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英文原文SmallholedrillinginengineeringplasticssheetanditsaccuracyestimationHirokiEndoandEtsuoMaruiAbstractInrecentmanufacturingprocesses,thesmalldiameterholedrillingprocessisfrequentlyusedowingtoitsgoodcharacteristics.Thedrillingprocesscaneasilybeadaptedtowidevariationsinlotsize,processingaccuracy,processingspotpatternswhereholesaremade,andsoon.Manymachineelements,whichhavesmalldiameterholes,aremanufacturedusingengineeringplasticsofsuperiormaterialandmachiningproperties.However,itisnoteasytodrillholeswithadiametersmallerthan1mm,inrecentmachiningtechnologyaswell.Inthisreport,1mmdiameterholesaredrilledontwoengineeringplasticssheetsandtheirdrillingaccuracyisdiscussed.KeywordsSmalldiameterholeDrillingEngineeringplasticsMachiningaccuracy1.IntroductionProcessingofsmalldiameterholesisdoneinvariousmaterials,correspondingtothetrendofdownsizingorhighaccuracyinpartsincorporatedintoelectronicequipments,medicalinstrumentsortextilemachineries.Manytechniquesareputtopracticaluse,includingdrilling,ultrasonicmachining,electricdischargemachining,electrolyticmachining,laserbeammachining,electronbeammachining,fluidorabrasivejetmachining,andchemicalblanking.Dependingontheworkpiecematerial,themachiningaccuracy,andthelotsize,thebestprocessformakingholesofsmalldiametermaybeappropriatelyselected.Withinthesevariousmachiningprocesses,thedrillingprocesscanreadilydealwithawidevarietyofmachiningconditions.However,therearesomedifficultproblemsindrillingholessmallerthan1mmindiameter.Forexample,alargeloadcannotbeputonsmalldrills,owingtotheirlowstrengthandrigidity.Thus,thefeedrateperunitdrillrotationmustbesetsmall.Theremovalofdrilledchipsisdifficultowingtothesmalldrillflutearea.Inmanycases,engineeringplasticsareusedinmakingvariousmachinepartsbecausetheyarelightandhavesuperiorspecificstrengththatis,theratiooftensilestrengthtodensitycomparedwithcarbonsteel.Also,thematerialcostofengineeringplasticsiscompetitiveandtheirmachinabilityisfairlygood.Withthesepointsasbackground,theorthogonalcuttingofengineeringplasticswasinvestigated1anditwassuggestedherethatthevisco–elasticpropertiesofengineeringplasticshavesomeeffectsonthemagnitudeofcuttingforceandthesurfaceroughnessofmachinedsurfaces.Thereisareviewpaper2regardingthemachiningofengineeringplastics.Inthisreviewpaper,drillingprocesswasalsotreated.Itwaspointedoutthattheheatingupoftheworkpieceduetobuildupofswarfondrillflutesisanobstacletothedrillingprocessofengineeringplastics.Recently,someexperimentshavebeenattemptedondrillingglass–fiberreinforcedengineeringplasticssheets3and4,andthethrustforceandtorqueduringdrillinghavebeenmeasured.Inthesepapers,itwasreportedthatthedelaminationphenomenondecreasesthedrilledholeintegrity,whenholesofabout5mmdiameteraredrilled.However,theinvestigationontheaccuracyinsmallholedrillingofengineeringplasticsisleftpending.Theninthispaper,smalldiameterholesof1mmaredrilledintwotypicalengineeringplasticssheets,andtheeffectofspindlespeedandfeedrateontheaccuracyradiuserrorisestimated.2.WorkpiecematerialsTwotypicalengineeringplasticssheets,polyacetalPOMandpolyetherimidePEI,weredrilled.ThematerialspropertiesarelistedinTable1.Table1.MaterialpropertiesofworkpieceengineeringplasticsPerformanceUnitPOMPEISpecificgravity1.411.27Rateofwater0.220.25Meltingpoint°C165210Coefficientoflinearthermalexpansioncm/cm/°C910−55.610−5TensilestrengthMPa61124TensileextensionYieldingpoint4023BendingstrengthMPa89157BendingelasticityGPa2.603.07PerformanceUnitPOMPEICompressivestrengthMPa103118IzoteimpactvalueJ/m7442RockwellhardnessMscale119127Polyacetalisacrystallizedengineeringplasticsmaterial.Themainrawmaterialsareacetalcopolymerandhomopolymer.POMhasgoodfatiguepropertiesandmachinability.Manycams,guidesandlinersaremadeofPOM.Veryhighaccuracyisneededinthesemachinedparts.PEIisanamorphousengineeringplastichavingsuperiorthermalresistancecharacteristics.Specialelectricalparts,forexample,electricinsulators,connectors,aremadeofPEI,whichissuperiorinmechanicalstrengthbutinferiorinmachinabilitytoPOM.Theworkpiecesizewaslength100mm,width50mmandthickness0.8mm.3.ExperimentalapparatusandprocedureThedrillingmachineusedisforsmalldiameterholes,andisequippedwithanautomaticfeedmechanism.Ahighfrequencyinductionmotorpositionedattheuppermostpositionofthemainspindledrivesthespindle.Maximumspindlespeedis12,500rpm.Thenetspindlespeedofthespindleduringthedrillingismeasuredbyatachometer,whichcountsnumberofthelaserbeamreflectedfromareflectivetapepastedonthescrollchuck.Aservomotorfordrillfeeddrivesthefeedmotionofthespindle.Thefeedisstepless,andadialgaugeequippedatthespindleheadmeasuresthelengthofthedrillmotioninthespindleaxisdirection.Astopwatchwasusedtomeasurethetimeneededforthislength.Theratioofthemovedlengthtothetimeisthesubstantialfeedrateperunittime.Thespindlespeedwasvariedbetween1250and12,500rpm.Andalsothefeedrateperunittimewasvariedbetween0.405and1.986mm/s.Spindlespeedwasvariedinkeepingwiththefeedrateperunittime.Hence,thefeedrateperunitdrillrotationbecamesmallwiththeincreaseinthespindlespeedofthedrill.Thedrillspindleendisattachedtothescrollchuck.Thedrillusedhereisaconventionaltwistdrillmadeofhighspeedsteelwithadiameterof1mm.Insomeextraexperiments,a0.3mmdiameterdrillwasalsoused.Suchdrillshavenosurfacetreatment.Adialgaugeestimatesdeflectionaccuracyofthedrillonthescrollchuckduringrotation.Extremecarewastakensothatthedrilldeflectionwassmallerthan5μm.Thesamedrillmadefiveholesunderthecuttingconditionofthesamespindlespeedandthesamefeedrate.Anotherdrillwasusedinthedrillingunderanothercuttingcondition.Ofcourse,thesizeaccuracyofthesedrillsexistswithintheabovementionedsizescattering.Anyevidenceofthewearofdrillsandthebuildupofswarfondrillfluteswerenotrecognizedafterfiveholesdrilling.Formerlymentionedworkpieceofengineeringplasticsweresetonthebaseofthedrillingmachinebyclampingbolts.Drycuttingwithoutfluidwasperformed.4.CalculationofdrilledholeshapesThe1mmdiameterholesdrilledonengineeringplasticssheetsbytheprocessdescribedabovearenotgeometricallytruecircles,buthaveasmallradialdeviation.Shapeaccuracyofthedrilledholesisestimatedbythefollowingprocess.Anopticalmicroscopeequippedwithdigitalmeasuringdevicemeasurestheshapeofthedrilledhole.Thecrosswireofthemicroscopeissetatthecircumferenceofthehole.Then,thecoordinatesx,yoftheholecircumferenceareread.Dividingthecircumferenceinto18equalparts,thesamemeasurementsarethenrepeatedoneachspotonthecircumference.Usingthese18setsofmeasuredqualities,theequationofthecirclethatfitscloselytothedrilledholeiscalculated.Thisiscalledaleastsquarecircle,andinthecalculation,theleastsquaresmethodisapplied.Theequationoftheleastsquarecircleisassumedasfollowsx2y2AxByC01Owingtotheshapeerrorofthehole,therighthandsideofEq.1doesnotbecomezerowhentheabovementionedmeasuredqualitiesxi,yiaresubstituted.Theresidualinthiscaseisviandthefollowingequationisobtained2Here,thecoefficientsA,B,CinEq.1aredeterminedasthesumofthesquaredvaluesoftheresidualvibecomesminimum.Valuesofthesecoefficientsareobtainedbysolvingthefollowingsimultaneouslinearequations.Inthecalculation,N18.
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