A method for evaluating spindle rotation errors of machine tools using a laser interferometer.docx
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A method for evaluating spindle rotation errors of machine tools using a laser interferometer.docx
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Amethodforevaluatingspindlerotationerrorsofmachinetoolsusingalaserinterferometer
Amethodforevaluatingspindlerotationerrorsofmachinetoolsusingalaserinterferometer
H.F.F.Castro
EscolaPolitécnica,UniversidadeFederaldaBahia,CTAI,RuaAristidesNovis,02–2°andar,Federação,Salvador,BA40210-630,Brazil
Received10July2006.Revised10June2007.Accepted11June2007.Availableonline21June2007.
http:
//dx.doi.org/10.1016/j.measurement.2007.06.002,HowtoCiteorLinkUsingDOI
CitedbyinScopus(7)
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Abstract
Thispaperpresentsamethodforassessingradialandaxialerrormotionsofspindles.ItusestheHewlettPackard5529Alaserinterferometer.Themeasurementismadeusingreflectiondirectlyfromahigh-precisionsphere.Suchobjectisusedastheopticalreflector.Thesphereisaffixedattheendofawobbledevice,whichisclampedinthespindle.Theprincipleofmeasurementissimilartothatofalinearinterferometer,exceptthatthehigh-precisionsphereisusedinplaceoftheusualretroreflector.Aconvergentlensisutilizedtofocusthelaserbeamtoasmallspotonthespheresurface.Thisminimizesthedispersionofthebeamduetothereflectiononthesphericalsurface.Asoftwarepackagehasbeendevelopedfordataacquisitionandpresentationoftheerrormotionpolarplotsofthespindle.ApplicationofthisspindleerrorcalibratoronaCNCmachiningcentreisundertaken.Theresultsarepresentedanddiscussed.
Keywords
Spindlerotationerrors;
Laserinterferometer;
Machineaccuracy;
Spindlemetrology
1.Introduction
Nowadays,thereisconstantpressureonmachinetoolbuilderstoimprovetheaccuracy,manufacturingcapabilitiesandproductivityoftheirmachines.Parttolerancesinconventionalmachininghavedecreaseddramaticallyoverthepasttwodecades.Also,bettersurfacefinishhasbeenrequiredformostapplications.Thegeometricalshapeandsurfaceroughnessofworkpiecesdependsubstantiallyontherotationalaccuracyofthemachinetoolspindle.
Inviewofthefactsmentionedabove,theevaluationofthespindlerotationerrorshasbecomeveryimportant.Sucherrorscausedegradationinsurfacefinish,roundness,featuresizeandfeaturelocation.Furthermore,theanalysisofspindlerotationerrorscanpredictthequalityofthemachinedpart.Itcanalsobeusedtoevaluatethemachinetoolprecisionforpurchasingandmaintenancepurposes.
Thespindleerrorscomprisetwopartsasfollows:
(a)Errormotions.Thesearesmalldeparturesoftheaxisofrotationrelativetoastationaryreferencecoordinateaxes(X,Y,Z).ThefivecomponentsoftheseerrormotionsarethetranslationsintheX,YandZdirectionsandtherotations(ortilting)abouttheXandY-axes[1]and[2].TheerrormotionsintheXandYdirectionsataspecifiedaxiallocationontheZ-axisarecalledradialerrormotions.AxialerrormotionistheerrormotioncolinearwiththeZ-axis.Tilt(angular)errormotionsaretheerrormotionsintheXandYangulardirectionsrelativetotheZ-axis[1]and[2].(b)Spindlethermaldriftsaremotionsoftheaxisofrotationduetothethermalexpansionorcontractionofthecomponentsbelongingtothestructuralloopassociatedwiththespindle.Therearethreetypesofspindlethermaldrifts:
axial,radialandtiltthermaldrifts[1]and[3].
Inthefollowing,abriefreviewoftherecentworksonthemeasurementofspindlerotationerrorsispresented.
AsimpletechniqueistousefiveLVDT(LinearVariableDisplacementTransducer)sensorsformeasuringtheradial,axialandangularerrormotions.Thisisachievedbymeansofamasterballandamasterplate,whichareemployedasreference[4].Theradialerrormotioncanbedeterminedbythree-point[5]andfour-pointmethods[6].Theyareabletoseparatetheradialerrormotionofthespindlefromtheroundnesserrorofaprecisionsphere.Furtherdevelopmentoftheseworksresultedinthemulti-pointmethod[7].Itpermitstheassessmentoftheradial,axialandtilterrormotionsofthespindle.
ParkandKim[8]developedanopticalMoirétechniquetoevaluatetheradialmotionofaspindlewithoutusingamechanicalmaster(forexample,masterball).AnotherapproachtoassessradialerrorisofferedbytheVectorIndicationMethod[9].Theradialerrorisevaluatedbymeansofthevectorofradialerrormotionofthespindle.AmeasuringsystembasedonaFizeauinterferometer[10]hasbeendevelopedforassessingtheaxialandangularerrormotionsofanultra-precisionairspindle.
Inanothertechnique,threecapacitance-typesensorswereemployedtomeasurethethree-dimensionalpositionsofamasterball.Thistechnique[11]tracesthecentreoftherotatingmasterballin3Dspaceusingpolarplot.Anangularthree-pointmethod[12]thatemploysthree2Dsurfaceslopesensorsiscapableofmeasuringsimultaneouslytheworkpieceout-of-roundnessandthespindleradialandangularerrormotions.Liuetal.[13]developedameasuringsystemforevaluatingtheradialandtilterrormotionsofthespindlewithoutusingamastersphereorcylinder.Thissystemusesarotationalfixturewithabuilt-inlaserdiodewhichismountedonthespindle.Twomeasuringdeviceswithtwopositionsensitivedetectors(PSD)arefixedonthemachinetableinordertomeasurethelaserpointpositionfromthelaserdiode.
Grejdaetal.[14]haveusedaportablemasteraxisofrotation(ultrahigh-precisionair-bearingspindle)andacapacitanceprobeforassessingradialandaxialerrormotionsofspindlesatthenanometerlevel.Donaldson[15]andEstler[16]reversaltechniquesareemployedtoseparatethemasteraxiserrormotionfromthatofthespindleundertest.Thisreversalisachievedbyusingarotarytableandareversalchuckwhicheliminatestheneedforrelocatingthecapacitanceprobe.
ManymethodsdescribedaboveuseLVDTandcapacitancesensorsthatmayinteractwiththedeviceundertestinducingnoiseinthemeasurementsignal,especiallyatspindlesthatrunatveryhighspeed.Otheronesarecapableofassessingonlyradialerrormotion.ThemeasuringsystemthatemploysaFizeauinterferometerisverycomplicatedandexpensive.Inthemethodthatusesthree2Dslopesensors,itisdifficulttoaligntheopticalcomponentsaccuratelyaroundthecylindricalworkpiecemountedonthespindle.Theseopticalcomponentsarelaserdiodes,beamsplitters,mirrors,autocollimatorlenses,quadrantphotodiodes,etc.ThemeasuringsystemdevisedbyLiuetal.[13]usesalsomanyopticsanddevicesthatneedtobemountedandalignedaccuratelyonthemachine.Thissystemisverytime-consuming,thusresultinginlargemachinedowntime.Finally,thetechniquethatemploysthemasteraxisrequiresarotarytableforsupportingandrotatingthestatorofthespindleundertest.However,formedium-andlarge-sizemachines,themountingofthespindleheadstockonarotarytableisnotfeasiblebecausethisheadstockisfixedonthemachinestructure.Therefore,thistechniqueisonlyapplicableforveryrestrictedcases,forexample,fortestingasmall-sizespindlethatcanbefixedonarotarytable.
Inordertoovercomesomeproblemsassociatedwiththemethodsmentionedabove,amethodbasedonalaserinterferometerisproposedinthispaper.Itiscapableofevaluatingtheradialandaxialerrormotionsofspindlesatveryhighspeedandatthenanometerlevel.
2.Methodforevaluatingspindlerotationerrors
Theprincipleofmeasurementissimilartothatofalinearinterferometer,exceptthatinsteadofhavingtheretroreflector,amasterballwithhigh-surfacefinishandaccuracyisusedtoreflecttheincidentbeambacktotheinterferometer.Aconvergentlensisutilizedtofocusthelaserbeamtoasmallspotonthespheresurface.Thiswillminimizethedispersionofthebeamduetothereflectiononthesphericalsurfaceofthemasterball.
AHewlettPackard(HP)5529Alaserinterferometersystem(nowadays,“AgilentTechnologiesCompany”)hasbeenusedinthisresearch.Itiscapableofperformingdynamiccalibrationduetoitshigh-samplingrate.Inthisapplication,thelaserinterferometermakesthemeasurementsbyusingtheTimeBaseGenerator(TBG).Thisisacircuitthatusesa10MHzcrystal-controlledoscillatortocreateatimercircuitthatcausesalaserpositionsampletooccurrepeatedlyatapredeterminedtimeinterval.TheTBGcanproduceanoutputfrequencyof10MHz/(4JK),whereJandKeachmaybesetbetween1and65535.WhenJandKassumevalue1,thefrequencywillbe2.5MHz[17].Thisisthemaximumsamplingrate.
Thelaserperformance(accuracy,resolution,etc.)inthisapplicationisapproximatelythesameasforastandardmeasurementwithalinearinterferometer.Theresolutionisabout1nm.Thismethodforevaluatingthespindleerrorsofferssomeadvantagesoverothernon-contactmeasurementtechniques.Theyarethefollowing:
(a)
Thelaserinterferometersprovidehigherresolutionandprecision.Thespindleerrorsaremeasuredinresolutionof1nm.
(b)
Asmentionedabove,byusingtheTBG,thesamplingrateof2.5MHzcanbeachieved.Therefore,averygoodpictureofthespindlerotationerrorscanbeobtainedevenatveryhigh-spindlespeed.
(c)
Thespotsize(measurementarea)maybemadeassmallasnecessary,whilstthemeasurementarea(spatialresolution)ofothermethodsisoftentoolarge[18].
(d)
Lasersdonotinteractwiththedevicesundertest,sotheydonotproducemeasurementdefectlikeothertechniques.Forexample,whenmeasuringradialrun-outofspindlesatveryhighspeed,theaircurrentsoftenset-upavortex(airturbulence)trailingoffmechanicaltransducers,inducingnoiseintherun-outsignal[18].Thisismoreprobabletooccur
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