中國機械工程學刊第二十七卷第期第707715頁(民國九十五)JournaloftheChineseSocietyofMechanicalEngineers,Vol.27,No.6,pp.707715(2006)-707-TheStudyofNanometer-GradeGrindingSurfaceforPrecisionMoldsHo-HuaChung*,K.L.Yang*andHsin-TzuLiao*Keywords:Precisiongrinding,Surfaceroughness,Grindingspeed.ABSTRACTTheSKHmoldsteelswerepre-processed,groundbytheprecisesharpeningstonetohaveRa=10-15mforsubsequentprecisegrindingallowance,andthenpreciselygroundbytheCBNgrindingwheel.Theinfluenceofthemainaxelrotationspeedandthesamplehardnessonthegrindingsituationwasdiscussedinthisstudy.TheresultshowsthatthesurfaceroughnessofthelowhardnessSKH51moldsteelcouldbeimprovedmarkedlyasrotationspeedincreased.Onthecontrary,thereisnoobviousimprovementforthehighhardnessSKH59moldsteel.Withincreasingrotationspeed,thesurfaceroughnessofworkpiecesimprovedsignificantlyduetotheincreasingofgrindingforce.However,athighrotationspeed,itwasunabletogetfurtherimprovementsincethegrindingforcenearlyremainingthesamelevel,causingthelessimprovementonthesurfaceroughnessofworkpieces.INTRODUCTIONCuttingmachiningtechnologyisamongthemosteffectivemethodsformoldmachining.Atpresent,machiningbyusingCNClatheswithmillingturretandendmillingmachininginaleadingCNCmachinecenterarethemostcommonmethodsforproductsmachining.However,aftertheroughmachining,thesurfaceofthemoldisextremelyrough.Tosmooththemoldsurface,itshouldbetreatedagainbythefinishmachiningandhandpolishing.Thisprocessreliesonexperiencedtechnician,anditcostsalongtime.Moreover,theprecisionandthequalityofmoldsaredifficulttobefurtherenhancedbythemanualoperation.Therefore,themaingoalofhigh-speedmoldprocessingistoobtainthehighprecisionoffreecurvesurface,toshortenthepolishingoperationtime,andtolowerthemanufacturingcost.Especially,intherecentyears,thecombinationofhigh-speedmoldprocessingwithhighhardnessmaterialsisusuallyusedinthemoldindustrytoenhancethecompetitivenessofprocessingtechnology.Inordertoobtainsubmicron-grade(10-7-10-8m)shapeaccuracyandnanometer-grade(10-9m)surfaceroughness,diamondmilling,diamondlathing,anddiamondgrindingareusuallyusedinultraprecisionmachining.ComputersimulationdonebyHasegawaandMiyazimapointedoutthattheimprovementofsurfaceroughnessofworkpieceswouldreachasaturationvalueunderalongtimepolish,anditshardtobefurtherimproved.Weckdevelopedanultraprecisionmachiningbyusingadiamond-cuttingtool,bywhichthesurfaceroughnessofworkpiecescouldbereducedtobelow5nm.Asaconsequence,themanufacturedminiaturecomponentscouldalsobeappliedinthesemiconductorindustry.Takeuchidevelopedafive-axleultraprecisionmachine,whichhavehydropressureballscrewsandaservomotorwithinitscap.Thismachineusuallyattachedasuper-resolutionopticalencoderwith6,400millionpulsesperrevolution.Thesurfaceroughnesscouldbereducedtoabout50nmunderextremelowfeedingrate,becausetherewerenofrictionforceandbacklashquestions.Itisnecessaryformachiningtoolstohaveexcellentdynamiccharacteristicsandhighrigidityespeciallyformachinigbrittlematerials.PatentTetraform“C”developedbyStephensonandCorbettreportedthatthesurfacefinishwasproducedbetterthan10nmbyusinga76mCBNwheelandtheelectrolyticin-processdressing(ELID)assistedgrinding.Combinationofthehigh-rigidityPaperReceivedAugust,2006.RevisedNovember,2006.AcceptedNovember,2006.AuthorforCorrespondence:Ho-HuaChung*CorrespondingAuthor:AssociateProfessor,DepartmentofMechanicalandAutomationEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Email:.tw*AssistantProfessor,DepartmentofElectronicsEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Email:.tw*AssociateProfessor,DepartmentofBiochemicalEngineering,KaoYuanUniversity,KaohsiungCounty,Taiwan82101,R.O.C.,Email:.twJ.CSMEVol.27,NO.6(2006)-708-machineandmanufacturingprocess,theremovingrateofultraprecisionmachiningcouldbeincreasedsignificantly.Shapeaccuracyandsurfaceroughnessarealsoveryimportantinthesemiconductorindustry.Surfaceroughnessdemandingforsiliconwaferisaboutatnanometergrade.Surfaceroughnessforoptoelectronicsandmemorydevicesisanimportantfactordirectlyaffectingtheperformanceofproduct.Inbrief,thecurrenttrendsinultraprecisemachiningaretoachievenanometer-grademillingandturninginhardenedsteels,toimprovetheprecisionofproducts,toextendthelifeofprecisionmolds,andtoreductionofsurfaceroughnessofworkpieces.Inordertosaveenergyandenvironmentalconservation,surfaceroughnessindegreeofnanometeroffastenersfortransmissionpartsusedinvehicleshasincreaseditsessentiality.Atpresent,hardenedmoldsteel,whichisacommonprecisionfastenermaterial,isusuallymanufacturedbyprecisegrinding.Inthefuture,high-efficiencyprecisegrindingandvariantnanometerhardturningtechnologieswillbedevelopedforgrindingprecisionmolds.Inthisresearch,SKHtoolsteelsarechosenasthemoldworkpieces,andtheeffectofgrindingspeed,materialhardness,grindingforceandfeedingrateonthesurfaceroughnessofworkpiecewillbestudied.EXPERIMENTMETHODEquipmentalset-upInthisstudy,ahighrotationspeedmicro-grinderinstallingtheperipheralmeasurementsystemandmanykindsofdifferentprocessingequipment(suchasnumericalcontrollathe/millingmachine,fourthaxle)wereusedtoconducttheresearch.AsillustratedinFig.1,theoperationproceduresofthishighrotationspeedautomaticprecisemicro-grinderwereshowed.Firstly,afterheattreatment,thehighhardnessmaterialwaspretreatedbydiamondmachininganddiamondcutting.Secondly,thesurfaceroughnessofworkpieceswasreducedtoRa=1015mforsubsequentprecisegrindingallowance.Thirdly,matchingCBNgrindingwheelatFigure1:Schematicillustrationofhighrotationspeedmicro-grinder.H.H.Chungetal.:TheStudyofNanometer-GradeGrindingSurface.-709-highrotationspeed,XYZaxleandthefourthaxlereciprocationwithmoves,thecurvedsurfaceandlinegrinding-truncatingbythediamondscribingtool,andconstructingtheoutlineofgrinding-truncatingbytheCAD/CAMwithparametersobtainedfromexperiments,theunwantedpartofthesurfaceofworkpiecewasremovedandtheincisiveextremelyhardultramicrondiamondburrorabrasiveparticleswererevealed.Finally,themovementpathofgrindingwheelproducedbythediamondscribingtoolwasduplicatedtotheheat-treatedmoldtoolsteeltohavethemicrometer-gradeprecisecontacttypegrindingandthenthesurfaceroughnesscanreachbelow50nm.ExperimentalprocedureThreetypesofmoldtoolsteels(SKH51,SKH55andSKH59)werechosenastheworksamples,andthenweremarkedwithdifferentcolorsinclassified.Theprocessingofworksampleswascarriedonasthefollowingsteps:(1)Cutting:Roughcuttingtoclosetothesamplelength.(2)Roughmachining:Contourmachiningbythediscorcylindergrindingwheel.Trimming:Trimtwosidesofthecuttingend.(3)CNCroughlathe:Lathetheouterdiameterfromthelatheprogram.(4)CNCroughmilling:Shapemillingfromthemillingprogram.(5)Thermaltreatment:Worksampleswereaustenizedat1165oC,1200oCand1165oC,respectively,andthenquenched.Subsequently,thesampleswereannealedat560oCthreetimestoobtaintheRockwellhardnessof60,63and67HRc.ThechemicalcompositionsofthesamplesweresummarizedinTable1.(6)Precisionlathing:Lathetostandardlength.(7)Precisiongrinding:GrindedtothegeometryasillustratedinFigure2,andthencleanedbeforeconductingtheexperiment.Theworkpeiceswereusingacetoneplacedinanultrasoniccleanercleanedforseveralminutes,andthenthesurfaceroughnessweremeasuredbythesurfaceroughnessmeasurementinstrument.Theworkpeiceswerefixedonthespeedmicro-grinderwithsuper-highrotationspeed(Fig.1).Inordertoinvestigatetheinfluenceofthecuttingspeedandthesamplehardnessonthegrindingsituation,theoperationvariableswererotationspeed,depthofcut,horizontalfeedingrate,coolingjet,concentrationandtemperatureofgrindingslurry.ThedetailedexperimentalconditionwasaslistedinTable2.Table1:ElementsanalysisofthemoldtoolsteelsafterthermaltreatmentChemicalComposition(wt%)TestsampleCCrMoWVCoSKH511.073.562.032.591.76-SKH551.113.922.162.831.745.51SKH591.173.573.990.591.149.34Figure2:Thegeometryoftheprecisiongrindedhexagonpunch.Table2:TheexperimentalconditionsforthegrindingprocessofmoldtoolsItemConditionsTestsamplematerialSKH51,SKH55,SKH59GrindingwheelCBN80N75B11V26,d=125mmRotationspeed(rpm)5000,6500,7000,8000,9000Worktablevelocity(mm/min)90-150Optimumdepthofcut(mm)0.03-0.2Grindingslurrytemperature(oC)Input:28-29；Output:30-31Grindingslurryconcentrationoriginalgrindingfluid/water=3:97CoolingjetVolumetricflowrate:20m3/h，pressure:8barJ.CSMEVol.27,NO.6(2006)-710-Undereachkindofrotationspeed,depthofcut,feedingrate,grindingforce,thevibrationfrequency,andthegrindingtemperatureofgrindingslurryweredeterminedbythehigh-precisionloadcell,thedigitalstoragevibrationsmeasuringmeterandthenon-contactthermometer.Aftergrinding,thesurfacemorphologyofworkpieceswasexaminedbythescanningelectronmicroscope(SEM).Moreover,anenergydispersionspectroscope,theRockwellhardnesstester,andthesurfaceroughnessmeasurementinstrumentwerealsousedtocharacterizethegrindingmechanismofSKHsteelprocessingsurface.Finally,theprocessingefficiencyofhardvehiclemillandprecisiongrindingwasassessedviatheexperimentalresult.RESULTSANDDISCUSSIONSSurfaceBehaviorofCBNGrindingWheelThemajorpurposeofhighspeedmoldprocessingistoobtainthehighaccuracyfreecurvedsurface,toreducethepolishingtime,andtolowerthecost.ThehighefficientgrindingisbytheCBNgrindingwheel,thehighmainaxlerotationspeed,andthehighrateofmovementanddeeperentering.Itscuttingquantitywasseveralhundredtimesthanthetraditionalgrinderandthegrindingwheelshapeisallthesamefromthebeginningtoendduringtheperiodofthewholegrindingprocess.Therefore,thenanometer-gradegrindingsurfacecouldbeobtainedbyomittingtherepairingandmaintainingprocedureofgrindingwheelandthechangeofcuttingcondition.ThehardnessofthemoldsurfacewasusuallyhigherthanHRc60andthegeneralcuttingtoolcouldnotprocessitwhenitssurfacewashardenedafterheattreatment.Moreover,therequestofmoldprecisionandsurfacesmoothnesswastoohightobeestablishedbythegeneralcuttingtool.TheCBNgrindingwheelwasthebestmethodusedfortheprecisionprocessingofmoldsurfacebecauseithadthemulti-cuttingsangleandtheregenerationproperty.Figure3showstherelationshipbetweengrindingforceandcuttingspeedfordifferentSKHmoldsteelmaterialswhenthefeedingrateandthedepthofcutweremaintainedat150mm/minand0.05mm,respectively.ForthethreekindsofSKHmoldsteelmaterials,itisclearthatthegrindingforcedecreasedFigure3:Thevariationofgrindingforceasafunctionofcuttingspeedundertheconditionofthefeedingrateof150m/minandthefeedamountof0.05mm.markedlywhenthecuttingspeedwasincreasedfrom2000to2750m/minandthenapproachedasaturationvalue.ThisresultisduetothefactthattheheatgeneratedfromthegrindingofmoldtoolcanbeeasilydissipatedformCBNgrindingwheelitselfandnottrans