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SheetMetalBending
Bendingofsheetmetalisacommonandvitalprocessinmanufacturingindustry.Sheetmetalbendingistheplasticdeformationoftheworkoveranaxis,creatingachangeinthepart'sgeometry.Similartoothermetalformingprocesses,bendingchangestheshapeoftheworkpiece,whilethevolumeofmaterialwillremainthesame.Insomecasesbendingmayproduceasmallchangeinsheetthickness.Formostoperations,however,bendingwillproduceessentiallynochangeinthethicknessofthesheetmetal.Inadditiontocreatingadesiredgeometricform,bendingisalsousedtoimpartstrengthandstiffnesstosheetmetal,tochangeapart'smomentofinertia,forcosmeticappearanceandtoeliminatesharpedges.
Figure:264
Metalbendingenactsbothtensionandcompressionwithinthematerial.Mechanicalprinciplesofmetals,particularlywithregardtoelasticandplasticdeformation,areimportanttounderstandingsheetmetalbendingandarediscussedinthefundamentalsofmetalformingsection.Theeffectthatmaterialpropertieswillhaveinresponsetotheconditionsofmanufacturewillbeafactorinsheetmetalprocessdesign.Usuallysheetmetalbendingisperformedcoldbutsometimestheworkmaybeheated,toeitherwarmorhotworkingtemperature.
Mostsheetmetalbendingoperationsinvolveapunchdietypesetup,althoughnotalways.Therearemanydifferentpunchdiegeometries,setupsandfixtures.Toolingcanbespecifictoabendingprocessandadesiredangleofbend.Bendingdiematerialsaretypicallygrayiron,orcarbonsteel,butdependingontheworkpiece,therangeofpunchdiematerialsvariesfromhardwoodtocarbides.Forceforthepunchanddieactionwillusuallybeprovidedbyapress.
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Aworkpiecemayundergoseveralmetalbendingprocesses.Sometimesitwilltakeaseriesofdifferentpunchanddieoperationstocreateasinglebend.Ormanyprogressivebendingoperationstoformacertaingeometry.
Sheetmetalisreferencedwithregardtotheworkpiecewhenbendingprocessesarediscussedinthissection.However,manyoftheprocessescoveredcanalsobeappliedtoplatemetalaswell.Referencestosheetmetalworkpiecesmayoftenincludeplate.Somebendingoperationsarespecificallydesignedforthebendingofdifferentlyshapedmetalpieces,suchasforcabinethandles.Tubeandrodbendingisalsowidelyperformedinmodernmanufacturing.
BendingProcessesBendingprocessesdifferinthemethodstheyusetoplasticallydeformthesheetorplate.Workpiecematerial,sizeandthicknessareimportantfactorswhendecidingonatypeofmetalbendingprocess.Alsoimportantisthesizeofthebend,bendradius,angleofbend,curvatureofbendandlocationofbendintheworkpiece.Sheetmetalprocessdesignshouldselectthemosteffectivetypeofbendingprocessbasedonthenatureofthedesiredbendandtheworkmaterial.Manybendscanbeeffectivelyformedbyavarietyofdifferentprocessesandavailablemachinerywilloftendeterminethebendingmethod.
OneofthemostcommontypesofsheetmetalmanufacturingprocessesisVbending.TheVshapedpunchforcestheworkintotheVshapeddieandhencebendsit.Thistypeofprocesscanbendbothveryacuteandveryobtuseangles,alsoanythinginbetween,including90degrees.
Figure:265
Edgebendingisanotherverycommonsheetmetalprocessandisperformedwithawipingdie.Edgebendinggivesagoodmechanicaladvantagewhenformingabend.However,anglesgreaterthan90degreeswillrequiremorecomplexequipment,capableofsomehorizontal
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forcedelivery.Also,wipingdieemployedinedgebendingmusthaveapressurepad.Theactionofthepressurepadmaybecontrolledseparatelythanthatofthepunch.Basicallythepressurepadholdsasectionoftheworkinplaceonthedie,theareaforthebendislocatedontheedgeofthedieandtherestoftheworkisheldoverspacelikeacantileverbeam.Thepunchthenappliesforcetothecantileverbeamsection,causingtheworktobendovertheedgeofthedie.
Figure:266
Rotarybendingformstheworkbyasimilarmechanismasedgebending.However,rotarybendingusesadifferentdesignthanthewipingdie.Acylinder,withthedesiredanglecutout,servesasthepunch.Thecylindercanrotateaboutoneaxisandissecurelyconstrainedinallotherdegreesofmotionbyitsattachmenttothesaddle.Thesheetmetalisplacedcantileveredovertheedgeofthelowerdie,similartothesetupinedgebending.Unlikeinedgebending,withrotarybending,thereisnopressurepad.Forceistransmittedtothepunchcausingittoclosewiththework.Thegrooveonthecylinderisdimensionedtocreatethecorrectlyangledbend.Thegroovecanbelessthanorgreaterthan90degreesallowingforarangeofacuteandobtusebends.ThecylindersVgroovehastwosurfaces.Onesurfacecontactstheworktransmittingpressureandholdingthesheetmetalinplaceonthelowerdie.Asforceistransmittedthroughthecylinderitrotates,causingtheothersurfacetobendtheworkovertheedgeofthedie,whilethefirstsurfacecontinuestoholdtheworkinplace.Rotarybendingprovidesagoodmechanicaladvantage.
Thisprocessprovidesbenefitsoverastandardedgebendingoperation,inthatiteliminatestheneedforapressurepadanditiscapableofbendingover90degreeswithoutanyhorizontallyactingequipment.Rotarybendingisrelativelynewandisgainingpopularityinmanufacturingindustry.
Figure:267
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Airbendingisasimplemethodofcreatingabendwithouttheneedforlowerdiegeometry.Thesheetmetalissupportedbytwosurfacesacertaindistanceapart.Apunchexertsforceatthecorrectspot,bendingthesheetmetalbetweenthetwosurfaces.
Figure:268
Punchanddiearemanufacturedwithcertaingeometries,inordertoperformspecificbends.Channelbendingusesashapedpunchanddietoformasheetmetalchannel.AUbendismadewithaUshapedpunchofthecorrectcurvature.
Figure:269
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Manybendingoperationshavebeendevelopedtoproduceoffsetsandformthesheetmetalforavarietyofdifferentfunctions.
Figure:270
Somesheetmetalbendingoperationsinvolvetheuseofmorethan2die.Roundtubes,forexample,canbebentfromsheetmetalusingamultipleactionmachine.Thehollowtubecanbeseamedorweldedforjoining.
Figure:271
Corrugatingisatypeofbendingprocessinwhichasymmetricalbendisproducedacrossthewidthofsheetmetalandataregularintervalalongitsentirelength.Avarietyofshapesareusedforcorrugating,buttheyallhavethesamepurpose,toincreasetherigidityofthesheetmetalandincreaseitsresistancetobendingmoments.Thisisaccomplishedbyaworkhardeningofthemetalandachangeinthesheet'smomentofinertia,causedbythebend'sgeometry.Corrugatedsheetmetalisveryusefulinstructuralapplicationsandiswidelyusedintheconstructionindustry.
Figure:272
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EdgeBendingProcessesSheetmetalofdifferentsizescanbebentaninnumerableamountofways,atdifferentlocations,toachievedesiredpartgeometries.Oneofthemostimportantconsiderationsinsheetmetalmanufactureistheconditionofthesheetmetal'sedges,particularlywithregardtothepartaftermanufacture.Edgebendingoperationsarecommonlyusedinindustrialsheetmetalprocessingandinvolvebendingasectionofthemetalthatissmallrelativetothepart.Thesesectionsarelocatedattheedges.Edgebendingisusedtoeliminatesharpedges,toprovidegeometricsurfacesforpurposessuchasjoining,toprotectthepart,toincreasestiffnessandforcosmeticappearance.
Flangingisaprocessthatbendsanedge,usuallytoa90degreeangle.
Figure:273
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Sometimesthesheetmetal'smaterialispurposelysubjectedtotensionsorcompressions,intheprocessesofstretchflangingandshrinkflangingrespectively.Inadditiontobendingtheedge,theseoperationsalsogiveitacurve.
Figure:274
Beadingiscommonintheedgetreatmentofsheetmetalpartsandcanalsobeusedtoformtheworkingstructureofparts,suchashinges.Beadingformsacurloverapart'sedge.Thisbeadcanbeformedoverastraightorcurvedaxis.Therearemanydifferenttechniquesforformingabead.Somemethodsformthebeadprogressively,withmultiplestages,usingseveraldifferentdiearrangements.Othersheetmetalbeadingprocessesproduceabeadwithasingledie.Inaprocesscalledwiring,themetal'sedgeisbentoverawire.Howthebeadisformedwilldependonthespecificrequirementsofthemanufacturingprocessandsheetmetalpart.
Figure:275
Hemmingisanedgebendingprocessinwhichtheedgeofthesheetisbentcompletelyoveronitself.
Figure:276
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Seamingisasheetmetaljoiningprocess.Seaminginvolvesbendingtheedgesoftwopartsoveroneachother.Thestrengthofthemetalresistsbreakingthejoint,becausethematerialisplasticallydeformedintoposition.Asthebendsarelockedtogether,eachbendhelpsresistthedeformationoftheotherbend,providingawellfortifiedjointstructure.Doubleseaminghasbeenemployedtocreatewatertightorairtightjointsbetweensheetmetalparts.
Figure:277
RollBendingRollbendingprovidesatechniquethatisusefulforrelativelythickwork.Althoughsheetsofvarioussizesandthicknessesmaybeused,thisisamajormanufacturingprocessforthemetalbendingoflargepiecesofplate.Rollbendingusesthreerollstofeedandbendtheplatetothedesiredcurvature.Thearrangementoftherollsdeterminestheexactbendofthework.Differentcurvesareobtainedbycontrollingthedistanceandanglebetweentherolls.Amoveablerollprovidestheabilitytocontrolthecurve.Theworkmayalreadyhavesomecurvetoit,oftenitwillbestraight.Beams,barsandotherstockmetalisalsobentusingthisprocess.
Figure:278
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SheetMetalRollFormingRollformingofsheetmetalisacontinuousmanufacturingprocess,thatusesrollstobendasheetmetalcrosssectionofacertaingeometry.Oftenseveralrollsmaybeemployed,inseries,tocontinuouslybendstock.Similartoshaperolling,butrollformingdoesnotinvolvematerialredistributionofthework,onlybending.Likeshaperolling,rollformingusuallyinvolvesbendingoftheworkinsequentialsteps.Eachrollwillformthesheetmetaltoacertaindegree,inpreparationforthenextroll.Thefinalrollcompletesthegeometry.
Channelsofdifferenttypes,gutters,sidingandpanelsforstructuralpurposesarecommonitemsmanufacturedinmassproductionbyrollforming.Rollsareusuallyfedfromasheetmetalcoil.Theentryrollissuppliedasthecoilunwindsduringtheprocess.Onceformed,continuousproductscanbecuttodesiredlengthstocreatediscreteparts.Closedsectionssuchassquaresandrectanglescanbecontinuouslybentfromsheetmetalcoil.Framesfordoorsandwindowsaremanufacturedbythismethod.Sheetmetalcoilisoftenrollbentintothinwalledpipethatisweldedtogether,atitsseam.Theweldingofthecontinuousproductisincorporatedintotherollingprocess.Rollformingofchannelsisacontinuousalternativetoadiscretechannelbendingprocess,suchastheoneillustratedinfigure269.Figure279showsasimplesequenceusedtoproduceachannel.
Figure:279
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Thischannelcouldbeproducedwithapunchanddie.However,inthatcase,thelengthofthechannelwouldbelimitedbythelengthofthepunchanddie.Rollformingallowsforacontinuouspart,(limitedpracticallytothelengthofthesheetmetalcoil),thatcanbecuttowhateversizeneeded.Productivityisalsoincreased,withtheeliminationofloadingandunloadingofwork.Rollsforsheetmetalrollformingaretypicallymadeofgreycastironorcarbonsteel.Lubricationisimportantandaffectsforcesandsurfacefinish.Sometimesrollswillbechromiumplatedtoimprovesurfacequality.
MechanicsOfSheetMetalBendingTounderstandthemechanicsofsheetmetalbending,anunderstandingofthematerialproperties,characteristicsandbehaviorsofmetal,isnecessary.Particularlyimportantisthetopicofelasticandplasticdeformationofmetal.Informationonthepropertiesofmetals,withrelationtomanufacturing,canbefoundinanearliersection,(metalforming).Itshouldbeunderstoodalsothatsheetmetalbendingproduceslocalizedplasticdeformationandessentiallynochangeinsheetthickness,formostoperations.Itdoesnotcreatemetalflowthataffectsregionsawayfromthebend.
Theforcerequiredtoperformabendislargelydependentuponthebendandthespecificmetalbendingprocess,becausethemechanicsofeachprocesscanvaryconsiderably.Properlubricationisessentialtocontrollingforcesandhasaneffectontheprocess.Inpunchanddieoperations,thesizeofthedieopeningisamajorfactorintheforcenecessarytoperformthebending.Increasingthesizeofthedieopeningwilldecreasethenecessarybendingforce.Asthesheetmetalisbent,theforceneededwillchange.Usuallyitisimportanttodeterminethemaximumnecessarybendingforce,toaccessmachinecapacityrequirements.
Theimportantfactorsinfluencingthemechanicsofbendingarematerial,sheetthickness,widthoverwhichbendoccurs,radiusofbend,bendangle,machinery,toolingandspecificmetalbendingprocess.Bendingasheetwillcreateforcesthatactinthebendregionandthroughthethicknessofthesheet.Thematerialtowardstheoutsideofthebendisintensionandthematerialtowardstheinsideisincompression.Tensionandcompressionareopposite,thereforewhenmovingfromonetotheotherazeroregionmustexist.Atthiszeroregionno
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forcesareexertedonthematerial.Whensheetmetalbending,thiszeroregionoccursalongacontinuousplanewithinthepart'sthickness,calledtheneutralaxis.Thelocationofthisaxiswilldependonthedifferentbendingandsheetmetalfactors.However,agenericapproximationforthelocationoftheaxiscouldbe40percentofsheetthickness,measuredfromtheinsideofthebend.Anothercharacteristicoftheneutralaxisisthatbecauseofthelackofforces,thelengthoftheneutralaxisremainsthesame.Fundamentally,toonesideoftheneutralaxisthematerialisintension,totheothersidethematerialisincompression.Themagnitudeofthetensionorcompressionincreaseswithincreasingdistancefromtheaxis.
Figure:280
Ifarelativelysmallamountofforceisexertedonametalpart,itwilldeformelasticallyandrecoveritsshape,whentheforceisremoved.Inorderforplasticdeformationofmetaltooccur,aminimumthresholdofforcemustbereached.Theforceactingontheneutralaxisiszeroandincreaseswithdistancefromthisregion.Theminimumthresholdofforcerequiredforplasticdeformationisnotreacheduntilacertaindistancefromtheneutralaxisineitherdirection.Thematerialbetweentheseregionsisonlyplasticallydeformed,duetothelowmagnitudeofforces.Theseregionsrunparallelto,andformanelasticcorearound,theneutralaxis.
Figure:281
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Whentheforceusedtocreatethebendisremoved,therecoveryoftheelasticregionresultsintheoccurrenceofspringback.Springbackisthepartialrecoveryoftheworkfromthebendtoitsgeometrybeforethebendingforcewasapplied.Themagnitudeofspringbackdependslargelyonthemodulusofelasticityandtheyieldstrengthofthematerial.Typicallytheresultsofspringbackwillonlyacttoincreasethebendanglebyafewdegrees,however,allsheetmetalbendingprocessesmustconsiderthefactorofspringback.
Figure:282
MethodsOfEliminatingSpringbackTechniqueshavebeendeveloped,inmanufacturingindustry,thatcaneliminatetheeffectsofspringback.Onecommontechniqueisoverbending.Theamountofspringbackiscalculated
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andthesheetmetalisoverbenttoasmallerbendanglethanneeded.Recoveryofthematerialfromspringbackresultsinacalculatedincreaseinbendangle.Thisincreasemakestherecoveredbendangleexactlywhatwasoriginallyplanned.
Figure:283
Anothermethodforeliminatingspringbackisbyplasticallydeformingthematerialinthebendregion.Localizedcompressiveforcesbetweenthepunchanddieinthatareawillplasticallydeformtheelasticcore,preventingspringback.Thiscanbedonebyapplyingadditionalforcethroughthetipofthepunchaftercompletionofbending.Atechniqueknownasbottoming,orbottomingthepunch.
Figure:284
Stretchformingisametalbendingtechniquethateliminatesmostofthespringbackinabend.Subjectingtheworktotensilestresswhilebendingwillforcetheelasticregiontobeplasticallydeformed.Stretchformingcannotbeperformedforsomecomplexbendsandforverysharpangles.Theamountoftensionmustbecontrolledtoavoidcrackingofthesheetmetal.Stretchformingisaprocessoftenusedintheaircraftbuildingindustry.
Figure:285
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SheetMetalBendabilityBendabilityofsheetmetalisthecharacteristicdegreetowhichaparticularsheetmetalpartcanbebentwithoutfailure.Bendabilityisrelatedtothemoregeneraltermofformability,discussedinthesheetmetalformingsection.Thebendabilitywillchangefordifferentmaterialsandsheetthicknesses.Also,themechanicsofthemanufacturingprocesswillaffectbendability,sincedifferenttoolingandsheetgeometrieswillcausedifferentforcedistributions.
Metalbendingtendstobealesscomplicatedprocessthandeepdrawingintheanalysisofforcesactingduringtheoperation.Onesimplemethodtoquantifybendabilityistobendarectangularsheetmetalspecimenuntilitcracksontheoutersurface.Theradiusofbendatwhichcrackingfirstoccursiscalledtheminimumbendradius.Minimumbendradiusisoftenexpressedintermsofsheetthickness,(ie.2T,4T).Thehighertheminimumbendradius,thelowerthebendability.Aminimumbendradiusof0indicatesthatthesheetcanbefoldedoveronitself.Anisotropyofthesheetmetalisanimportantfactorinbending.Ifthesheetisanisotropicthebendingshouldbeperformedinthepreferreddirection.Atesttodetermineanisotropyisdiscussedinthesheetmetalformingsection.
Theconditionofasheetmetal'sedgeswillinfluencebendability.Oftencracksmaypropagatefromtheedges.Roughedgescandecreasethebendabilityofasheetmetalpart.Coldworkingattheedges,orwithinapart,canalsoreducebendability.Vacancieswithinsheetmetalcanbeanothersourceofmaterialfailurewhilebending.Thepresenceofvacancieswillreducemetalbendability.Impuritiesinthematerial,particularlyintheformofinclusions,canalsopropagatecracksandwilldecreasebendability.Pointedorsharplyshapedinclusionsaremoredetrimentaltobendabilitythanroundinclusions.Surfacequalityofthesheetmetalcanmakeadifferenceinbendingmanufacture.Roughsurfacescanincreasethelikelihoodofthesheetcrackingunderforce.
Tomitigatetheseproblems,andoptimizethebendabilityofsheetmetal,careshouldbetakenallthewaythroughthemanufacturingprocess.Highqualitysheetmetalcomesfromhigh
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qualitymetal.Effectiverefiningtechniques,alongwithasoundsheetmetalrollingprocessshouldcloseupvacancies,breakuporeliminateinclusionsandprovideasheetmetalproductwithasmoothsurface.Edgetreatmentsuchastrimming,orfineblanking,canimproveedgequality.Sometimescoldworkedareascanbemachinedout.Annealingtheparttoeliminateregionsofcoldworkingandincreaseductilityalsoimprovesmetalbendability.Bendingoperationsaresometimesperformedonheatedparts,becauseheatingwillcausethemetal'sbendabilitytogoup.Sheetmetalmayalso,onoccasion,beformedinahighpressureenvironment,whichisanotherwaytomakeitmorebendable.
CuttingAndBendingProcessesSomemanufacturingprocessesinvolvebothcuttingandbendingofthesheetmetal.Lancingisaprocessthatcutsandbendsthesheettocreatearaisedgeometry.Lancingmaybeusedtoincreasetheheatdissipationcapacityofsheetmetalparts,forexample.Anothercommonprocessthatemploysbothcuttingandbendingispiercing.Nottobeconfusedwiththeforgingprocessofpiercing.Piercingisusedtocreateaholeinasheetmetalpart.Unlikeblanking,whichcreatesaslug,piercingdoesnotremovematerial.Thepunchispointedandcanpiercethesheet.Asthepunchwidenstheholethematerialisbentintoaninternalflangeforthehole.Thisflangemaybeusefulforsomeapplications.
Figure:286
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MetalTubeBulgingTubebulgingisasheetmetalmanufacturingprocessinwhichsomepartoftheinternalgeometryofahollowmetaltubeissubjectedtopressure,causingthetubetobulgeoutward.Theareabeingbulgedisusuallyconstrainedwithinadiethatcancontrolitsgeometry.Totallengthofthetubewillbedecreasedbecauseofthewideningofthebulgingarea.Therearedifferentmetalbulgingtechniquesemployedinmanufacturingindustry.
Onemaingroupofprocessesusesanelastomerplug,usuallypolyurethane.Thisplugisplacedwithinthetube.Pressureisappliedtotheelastomercausingittobulge.Expandingoutward,theplugbendsthesheetmetaltube.Uponremovaloftheforce,theelastomerplugreturnstoitsoriginalshapeandcanbeeasilyremoved.Polyurethaneplugsaredurableandwillcreateagoodpressuredistributionoverthesurfaceduringbending.Hydraulicpressuremayalsobeusedtoproducethesamebulgingeffect.Howeverelastomerplugsarecleaner,easytoremoveandrequirelesscomplicatedtooling.Splitdiesareusedtofacilitatetheremovalofthepart.
Figure:287
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MetalTubeBendingTubes,rods,barsandothercrosssectionsarealsosubjecttometalbendingoperations.Itshouldberememberedthatwhenbendingametalpart,springbackisalwaysafactor.Severalspecialmanufacturingprocesseshavebeendevelopedforthebendingofhollowtubes.Theseoperationscanalsobeusedonsolidrods.Hollowtubeshavethecharacteristicthattheymaycollapsewhenbent.Tubesmayalsocrackortear,thematerial'sductilityisimportantwhenconsideringtubefailure.
Asthebendradiusgoesdown,thetendencytocollapseincreases.Bendradiusinmetaltubebendingismeasuredfromthetube'scenterline.Theothermajorfactordeterminingcollapseisthewallthicknessofthetube.Tubeswithagreaterwallthicknessarelesslikelytocollapse.Bendingathickwalledtubetoalargeradiusisusuallynotaproblem,asfarascollapseisconcerned.However,aswallthicknessdecreasesand/orbendradiusgoesdown,solutionsmustbefoundtopreventtubecollapse.Onesolutionistofillthetubewithsandbeforebending.Anothermethodwouldbetoplaceaplasticplugofsomesortinthetube,thenbendit.Boththesandandtheplasticplugacttoprovideinternalstructuralsupport,greatlyincreasingtheabilitytobendthetubewithoutcollapse.
Stretchbendingisaprocessinwhichatubeisformedbyastretchingforceparalleltothetube'saxisandasimultaneousbendingforceactingtopullthetubeoveraformblock.Theblockisfixedandtheforcesareappliedtotheendsofthetube.
Figure:288
Drawbendinginvolvesclampingthetubenearitsendtoarotatingformblock.Apressurepadisalsousedtoholdthetubestock.Astheformblockrotates,thetubeisbent.
Figure:289
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Compressionbendingisatubebendingprocessthathassomesimilaritiestoedgebendingofsheetmetalwithawipingdie.Thetubestockisheldbyforcetoafixedformblock.Awiperlikedieappliesforce,bendingthetubeovertheformblock.
Figure:290
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