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中國機械工程學刊第二十七卷第六期第707715頁民國九十五年JournaloftheChineseSocietyofMechanicalEngineers,Vol.27,No.6,pp.7077152006707TheStudyofNanometerGradeGrindingSurfaceforPrecisionMoldsHoHuaChung,K.L.YangandHsinTzuLiaoKeywordsPrecisiongrinding,Surfaceroughness,Grindingspeed.ABSTRACTTheSKHmoldsteelswerepreprocessed,groundbytheprecisesharpeningstonetohaveRa1015μmforsubsequentprecisegrindingallowance,andthenpreciselygroundbytheCBNgrindingwheel.Theinfluenceofthemainaxelrotationspeedandthesamplehardnessonthegrindingsituationwasdiscussedinthisstudy.TheresultshowsthatthesurfaceroughnessofthelowhardnessSKH51moldsteelcouldbeimprovedmarkedlyasrotationspeedincreased.Onthecontrary,thereisnoobviousimprovementforthehighhardnessSKH59moldsteel.Withincreasingrotationspeed,thesurfaceroughnessofworkpiecesimprovedsignificantlyduetotheincreasingofgrindingforce.However,athighrotationspeed,itwasunabletogetfurtherimprovementsincethegrindingforcenearlyremainingthesamelevel,causingthelessimprovementonthesurfaceroughnessofworkpieces.INTRODUCTIONCuttingmachiningtechnologyisamongthemosteffectivemethodsformoldmachining.Atpresent,machiningbyusingCNClatheswithmillingturretandendmillingmachininginaleadingCNCmachinecenterarethemostcommonmethodsforproductsmachining.However,aftertheroughmachining,thesurfaceofthemoldisextremelyrough.Tosmooththemoldsurface,itshouldbetreatedagainbythefinishmachiningandhandpolishing.Thisprocessreliesonexperiencedtechnician,anditcostsalongtime.Moreover,theprecisionandthequalityofmoldsaredifficulttobefurtherenhancedbythemanualoperation.Therefore,themaingoalofhighspeedmoldprocessingistoobtainthehighprecisionoffreecurvesurface,toshortenthepolishingoperationtime,andtolowerthemanufacturingcost.Especially,intherecentyears,thecombinationofhighspeedmoldprocessingwithhighhardnessmaterialsisusuallyusedinthemoldindustrytoenhancethecompetitivenessofprocessingtechnology.Inordertoobtainsubmicrongrade107108mshapeaccuracyandnanometergrade109msurfaceroughness,diamondmilling,diamondlathing,anddiamondgrindingareusuallyusedinultraprecisionmachining.ComputersimulationdonebyHasegawaandMiyazimapointedoutthattheimprovementofsurfaceroughnessofworkpieceswouldreachasaturationvalueunderalongtimepolish,anditshardtobefurtherimproved.Weckdevelopedanultraprecisionmachiningbyusingadiamondcuttingtool,bywhichthesurfaceroughnessofworkpiecescouldbereducedtobelow5nm.Asaconsequence,themanufacturedminiaturecomponentscouldalsobeappliedinthesemiconductorindustry.Takeuchidevelopedafiveaxleultraprecisionmachine,whichhavehydropressureballscrewsandaservomotorwithinitscap.Thismachineusuallyattachedasuperresolutionopticalencoderwith6,400millionpulsesperrevolution.Thesurfaceroughnesscouldbereducedtoabout50nmunderextremelowfeedingrate,becausetherewerenofrictionforceandbacklashquestions.Itisnecessaryformachiningtoolstohaveexcellentdynamiccharacteristicsandhighrigidityespeciallyformachinigbrittlematerials.PatentTetraformCdevelopedbyStephensonandCorbettreportedthatthesurfacefinishwasproducedbetterthan10nmbyusinga76μmCBNwheelandtheelectrolyticinprocessdressingELIDassistedgrinding.CombinationofthehighrigidityPaperReceivedAugust,2006.RevisedNovember,2006.AcceptedNovember,2006.AuthorforCorrespondenceHoHuaChungCorrespondingAuthorAssociateProfessor,DepartmentofMechanicalandAutomationEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Emailhohuacc.kyu.edu.twAssistantProfessor,DepartmentofElectronicsEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Emailklyangcc.kyu.edu.twAssociateProfessor,DepartmentofBiochemicalEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Emailhtliawcc.kyu.edu.twJ.CSMEVol.27,NO.62006708machineandmanufacturingprocess,theremovingrateofultraprecisionmachiningcouldbeincreasedsignificantly.Shapeaccuracyandsurfaceroughnessarealsoveryimportantinthesemiconductorindustry.Surfaceroughnessdemandingforsiliconwaferisaboutatnanometergrade.Surfaceroughnessforoptoelectronicsandmemorydevicesisanimportantfactordirectlyaffectingtheperformanceofproduct.Inbrief,thecurrenttrendsinultraprecisemachiningaretoachievenanometergrademillingandturninginhardenedsteels,toimprovetheprecisionofproducts,toextendthelifeofprecisionmolds,andtoreductionofsurfaceroughnessofworkpieces.Inordertosaveenergyandenvironmentalconservation,surfaceroughnessindegreeofnanometeroffastenersfortransmissionpartsusedinvehicleshasincreaseditsessentiality.Atpresent,hardenedmoldsteel,whichisacommonprecisionfastenermaterial,isusuallymanufacturedbyprecisegrinding.Inthefuture,highefficiencyprecisegrindingandvariantnanometerhardturningtechnologieswillbedevelopedforgrindingprecisionmolds.Inthisresearch,SKHtoolsteelsarechosenasthemoldworkpieces,andtheeffectofgrindingspeed,materialhardness,grindingforceandfeedingrateonthesurfaceroughnessofworkpiecewillbestudied.EXPERIMENTMETHODEquipmentalsetupInthisstudy,ahighrotationspeedmicrogrinderinstallingtheperipheralmeasurementsystemandmanykindsofdifferentprocessingequipmentsuchasnumericalcontrollathe/millingmachine,fourthaxlewereusedtoconducttheresearch.AsillustratedinFig.1,theoperationproceduresofthishighrotationspeedautomaticprecisemicrogrinderwereshowed.Firstly,afterheattreatment,thehighhardnessmaterialwaspretreatedbydiamondmachininganddiamondcutting.Secondly,thesurfaceroughnessofworkpieceswasreducedtoRa1015μmforsubsequentprecisegrindingallowance.Thirdly,matchingCBNgrindingwheelatFigure1Schematicillustrationofhighrotationspeedmicrogrinder.H.H.Chungetal.TheStudyofNanometerGradeGrindingSurface.709highrotationspeed,XYZaxleandthefourthaxlereciprocationwithmoves,thecurvedsurfaceandlinegrindingtruncatingbythediamondscribingtool,andconstructingtheoutlineofgrindingtruncatingbytheCAD/CAMwithparametersobtainedfromexperiments,theunwantedpartofthesurfaceofworkpiecewasremovedandtheincisiveextremelyhardultramicrondiamondburrorabrasiveparticleswererevealed.Finally,themovementpathofgrindingwheelproducedbythediamondscribingtoolwasduplicatedtotheheattreatedmoldtoolsteeltohavethemicrometergradeprecisecontacttypegrindingandthenthesurfaceroughnesscanreachbelow50nm.ExperimentalprocedureThreetypesofmoldtoolsteelsSKH51,SKH55andSKH59werechosenastheworksamples,andthenweremarkedwithdifferentcolorsinclassified.Theprocessingofworksampleswascarriedonasthefollowingsteps1CuttingRoughcuttingtoclosetothesamplelength.2RoughmachiningContourmachiningbythediscorcylindergrindingwheel.TrimmingTrimtwosidesofthecuttingend.3CNCroughlatheLathetheouterdiameterfromthelatheprogram.4CNCroughmillingShapemillingfromthemillingprogram.5ThermaltreatmentWorksampleswereaustenizedat1165oC,1200oCand1165oC,respectively,andthenquenched.Subsequently,thesampleswereannealedat560oCthreetimestoobtaintheRockwellhardnessof60,63and67HRc.ThechemicalcompositionsofthesamplesweresummarizedinTable1.6PrecisionlathingLathetostandardlength.7PrecisiongrindingGrindedtothegeometryasillustratedinFigure2,andthencleanedbeforeconductingtheexperiment.Theworkpeiceswereusingacetoneplacedinanultrasoniccleanercleanedforseveralminutes,andthenthesurfaceroughnessweremeasuredbythesurfaceroughnessmeasurementinstrument.TheworkpeiceswerefixedonthespeedmicrogrinderwithsuperhighrotationspeedFig.1.Inordertoinvestigatetheinfluenceofthecuttingspeedandthesamplehardnessonthegrindingsituation,theoperationvariableswererotationspeed,depthofcut,horizontalfeedingrate,coolingjet,concentrationandtemperatureofgrindingslurry.ThedetailedexperimentalconditionwasaslistedinTable2.Table1ElementsanalysisofthemoldtoolsteelsafterthermaltreatmentChemicalCompositionwtTestsampleCCrMoWVCoSKH511.073.562.032.591.76SKH551.113.922.162.831.745.51SKH591.173.573.990.591.149.34Figure2Thegeometryoftheprecisiongrindedhexagonpunch.Table2TheexperimentalconditionsforthegrindingprocessofmoldtoolsItemConditionsTestsamplematerialSKH51,SKH55,SKH59GrindingwheelCBN80N75B11V26,d125mmRotationspeedrpm5000,6500,7000,8000,9000Worktablevelocitymm/min90150Optimumdepthofcutmm0.030.2GrindingslurrytemperatureoCInput2829Output3031Grindingslurryconcentrationoriginalgrindingfluid/water397CoolingjetVolumetricflowrate20m3/h，pressure8barJ.CSMEVol.27,NO.62006710Undereachkindofrotationspeed,depthofcut,feedingrate,grindingforce,thevibrationfrequency,andthegrindingtemperatureofgrindingslurryweredeterminedbythehighprecisionloadcell,thedigitalstoragevibrationsmeasuringmeterandthenoncontactthermometer.Aftergrinding,thesurfacemorphologyofworkpieceswasexaminedbythescanningelectronmicroscopeSEM.Moreover,anenergydispersionspectroscope,theRockwellhardnesstester,andthesurfaceroughnessmeasurementinstrumentwerealsousedtocharacterizethegrindingmechanismofSKHsteelprocessingsurface.Finally,theprocessingefficiencyofhardvehiclemillandprecisiongrindingwasassessedviatheexperimentalresult.RESULTSANDDISCUSSIONSSurfaceBehaviorofCBNGrindingWheelThemajorpurposeofhighspeedmoldprocessingistoobtainthehighaccuracyfreecurvedsurface,toreducethepolishingtime,andtolowerthecost.ThehighefficientgrindingisbytheCBNgrindingwheel,thehighmainaxlerotationspeed,andthehighrateofmovementanddeeperentering.Itscuttingquantitywasseveralhundredtimesthanthetraditionalgrinderandthegrindingwheelshapeisallthesamefromthebeginningtoendduringtheperiodofthewholegrindingprocess.Therefore,thenanometergradegrindingsurfacecouldbeobtainedbyomittingtherepairingandmaintainingprocedureofgrindingwheelandthechangeofcuttingcondition.ThehardnessofthemoldsurfacewasusuallyhigherthanHRc60andthegeneralcuttingtoolcouldnotprocessitwhenitssurfacewashardenedafterheattreatment.Moreover,therequestofmoldprecisionandsurfacesmoothnesswastoohightobeestablishedbythegeneralcuttingtool.TheCBNgrindingwheelwasthebestmethodusedfortheprecisionprocessingofmoldsurfacebecauseithadthemulticuttingsangleandtheregenerationproperty.Figure3showstherelationshipbetweengrindingforceandcuttingspeedfordifferentSKHmoldsteelmaterialswhenthefeedingrateandthedepthofcutweremaintainedat150mm/minand0.05mm,respectively.ForthethreekindsofSKHmoldsteelmaterials,itisclearthatthegrindingforcedecreasedFigure3Thevariationofgrindingforceasafunctionofcuttingspeedundertheconditionofthefeedingrateof150m/minandthefeedamountof0.05mm.markedlywhenthecuttingspeedwasincreasedfrom2000to2750m/minandthenapproachedasaturationvalue.ThisresultisduetothefactthattheheatgeneratedfromthegrindingofmoldtoolcanbeeasilydissipatedformCBNgrindingwheelitselfandnottransfertotheworkpiecebecauseCBNhashighthermalconductivity.Therefore,alongwithanincreasingofcuttingspeed,thecuttingtemperaturewaseasytoconcentrateonthegrindingparticlesofCBNgrindingwheelandleadingtoreducethegrindingforcegradually.Ontheotherhand,itwasalsofoundfromFigure3that,underthesamevalueofcuttingspeed,thegrindingforceofSKH59moldsteelissmallerthanthatofSKH51andSKH55duetothehighercarboncontentandthehigherhardness.ThisresultcanbeexplainedbythephenomenonthatSKH59haslowersofttemperatureandthecarbidecanbeseparatedafterquenching.Theseparatedcarbideaffectsthefrictionfactorofcuttingsurfaceandthenleadingtoreducethegrindingforce.Theanalysisoftheseparatedmaterialwillbefurtherdiscussedinthenextsection.AspinwallandNgstudiedthevariationofgrindingforcewithcuttingspeedforAISIH13moldsteelwithdifferenthardenabilityandtheresultshowedthat,withthesamecuttingconditionandthesamematerial,thegrindingforcewasnotnecessarilylargeforthehigherhardenabilityworkpiece.Intheaspecttogrindingforce,thecuttingtemperaturewillincreasetosomedegreewithanincreasingofcuttingspeedandthentheheatconcentratesonthegrindingparticlesoftheCBNgrindingwheel.Whenthequantityofconcentrated