-peak displacement at thatrotation rate of the grinding spindle. These data were obtainedsubsequent to fine (single plane) balancing of the grinding644J. Corbett et al.Fig. 8. Grinding spindle vertical amplitude response to out-of-balance forces.spindle. A strong resonant response can be seen at around1200 rpm (or 20 Hz). Figure 8 shows the amplitude responsefor the balance (displacement) sensor in a vertical orientation.The resonant behaviour is entirely absent. Subsequent investi-gation has revealed that the source of the resonance is com-pliance in the “B” grinding wheel tilt axis (a vertical axis), asshown in Fig. 3, which is why this is only apparent to ahorizontally oriented sensor.The truing wheel balance is also critical. Figure 9 showsthe horizontal response of the truing spindle, subsequent tofine balancing. Again a small resonance is apparent in thehorizontal direction, at around 4000 rpm (67 Hz). The truingspindle is mounted on the X-axis carriage, and this horizontalresonance is in the direction of the X-axis motion. Once morethis is due to the compliance of the motion system in its drivedirection. This has a lower impact on grinding performancethan grinding spindle balance, although truing the spindle out-Fig. 9. Truing spindle horizontal amplitude response to out-of-balance forces.of-balance motion will impart a small-scale cyclic topographyon to grinding wheels, which in turn affects grinding quality.5.5Wheel ConditionOn this machine, wheel form is imparted through a truingoperation, and wheel condition is maintained through sub-sequent dressing, in between relatively infrequent truing oper-ations. The truing (forming) operation specified in the machinedesign involves a plunge “grind” similar to the wafer grindingoperation, although in this case the cup grinding wheel andparallel truing wheel make contact at their periphery. Thegrinding wheel form was found to be in error by 0.2 (12minutes of arc). The truing operation was amended to rectifythis. Truing is now accomplished by a plunge and a subsequenttraverse of the X-axis. Correctly applied, this can produce theUltraprecision Face Grinding Machine645correct (planar) wheel form, since the grinding spindle axishad previously been set accurately to be perpendicular to theX-axis motion.5.6Coolant ApplicationConsiderable effort was concentrated in the alignment of thecoolant nozzles, in order to deliver sufficient coolant into thegrinding interface. This is particularly important here wherethe grinding arc of contact is so long, at approximately 200 mmfor a 200 mm wafer.5.7Motion ControlNew grinding routines and complex motion profiles weredeveloped for the grinding. A full wafer grinding cycle consistsof an initial rough grind, followed by a finish grind. In eachgrinding phase, the work and grinding spindles are set torotate, coolant is then applied, and the grinding spindle is fedinto the work rapidly until the acoustic emission sensor detectsa touch. Following rapid deceleration, the spindle is fed infurther, in three stages, at successively lower feedrates, andfor successively smaller feed depths. Finally, after a dwell(spark out) time, the grinding spindle is retracted.This in-feed motion sequence is complicated by the simul-taneous slaved motion of the A and B tilt axes. Completelycoplanar plunge grinding is likely to result in non-planargrinding results. In order to achieve planarity in the finishedsurface, a slight angle must be maintained between spindleaxes at first contact, and this angle is gradually reduced tozero (nominally) at the conclusion of grinding. A furthermodification to the infeed motion is imposed by the results ofthe three-point gauging, which monitors, in-process, the deflec-tion of the machine due to the high static grinding forces. Themeasurements of the gauges modify the demanded anglebetween spindle axes throughout the grinding process.6.Initial Grinding TrialsInitial grinding trials were conducted on monocrystalline sili-con, using 200 mm wafers. Initial grinding trials were conduc-ted, using parameters selected on the basis of extensive researchexperience in silicon grinding. Wheel speeds were selected togive minimum excitation of machine resonances (as indicatedin Figs 6 and 9) i.e. ?2000 rpm for the grinding wheel and5100 rpm for the truing wheel, whilst still maintaining adequategrinding efficiency. The requirement for the total grindingdepth for roughing is set by the need to remove any swashon the wafer, and the requirement for total grinding depth forfinishing is set by the need to remove subsurface damagecaused by the roughing process. Relative wheel speeds duringtruing were set so to give a non-integer ratio. The parametersfor truing, rough grinding and fine grinding are shown inTables 13.A Talysurf form trace indicated that the surface roughnessachieved in the roughing operation was approximately 200 nmRa. Other measurements indicate a sub-surface damage depthTable 1. Truing parameters.Truing wheel typePlated wheel ZD107N200gTruing wheel diameter180 mmTruing wheel speed5100 RPM (48.05 m s?1)Feedrate to first touch40 ?m s?1First grinding feedrate5 ?m s?1Second grinding feedrate0.3 ?m s?1Total machining depth20 ?mGrinding wheel speed2000 rpmTable 2. Rough grinding parameters.Wheel type46 ?m resin VD46-C75-B117Wheel diameter370 mmWheel speed2000 rpm (38.75 m s?1)Feed rate to first touch150 ?m s?1First grinding feedrate2 ?m s?1Second grinding feedrate1 ?m s?1Third grinding feedrate0.5 ?m s?1Total machining depth15 ?mWork speed10 rpmTable 3. Fine grinding parameters.Wheel type6/12 ?, resin bond AD6/12-C75-B118Wheel diameter305 mmWheel speed1800 rpm (28.75 m s?1)Feedrate to first touch80 ?m s?1First grinding feedrate1 ?m s?1Second grinding feedrate0.2 ?m s?1Third grinding feedrate0.05 ?m s?1Total machining depth8 ?mWork speed200 rpmof around 5 ?m. It is this subsurface damage depth that dictatesthe total machining depth for the fine grinding operation.The appearance of the finish ground wafer surface was verygood (Fig. 10) for these initial machining trials. Very faint“ghost” arc lines are discernible, although these are not evidenton the Talysurf traces, nor are they visible on the photograph.They do, however, show a cyclic pattern, which is a productof the grinding wheel rotation rate and the workpiece rotationrate i.e. their effect is exacerbated by any out-of-balanceeffect of the grinding spindle. The cosmetic appearance isoptimised by maximising the rotation rate of the wafer duringgrinding, although this slightly worsens the measured surfaceroughness. The grinding parameters given above represent acompromise; the best surface roughness figures (around 10 nmRa using the current configuration and tools) can be obtainedfor very low workpiece rotation rates (?1 rpm) although thecosmetic appearance is worse, owing to the more prominentappearance of the cyclic pattern.7.Assessment of the MachineThe wafer face grinder has produced some very impressiveresults, following limited process development due to the short646J. Corbett et al.Fig. 10. 200 mm wafer as finish ground 15 nm Ra.timescale of the programme. There are some areas of poten-tial improvement.The impact of servo compliance in B- and X-axes as indi-cated by the displacement measurements for spindle balance(Figs 69) suggest that improvements in servo dynamic stiff-ness and improvements to spindle balance can have immediateand significant effects on ground surface quality given thatthe “once-per-rev” signature from the grinding spindle is amajor factor in the machined surface topography.8.Research ProgrammesThe machine is currently undergoing final commissioningwithin Cranfield Universitys Precision Engineering Laboratoryand a number of research programmes, including those outlinedbelow, have been identified which will benefit directly fromthe availability of the face grinding machine.8.1Micromachining and Planarisation ofPiezoelectric CeramicsPiezoelectric ceramics such as those based on the lead zirconatetitanate system offer the capability for high-accuracy fine-scalemovement coupled with high stiffness and the ability to gener-ate high forces. Such materials are widely used in a host ofdevices ranging from low-cost mass-market applications likebuzzers through inkjet printers 3 to high-end products suchas tunable etalons 4. The latter devices show considerablepromise for channel selection in wavelength division mul-tiplexing 4. There is also considerable interest in the inte-gration of piezoelectric materials with silicon for microsystemdevices. Much work is currently focused on the use of directlydeposited thin- and thick-film materials 5. However, thereare advantages to the use of high-temperature sintered and hot-pressed bulk materials. Firstly, the piezoelectric coefficientswhich can be achieved are much higher. Secondly, the use ofbulk materials opens the possibility of using thicker materials,leading to greater longitudinal displacements for a given appliedfield. Thirdly, and perhaps most interestingly, it opens thepossibility of using multilayer actuator technology, an optionthat is not currently available in directly deposited material.(In this technology the piezoelectric ceramic is made as amultilayer cofired stack with interleaved metal electrodes, giv-ing higher displacement for a given drive voltage than amonolithic device.) One can consider using a process such asanodic bonding to bond a very flat PZT surface (formed oneither a bulk or multilayer material) directly to a silicon wafer,then machine-down the opposite face to leave a flat parallelmaterial bonded to silicon, but exhibiting all the advantagesof the bulk material. Clearly such an idea could also be appliedto the bonding of PZT to other materials such as optical glasses.Thus there are many areas where piezoelectric materials areused or have the potential to be used. In many cases, forexample for the optical applications, there is a need to makehigh-flatness, low-damage surfaces at high speed and lowcost. Work at Cranfield University 69 has demonstrated thepotential for using ductile mode machining for achieving verylow damage, excellent surface finishes in hard and soft PZTs.The intention will be to apply the machine to the preparationof such surfaces for practical applications.8.2Monitoring of Surface Integrity During GrindingThe aim of this programme is to optimise the grinding para-meters to maximise material removal rate whilst ensuring thatductile material removal mechanisms dominate. Further, thedynamics of the grinding process ensure that the optimumgrinding parameters will change, for example, as the grindingwheelwears,thegrindingforceincreasesandthegrit/workpiece interaction changes. There is clearly a need tomonitor the grinding process with the aim of optimising theductile regime material removal conditions.A promising method which could be applied for the continu-ous monitoring of the grinding process, is acoustic emission(AE). The boundary conditions between the ductile and brittlegrinding regimes are distinct and reflect a change in materialremoval mechanism from the shear mode “ductile” (plasticitydominated) to the brittle, fracture-controlled processes. Thischange in mechanism is abrupt as demonstrated by the sharptransitions observed at critical depths of cut. Such a change inthe mechanism should provide a change in AE signal whichcan be used to control the grinding parameters in real time.The wafer grinder is fitted with an AE system and includesthree sensors, one in the grinding spindle, a ring sensor aroundUltraprecision Face Grinding Machine647the workhead and a ring sensor around the truing spindle. TheAE system is highly sensitive and detects a wide range ofevents that influence the grinding process. These events canbe isolated, and because of the very short response time, real-time adjustment of the grinding process is possible.AE signals will be analysed and correlated with the grindingconditions and surface integrity of the workpiece material. Inaddition the AE signal will be related to the force, stockremoval rate and specific grinding energies. This should enablethe change in contact conditions within the grinding zone tobe monitored and the approach of the ductile/brittle transitionidentified. Thus, avertive action can be taken as the criticaltransition is approached.8.3High-Efficiency Precision Grinding of HardMetalsAmongst the most difficult materials to finish are hardenedtool steels and bimetallics that are often used for applicationsrequiring an exceptional combination of wear resistance andtoughness. A good example is their use as barrels and dies inextruder technology 10. These components are exposed to avariety of degradation mechanisms but must maintain theirsurface finish and dimensional control throughout their lifetime.Not only is surface finish important, but also surface integrity,since machining damage introduced into the surface can havea deleterious effect on the wear resistance and thence thecomponent performance and lifetime.Low-stress grinding (LSG) of these hard metallic alloys ispossible in order to minimise microcracking and surface burnand phase transformations. However, LSG requires the use ofa rigid, vibration-free machine tool with soft wheels, low wheelspeeds and frequent wheel dressing. This results in lower ratesof material removal, low grinding ratios (typically 15) and asignificant increase in production costs 11.Manufacturers have investigated the use of grinding to finishmachine barrels 12, but the use of resin-bonded diamond orCBN form wheels was found to be uneconomic due to thefrequent need for wheel dressing. Attempts to increase materialremovalratesresultedinsurfacedamage,inparticularmicrocracking, which is a common problem in these types ofmaterial 13. For this reason, current practice in finish machin-ing of bimetallic extruder barrels uses single-point CBN tool-ing, which itself is slow and expensive. Typically, finish mach-ining accounts for over 50% of the total manufacturing costs12.The problem of machining hard, tough tool steels and bimet-allics is common to many industries and there is clearly arequirement for a grinding process that will provide the requiredlevel of surface integrity and finish, whilst removing metal ateconomic rates. A current research project aims to achieve thisthrough the application of electrolytic in-process dressing(ELID) to the grinding process. ELID has been shown to bean effective method for maintaining cutting grit protrusionand preventing wheel loading. The results to date have beenoutstanding, demonstrating the ability to produce surface finishat the nanometre level, at realistic material removal rates 1.This work has been undertaken on small flat test pieces andthe next stage will be to scale up the process to demonstratea capability on larger components of more complex form. Thiswill require both roughing and finish grinding cycles. The facegrinder will be used to investigate the interaction between theroughing and finishing stages, to grind components in a singlegrinding cycle, using coarse CBN resin bond wheels for theroughing stage and finer metal bond CBN/ELID-assisted grind-ing for the finishing to optical quality. The primary aim willbe to optimise the roughing stage to maximise material removalwhilst minimising the introduction of subsurface damage to alevel that is easily removed by the finishing cycle.9.Concluding RemarksThe availability of an advanced machine tool with such exten-sive machine/process monitoring and automatic supervisionfeatures will enable the development of optimised processmodels for a wide range of advanced materials and components.The requirements for nanometre regime surface finishes andminimum surface/subsurface damage are becoming increasinglydemanding and novel approaches are required, including thedevelopment of a new range of specifically designed ultrapreci-sion machine tools for various products. A major beneficiary,following the development of optimised process mode
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