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MachiningTechniquesAndToolDrillingandDrillsDrillinginvolvesproducingthroughorblindholesinaworkpiecebyforcingatool,whichrotatesarounditsaxis,againsttheworkpiece.Consequently,therangeofcuttingfromthataxisofrotationisequaltotheradiusoftherequiredhole.Inpractice,twosymmetricalcuttingedgesthatrotateaboutthesameaxisareemployed.Drillingoperationscanbecarriedoutbyusingeitherhanddrillsordrillingmachines.Thelatterdifferinsizeandconstruction.Nevertheless,thetoolalwaysrotatesarounditsaxiswhiletheworkpieceiskeptfirmlyfixed.Thisiscontrarytodrillingonalathe.CuttingToolforDrillingOperationIndrillingoperations,acylindricalrotaryendcuttingtool,calledadrill,isemployed.Thedrillcanhaveeitheroneormorecuttingedgesandcorrespondingflutes,whichcanbestraightorhelical.Thefunctionoftheflutesistoprovideoutletpassagesforthechipsgeneratedduringthedrillingoperationandalsotoallowlubricantsandcoolantstoreachthecuttingedgesandthesurfacebeingmachined.Followingisasurveyofthecommonlyuseddrills.Twistdrill.Thetwistdrillisthemostcommontypeofdrill.Ithastwocuttingedgesandtwohelicalflutesthatcontinueoverthelengthofthedrillbody,.Thedrillalsoconsistsofaneckandashankthatcanbeeitherstraightortapered.Inthelattercase,theshankisfittedbythewedgeactionintothetaperedsocketofthespindleandhasatang,whichgoesintoaslotinthespindlesocket,thusactingasasolidmeansfortransmittingrotation.Ontheotherhand,straightshankdrillsareheldinadrillchuckthatis,inturn,fittedintothespindlesocketinthesamewayastaperedshankdrills.Thetwocuttingedgesarereferredtoasthelips,andareconnectedtogetherbyawedge,whichisachisellikeedge.Thetwistdrillalsohastwomargins,whichenableproperguidanceandlocatingofthedrillwhileitisinoperation.ThetoolpointangleTPAisformedbythetwolipsandischosenbasedonthepropertiesofthematerialtobecut.TheusualTPAforcommercialdrillsis118°,whichisappropriatefordrillinglowcarbonsteelsandcastirons.Forharderandtoughermetals,suchashardenedsteel,brassandbronze,largerTPAs130°or140°givebetterperformance.Thehelixangleoftheflutesofthecommonlyusedtwistdrillsrangesbetween24°and30°.Whendrillingcopperorsoftplastics,highervaluesforthehelixanglearerecommendedbetween35°and45°.Twistdrillsareusuallymadeofhighspeedsteel,althoughcarbidetippeddrillsarealsoavailable.Thesizesoftwistdrillsusedinindustrialpracticerangefrom0.01upto3.25in.i.e.,0.25upto80mm.Coredrills.Acoredrillconsistsofthechamfer,body,neck,andshank.Thistypeofdrillmayhaveeitherthreeorfourflutesandequalnumberofmargins,whichensuresuperiorguidance,thusresultinginhighmachiningaccuracy.thatacoredrillhasflatend.Thechamfercanhavethreeorfourcuttingedgesorlips,andthelipanglemayvarybetween90°and120°.Coredrillsareemployedforenlargingpreviouslymadeholesandnotfororiginatingholes.Thistypeofdrillischaracterizedbygreaterproductivity,highmachiningaccuracy,andsuperiorqualityofthedrilledsurfaces.Gundrills.Gundrillsareusedfordrillingdeepholes.Allgundrillsarestraightfluted,andeachhasasinglecuttingedge.Aholeinthebodyactsasaconduittotransmitcoolantunderconsiderablepressuretothetipofthedrill.Therearetwokindsofgundrills,namely,thecentercutgundrillusedfordrillingblindholesandthetrepanningdrill.Thelatterhasacylindricalgrooveatitscenter,thusgeneratingasolidcore,whichguidesthetoolasitproceedsduringthedrillingoperation.Spadedrills.Spadedrillsareusedfordrillinglargeholesof3.5in.90mmormore.Theirdesignresultsinamarkedsavingincostofthetoolaswellasatangiblereductioninitsweight,whichfacilitatesitshandling.Moreover,thistypeofdrilliseasytogrind.MillingandMillingCutterMillingisamachiningprocessthatiscarriedoutbymeansofamultiedgerotatingtoolknownasamillingcutter.Inthisprocess,metalremovalisachievedthroughcombiningtherotarymotionofthemillingcutterandlinearmotionsoftheworkpiecesimultaneously.Millingoperationsareemployedinproducingflat,contouredandhelicalsurfacesaswellasforthreadandgearcuttingoperation.Eachofthecuttingedgesofamillingcutteractsasanindividualsinglepointcutterwhenitengageswiththeworkpiecemetal.Therefore,eachofthosecuttingedgeshasappropriaterakeandreliefangles.Sinceonlyafewofthecuttingedgesareengagedwiththeworkpieceatatime,heavycutscanbetakenwithoutadverselyaffectingthetoollife.Infact,thepermissiblecuttingspeedsandfeedsformillingarethreetofourtimeshigherthanthoseforturningordrilling.Moreover,thequalityofthesurfacesmachinedbymillingisgenerallysuperiortothequalityofsurfacesmachinedbytrning,shaping,ordrilling.Awidevarietyofmillingcuttersisavailableinindustry.This,togetherwiththefactthatamillingmachineisaveryversatilemachinetool,makesthemillingmachinethebackboneofamachiningworkshop.Asfarasthedirectionofcutterrotationandworkpiecefeedareconcerned,millingisperformedbyeitherofthefollowingtwomethods.Upmillingconventionalmilling.Inupmillingtheworkpieceisfedagainstthedirectionofcutterrotation.Aswecanseeinthatfigure,thedepthofcutandconsequentlytheloadgraduallyincreasesonthesuccessivelyengagedcuttingedges.Therefore,themachiningprocessinvolvesnoimpactloading,thusensuringsmootheroperationofthemachinetoolandlongertoollife.Thequalityofthemachinedsurfaceobtainedbyupmillingisnotveryhigh.Nevertheless,upmillingiscommonlyusedinindustry,especiallyforroughcuts.Downmillingclimbmilling.Indownmillingthecutterrotationcoincideswiththedirectionoffeedatthecontactpointbetweenthetoolandtheworkpiece.Itcanalsobeseenthatthemaximumdepthofcutisachieveddirectlyasthecutterengageswiththeworkpiece.Thisresultsinakindofimpact,orsuddenloading.Therefore,thismethodcannotbeusedunlessthemillingmachineisequippedwithabacklasheliminatoronthefeedscrew.Theadvantagesofthismethodincludehigherqualityofthemachinedsurfaceandeasierclampingofworkpieces,sincethecuttingforcesactdownward.TypesofMillingCuttersThereisawidevarietyofmillingcuttershapes.Eachofthemisdesignedtoperformeffectivelyaspecificmillingoperation.Generally,amillingcuttercanbedescribedasamultiedgecuttingtoolhavingtheshapeofasolidofrevolution,withthecuttingteetharrangedeitherontheperipheryoronanendfaceoronboth.Followingisaquicksurveyofthecommonlyusedtypesofmillingcutters.Plainmillingcutter.Aplainmillingcutterisadiskshapedcuttingtoolthatmayhaveeitherstraightorhelicalteeth.Thistypeisalwaysmountedonhorizontalmillingmachinesandisusedformachiningflatsurfaces.Facemillingcutter.Afacemillingcutterisalsousedformachiningflatsurfaces.Itisboltedattheendofashortarbor,whichisinturnmountedonaverticalmillingmachine.Plainmetalslittingsawcutter.Indicatesaplainmetalslittingsawcutter.wecanseethatitactuallyinvolvesaverythinplainmillingcutter.Sidemillingcutter.Asidemillingcutterisusedforcuttingslots,grooves,andsplines.Itisquitesimilartotheplainmillingcutter,thedifferencebeingthatthistypehasteethonthesides.Asisthecasewiththeplaincutter,thecuttingteethcanbestraightorhelical.Anglemillingcutter.Ananglemillingcutterisemployedincuttingdovetailgrooves,ratchetwheels.Tslotcutter.ATslotcutterinvolvesaplainmillingcutterwithanintegralshaftnormaltoit.Asthenamesuggests,thistypeisusedformillingTslots.Endmillcutter.Endmillcuttersfindcommonapplicationsincuttingslots,grooves,flutes,splines,pocketingwork,andthelike.Indicatesanendmillcutter.Thelatterisalwaysmountedonaverticalmillingmachineandcanhavetwoorfourflutes,whichmaybeeitherstraightorhelical.Formmillingcutter.Theteethofaformmillingcutterhaveacertainshape,whichisidenticaltothesectionofthemetaltoberemovedduringthemillingoperation.Examplesofthistypeincludegearcutters,gearhobs,convexandconcavecutters,andthelike.Frommillingcuttersaremountedonhorizontalmillingmachines.MaterialsofMillingCuttersThecommonlyusedmillingcuttersaremadeofhighspeedsteel,whichisgenerallyadequateformostjobs.Millingcutterstippedwithsinteredcarbidesorcastnonferrousalloysascuttingteethareusuallyemployedformassproduction,whereheaviercutsand/orhighcuttingspeedsarerequired.Ithasalreadybeenstatedthattheworkpiecemustbelocatedrelativetothecuttingtool,andbesecuredinthatposition.Aftertheworkpiecehasbeenmarkedout,
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