Designofultraprecisionmodular,FreeformmachinetoolR.KoningandM.WalterPrecitechInc,Keene,NH03431Keywords:ultraprecisionmachining,freeformoptics,diamondmilling,modularAbstractTheFreeform3000ultraprecisionmachinetoolhasbeendevelopedtosatisfyaneedforaunifiedaffordablemulti-axismachinewiththecapabilityofmilling,grindingandturningultraprecisionsurfaces.Demandforauniversalultra-precisionmachineemergesfromdevelopmentsingrindingandmillingconformaloptics,lensarrays,groovedpatternsanddiamondturning.ThispaperdiscussesthedesignchallengesoftheFreeform3000machineandtheculminationofenablingtechnologies.Thefinalmachineconfigurationwasdeterminedbyevaluatingprecision,stiffnessandmodularity.Ultraprecisionmachineelementshavebeenreinventedtoaccommodatethenewconfiguration.Finiteelementmodelswereusedtoevaluatestaticanddynamicperformanceofmachinecomponents.Asteadystatethermalanalysisofthemachinehasbeencompiledwhichpinpointedareasforthermalerrorreduction.Athermalenvironmenthasbeencreatedtoholdthemachinetemperaturestabletolessthan.05C(.1F).Cuttingtestsdisplaythatthemachineisappropriateformillingthreedimensionalforms,diamondturningaxis-symmetriccomponentsandgrindinginaflood-cooledenvironment.ConfigurationSeveralmachineconfigurationshavebeenevaluatedforthefollowingcriteria:Floodcoolantcompatible,commonalityofparts,easeofbuild,abilitytosinglepointdiamondturn,abilitytodiamondmill,andmodularity.Itiscommontothinkofmachineconfigurationsinaconventionalmanner,howeverthiscanleadtheultra-precisiondesignerdownthewrongpath.Fundamentaltoultra-precisionmachinedesignisthatrelativemotionbetweenmovingelementsbefacilitatedbyafluidfilmandbefreeofcontact.Thefluidfilmprovidesanaveragingeffect,whichinturnproducesstraightnessandtruth-of-motioninexcessoftheindividualelements.Contactbetweenmovingelementsthroughasealorserviceloopproducesundesirableforcesanderrormotion.Sealingslidesfrommachiningdebrismustbefacilitatedthroughtheuseofnon-contactinglabyrinths.Serviceloopsmustbesuppleandthenumberofcablesminimizedwheneverpossible.Finally,cablesmustbebundledinamannertoprovideaneutralaxisthatremainsparallelthroughouttheslidetravel.Figure-1Freeform-3000machineGuardsandcoversremovedToproducethreeaxesofmotion,atleastoneaxismustbemountedontoanother.However,ifthemovingslidewaycausesatransferofmass,thenacompromiseinslidestraightnesswillresult.TheFreeform3000wasdesignedwitha150mmverticalslidemountedontoa350mmhorizontalslideandathird250mmhorizontalslidemountsorthogonaltotheothertwoslidestoproducethefinalaxisofmotion.DuringtheconceptualphaseoftheFreeform3000,itwasdeterminedthatanon-contactingbulkheaddesignwouldseparatethecuttingprocessfromthecombinationslideways,toprovideisolationforfloodcoolant1.TheFreeform3000isbasedonamodulardesign.Themachinebase,oilhydrostaticboxslides,frame,electricalandpneumaticcabinetsweredesignedtobecommonforafamilyofthreeseparatemachinetools,Nanoform350,Nanoform700andtheFreeform3000.Thecommonmodulardesignfacilitateseconomiesofscaleforthecomponentsandanabilitytobuildupthemachinesubassembliestoalevelandthenquicklyreacttocustomerdemands.MachineElementsAllthreelinearslidewaysutilizeoilhydrostaticpads,atasupplypressureof250psi,andlinearmotorstoproduceultra-precisioncontouringandpositioning.Hydrostatic“boxtype”slidewayswerechosenineachaxisfortheirstiffnessandabilitytoprovidestraightslidetravel.Asmallhighstiffnessverticalslidewasdevelopedtomountontoahorizontalaxisviaananglebracket.Emphasisfortheverticalslidedesignwasplacedonreducingthemovingmass,whichwouldenabletheservobandwidthtoremainhigh.Severalcounterbalancedesignconceptswereevaluatedfortheirabilitytoproduceaneutralizingforce,benon-influencing,convenientandinexpensive.Acounter-balanceairpistonwaschosenbecauseitwouldnotcontributeasignificantamountofinertia.Otherfrictionlessdesignssuchasacablemasscounterbalancedoublethemovingmassandcreateasecondorderharmonic.Finally,theairpistoncounterbalancewouldbeconvenientandinexpensivetoimplement.Astandardcommerciallyavailablecylinderwasinitiallyusedwhichwasratedforafrictionalforceequalto1-2%ofappliedload.Duringimplementationofthecylindersontheverticalslide,itwasfoundthatthefrictionproducedanundesirableslideerror.Severaleffortstoreducethefrictionresultedinadesignofanon-contactingaerostaticpistonthatwasnon-influencing2(Figure2).Toavoidreversalerrorandhysteresis,adesignwasemployedwhichprovidesconstantpressuretotheairpiston.Thesystemincludesaprecisionregulator,whichmaintainsconstantpressuretoableedvalve.Thebleedvalveallowsanexcessof10timesthevolumedisplacedbythemaximumtraverserate,tobeexhaustedtotheenvironment(Figure-3).The1AlexanderH.Slocum,“PrecisionMachineDesign”,PrenticeHall,1992,P.372JuliusC.Boonshaft,U.S.PatentNo.3,319,534,1967Figure-2FrictionlessairpistoncounterbalanceFigure-3Counterbalanceairpistonschematicconstantnon-reversingflowthroughthesystemreducesvariationsinforceasafunctionofslidetraveldirection.Thenewgenerationoflensandreflectorelementsisthefreeformsurfaceshape,thathasnoaxisofrevolutionbutceshapethatcouldbedefinedbyamapofpoints,anequation,oracomputerepresentedinsolidfFreeformcomponentsrequirepartcutsofuptoseveraldaysandcompletethermalstabilization.TheFreeform3000requiredathermalenclosureandtemperaturecontrollerwhichproducedanenvironmentwithin.05C(.1F)(Figure-4).KeymachineelementssuchasthespindleriserandflycutterwheelweremanufacturedwithInvar-36,alowcoefficientofexpansionnickelbasedalloy.Withadiamondtoolholdermountedontothemillingspindleriser(Figure-5)andaspindlemountedontothehorizonFreeform3000canbeconfiguredasalathe.Atoolholderdesignwasevelementanalysisforstiffnessandmodesofvibration.Baselinedesignparknowndiamondturninglathehaveastiffnessgreaterthan150,000lbs/inandfirstmodeofvibrationgreaterthan700Hz.Severaliterationswererequiredtoproduceatoolholder,whichmetthesecriteriaandstillmaintainedasmallmovingmass.(Figure-6).3P.D.Brehm,TheSinglePointDiamondMachiningofOpticalMicrostructuresandFreefConferenceProceedings,Vol.2,199920.52121.52222.52323.521:00:0021:40:0022:20:0023:00:0023:40:000:20:001:00:001:40:002:20:003:00:003:40:00440:0020:00:00:0040:00TIMETEMPERATURECROOMTEMPPRECITEMPDUCTABOVESPINDLEAIRSUPPLYFigu10HrmachinetemperatureFigure-5Freeform3000shownasadiamondturninglatheFigure-6Toolholderfordiamondturning:20:0000:00isinsteadasurfarmodelorm3.talslide,thealuatedusingfiniteametersderivedfromaormSurfaces,Euspen5:5:6:77:re-4stabilitychart714HzMachiningResultsDiamondmilledsurfaceshavebeengeneratedtoaformaccuracyof0.204mper1500mm2inan8-hrcyclewithasurfacefinishof15nmRMS.Diamondturnedsurfaceshavebeencreatedtoaformaccuracyof.057mP-Vwithasurfacefinishof3nmona75mm(250mmconvexradius)sphericalaluminumpart.Peripheralgroundpartshavebeengeneratedtoaformaccuracyof.214mandasurfacefinishof4.4nmRMS.Arepeatabilitytestdisplayedthatfreeformpartscanbecreatedconsistent