英文原文Small-holedrillinginengineeringplasticssheetanditsaccuracyestimationHirokiEndoandEtsuoMaruiAbstractInrecentmanufacturingprocesses,thesmalldiameterholedrillingprocessisfrequentlyusedowingtoitsgoodcharacteristics.Thedrillingprocesscaneasilybeadaptedtowidevariationsinlotsize,processingaccuracy,processingspotpatternswhereholesaremade,andsoon.Manymachineelements,whichhavesmalldiameterholes,aremanufacturedusingengineeringplasticsofsuperiormaterialandmachiningproperties.However,itisnoteasytodrillholeswithadiametersmallerthan1mm,inrecentmachiningtechnologyaswell.Inthisreport,1-mmdiameterholesaredrilledontwoengineeringplasticssheetsandtheirdrillingaccuracyisdiscussed.Keywords:Smalldiameterhole；Drilling；Engineeringplastics；Machiningaccuracy1.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,engineeringplasticsareusedinmakingvariousmachinepartsbecausetheyarelightandhavesuperiorspecificstrength(thatis,theratiooftensilestrengthtodensity)comparedwithcarbonsteel.Also,thematerialcostofengineeringplasticsiscompetitiveandtheirmachinabilityisfairlygood.Withthesepointsasbackground,theorthogonalcuttingofengineeringplasticswasinvestigated1anditwassuggestedherethattheviscoelasticpropertiesofengineeringplasticshavesomeeffectsonthemagnitudeofcuttingforceandthesurfaceroughnessofmachinedsurfaces.Thereisareviewpaper2regardingthemachiningofengineeringplastics.Inthisreviewpaper,drillingprocesswasalsotreated.Itwaspointedoutthattheheatingupoftheworkpieceduetobuild-upofswarfondrillflutesisanobstacletothedrillingprocessofengineeringplastics.Recently,someexperimentshavebeenattemptedondrillingglassfiber-reinforcedengineeringplasticssheets3and4,andthethrustforceandtorqueduringdrillinghavebeenmeasured.Inthesepapers,itwasreportedthatthedelaminationphenomenondecreasesthedrilledholeintegrity,whenholesofabout5-mmdiameteraredrilled.However,theinvestigationontheaccuracyinsmallholedrillingofengineeringplasticsisleftpending.Theninthispaper,smalldiameterholesof1mmaredrilledintwotypicalengineeringplasticssheets,andtheeffectofspindlespeedandfeedrateontheaccuracy(radiuserror)isestimated.2.WorkpiecematerialsTwotypicalengineeringplasticssheets,polyacetal(POM)andpolyetherimide(PEI),weredrilled.ThematerialspropertiesarelistedinTable1.Table1.MaterialpropertiesofworkpieceengineeringplasticsPerformanceUnitPOMPEISpecificgravity1.411.27Rateofwater%0.220.25MeltingpointC165210Coefficientoflinearthermalexpansioncm/cm/C91055.6105TensilestrengthMPa61124Tensileextension(Yieldingpoint)%4023BendingstrengthMPa89157BendingelasticityGPa2.603.07PerformanceUnitPOMPEICompressivestrengthMPa103118IzoteimpactvalueJ/m7442RockwellhardnessMscale119127Polyacetalisacrystallizedengineeringplasticsmaterial.Themainrawmaterialsareacetalco-polymerandhomo-polymer.POMhasgoodfatiguepropertiesandmachinability.Manycams,guidesandlinersaremadeofPOM.Veryhighaccuracyisneededinthesemachinedparts.PEIisanamorphousengineeringplastichavingsuperiorthermalresistancecharacteristics.Specialelectricalparts,forexample,electricinsulators,connectors,aremadeofPEI,whichissuperiorinmechanicalstrengthbutinferiorinmachinabilitytoPOM.Theworkpiecesizewas:length100mm,width50mmandthickness0.8mm.3.ExperimentalapparatusandprocedureThedrillingmachineusedisforsmalldiameterholes,andisequippedwithanautomaticfeedmechanism.Ahigh-frequencyinductionmotorpositionedattheuppermostpositionofthemainspindledrivesthespindle.Maximumspindlespeedis12,500rpm.Thenetspindlespeedofthespindleduringthedrillingismeasuredbyatachometer,whichcountsnumberofthelaserbeamreflectedfromareflectivetapepastedonthescrollchuck.Aservomotorfordrillfeeddrivesthefeedmotionofthespindle.Thefeedisstepless,andadialgaugeequippedatthespindleheadmeasuresthelengthofthedrillmotioninthespindleaxisdirection.Astopwatchwasusedtomeasurethetimeneededforthislength.Theratioofthemovedlengthtothetimeisthesubstantialfeedrateperunittime.Thespindlespeedwasvariedbetween1250and12,500rpm.Andalsothefeedrateperunittimewasvariedbetween0.405and1.986mm/s.Spindlespeedwasvariedinkeepingwiththefeedrateperunittime.Hence,thefeedrateperunitdrillrotationbecamesmallwiththeincreaseinthespindlespeedofthedrill.Thedrillspindleendisattachedtothescrollchuck.Thedrillusedhereisaconventionaltwistdrillmadeofhigh-speedsteelwithadiameterof1mm.Insomeextraexperiments,a0.3mm-diameterdrillwasalsoused.Suchdrillshavenosurfacetreatment.Adialgaugeestimatesdeflectionaccuracyofthedrillonthescrollchuckduringrotation.Extremecarewastakensothatthedrilldeflectionwassmallerthan5m.Thesamedrillmadefiveholesunderthecuttingconditionofthesamespindlespeedandthesamefeedrate.Anotherdrillwasusedinthedrillingunderanothercuttingcondition.Ofcourse,thesizeaccuracyofthesedrillsexistswithintheabove-mentionedsizescattering.Anyevidenceofthewearofdrillsandthebuild-upofswarfondrillfluteswerenotrecognizedafterfiveholesdrilling.Formerlymentionedworkpieceofengineeringplasticsweresetonthebaseofthedrillingmachinebyclampingbolts.Drycuttingwithoutfluidwasperformed.4.CalculationofdrilledholeshapesThe1-mmdiameterholesdrilledonengineeringplasticssheetsbytheprocessdescribedabovearenotgeometricallytruecircles,buthaveasmallradialdeviation.Shapeaccuracyofthedrilledholesisestimatedbythefollowingprocess.Anopticalmicroscopeequippedwithdigitalmeasuringdevicemeasurestheshapeofthedrilledhole.Thecrosswireofthemicroscopeissetatthecircumferenceofthehole.Then,thecoordinates(x,y)oftheholecircumferenceareread.Dividingthecircumferenceinto18equalparts,thesamemeasurementsar