模具专业英语Blanking-Technique2

Lesson8BlankingTechniqueCutting(shearing)OperationsInthefollowingdiscussion,certaindieterminologywillbeusedfrequently,Figure8-1presentsthetermsmostcommonlyencountered.ShearActioninDieCuttingOperationsThecuttingofmetalbetweendiecomponentsisashearingprocessinwhichthemetalisstressedinshearbetweentwocuttingedgestothepointoffracture,orbeyonditsultimatestrength.Themetalissubjectedtobothtensileandcompressivestresses(Figure8-2);stretchingbeyondtheelasticlimitoccurs,thenplasticdeformation,reductioninarea,andfinally,fracturingstartsthroughcleavageplanesinthereducedareaandbecomescomplete.ThefundamentalstepsinshearingorcuttingareshowninFigure8-3.Thepressureappliedbythepunchonthemetaltendstodeformitintothedieopening.Whentheelasticlimitisexceededbyfurtherloading,aportionofthemetalwillbeforcedintothedieopeningintheformofanembossedpadonthelowerfaceofthematerial.Acorrespondingdepressionresultsontheupperface,asindicatedatFigure8-3a.Astheloadisfurtherincreased,thepunchwillpenetratethemetaltoacertaindepthandforceanequalportionofmetalthicknessintothedie,asindicatedatFigure8-3b.Thispenetrationoccursbeforefracturingstartsandreducesthecross-sectionalareaofmetalthroughwhichthecutisbeingmade.Fracturewillstartinthereducedareaatbothupperandlowercuttingedges,asindicatedatFigure8-3c.Iftheclearanceissuitableforthematerialbeingcut,thesefractureswillspreadtowardeachotherandeventuallymeet,causingcompleteseparation.FurthertravelofthepunchwillcarrythecutportionthroughthestockandintothedieopeningCenterofpressureIfthecontourtobeblankedisirregularlyshaped,thesummationofshearingforcesononesideofthecenteroftherammaygreatlyexceedtheforcesontheothersideSuchirregularityresultsinabendingmomentinthepressram,andundesirabledeflectionsandmisalignment.ItisthereforenecessarytofindapointaboutwhichthesummationofshearingforceswillbesymmetricalThispointiscalledthecenterofpressure,andisthecenterofgravityofthelinethatistheperimeteroftheblank.Itisnotthecenterofgravityofthearea.Thepresstoolwillbedesignedsothatthecenterofpressurewillbeontheaxisofthepressramwhenthetoolismountedinthepress.clearancesClearanceisthespacebetweenthematingmembersofadieset.ProperClearancesbetweencuttingedgesenablethefracturestomeet.Thefracturedportionoftheshearededgewillhaveacleanappearance.Foroptimumfinishofacutedge,properclearanceisnecessaryandisafunctionofthetype,thickness,andtemperoftheworkmaterial.Clearance,penetration,andfractureareshownschematicallyinFigure8-4.InFigure8-5,characteristicsofthecutedgeonstockandblank,withnormalclearance,areshownschematically.Theuppercornerofthecutedgeofthestock(indicateda')andthelowercorneroftheblank(indicatedbya'-1)willhavearadiuswherethepunchanddieedges,respectively,makecontactwiththematerial.Thisradiusisduetotheplasticdeformationtakingplace,andwillbemorepronouncedwhencuttingsoftmetals.Excessiveclearancewillcausealargeradiusatthesecorners,aswellasaburronoppositecorners.Thisisatotalforcesrequiredtocutandformthepartwitha30%safetyfactoradded.Inmanycases,youwillhavetoaddstrippingforceifstrippingisbeingdonewithaspring-loadedstripper,becausethepresshastocompressthespringswhilecuttingthematerial.Likewise,anyspringpressureforforming,drawpads,andthelike,willhavetobeadded.Theforceneededtopuncha2(50.8mm)dia,holein1/8“(3.18mm)thickSAE1020steelhavingashearstrengthof60000psi(413.7N/mm2).PressTonnage