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Unit18Unit18TEXTLEARNTOSPEAKNEWWORDSANDEXPRESSIONSNOTESEXERClSESREADINGMATERIALPROFESSIONALWORDSANDPHRASES DrillsandDrillingMachinesThetwistdrillisaveryefficienttool.Itisgenerallyformedbyforgingandtwistinggroovesinaflatstripofsteelorbymillingacylindricalpieceofsteel,high-speedsteelbeingcommonlyused.High-speedsteelcostsmorebuttoolsmadeofitwithstandheatmuchbetterthanthosemadeofordinarytoolsteel.Thetwistdrillmaybedividedintothreeprincipalparts:body,shank,andpoint.Theflutesarethespiralgroovesthatareformedonthesideofadrill,drillsbeingmadewithtwo,three,orfourflutes.Thosehavingthreeorfourflutesareusedforfollowingsmallerdrillsorforenlargingholesalreadydrilled,andarenotsuitedfordrillingintosolidstock.Fig.18-1ThetwistdrillSpiralfluteshavefourmainadvantages:(1)?Theygivethecorrectraketothelipofadrill.(2)?Theycausethechiptocurlsotightlythatitoccupiestheminimumamountofspace.(3)?Theyformchannelsthroughwhichchipsescapefromthehole.(4)?Theyallowthelubricanttofloweasilydowntothecuttingedge.Themarginisthenarrowstriponthecuttingedgeoftheflute.Itispracticallythefulldiameterofthedrillandextendstheentirelengthoftheflute,itssurfacebeingapartofacylinder.Theportionofthebodynexttothemarginisoflessdiameterthanthemargin.Thislesseneddiameter,calledbodyclearance,reducesthefrictionbetweenthedrillandthewallsofthehole,whilethemargininsurestheholebeingofaccuratesize.Theshankistheendofthedrillwhichfitsintothesocket,spindle,orchuckofthedrillpress.Thetangisusuallyfoundonlyontaperedshanktools.。Thedrillingmachineisthesecondoldestmachinetools,havingbeeninventedshortlyafterthelathe,andisoneofthemostcommonandusefulmachines.Thedrill
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