sheet metal bending

3/23/2015 SheetMetalBending

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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.



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



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



Airbendingisasimplemethodofcreatingabendwithouttheneedforlowerdiegeometry.Thesheetmetalissupportedbytwosurfacesacertaindistanceapart.Apunchexertsforceatthecorrectspot,bendingthesheetmetalbetweenthetwosurfaces.

Figure:268

Punchanddiearemanufacturedwithcertaingeometries,inordertoperformspecificbends.Channelbendingusesashapedpunchanddietoformasheetmetalchannel.AUbendismadewithaUshapedpunchofthecorrectcurvature.

Figure:269



Manybendingoperationshavebeendevelopedtoproduceoffsetsandformthesheetmetalforavarietyofdifferentfunctions.

Figure:270

Somesheetmetalbendingoperationsinvolvetheuseofmorethan2die.Roundtubes,forexample,canbebentfromsheetmetalusingamultipleactionmachine.Thehollowtubecanbeseamedorweldedforjoining.

Figure:271

Corrugatingisatypeofbendingprocessinwhichasymmetricalbendisproducedacrossthewidthofsheetmetalandataregularintervalalongitsentirelength.Avarietyofshapesareusedforcorrugating,buttheyallhavethesamepurpose,toincreasetherigidityofthesheetmetalandincreaseitsresistancetobendingmoments.Thisisaccomplishedbyaworkhardeningofthemetalandachangeinthesheet'smomentofinertia,causedbythebend'sgeometry.Corrugatedsheetmetalisveryusefulinstructuralapplicationsandiswidelyusedintheconstructionindustry.

Figure:272



EdgeBendingProcessesSheetmetalofdifferentsizescanbebentaninnumerableamountofways,atdifferentlocations,toachievedesiredpartgeometries.Oneofthemostimportantconsiderationsinsheetmetalmanufactureistheconditionofthesheetmetal'sedges,particularlywithregardtothepartaftermanufacture.Edgebendingoperationsarecommonlyusedinindustrialsheetmetalprocessingandinvolvebendingasectionofthemetalthatissmallrelativetothepart.Thesesectionsarelocatedattheedges.Edgebendingisusedtoeliminatesharpedges,toprovidegeometricsurfacesforpurposessuchasjoining,toprotectthepart,toincreasestiffnessandforcosmeticappearance.

Flangingisaprocessthatbendsanedge,usuallytoa90degreeangle.

Figure:273



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



Seamingisasheetmetaljoiningprocess.Seaminginvolvesbendingtheedgesoftwopartsoveroneachother.Thestrengthofthemetalresistsbreakingthejoint,becausethematerialisplasticallydeformedintoposition.Asthebendsarelockedtogether,eachbendhelpsresistthedeformationoftheotherbend,providingawellfortifiedjointstructure.Doubleseaminghasbeenemployedtocreatewatertightorairtightjointsbetweensheetmetalparts.

Figure:277

RollBendingRollbendingprovidesatechniquethatisusefulforrelativelythickwork.Althoughsheetsofvarioussizesandthicknessesmaybeused,thisisamajormanufacturingprocessforthemetalbendingoflargepiecesofplate.Rollbendingusesthreerollstofeedandbendtheplatetothedesiredcurvature.Thearrangementoftherollsdeterminestheexactbendofthework.Differentcurvesareobtainedbycontrollingthedistanceandanglebetweentherolls.Amoveablerollprovidestheabilitytocontrolthecurve.Theworkmayalreadyhavesomecurvetoit,oftenitwillbestraight.Beams,barsandotherstockmetalisalsobentusingthisprocess.

Figure:278



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



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



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



Whentheforceusedtocreatethebendisremoved,therecoveryoftheelasticregionresultsintheoccurrenceofspringback.Springbackisthepartialrecoveryoftheworkfromthebendtoitsgeometrybeforethebendingforcewasapplied.Themagnitudeofspringbackdependslargelyonthemodulusofelasticityandtheyieldstrengthofthematerial.Typicallytheresultsofspringbackwillonlyacttoincreasethebendanglebyafewdegrees,however,allsheetmetalbendingprocessesmustconsiderthefactorofspringback.

Figure:282

MethodsOfEliminatingSpringbackTechniqueshavebeendeveloped,inmanufacturingindustry,thatcaneliminatetheeffectsofspringback.Onecommontechniqueisoverbending.Theamountofspringbackiscalculated



andthesheetmetalisoverbenttoasmallerbendanglethanneeded.Recoveryofthematerialfromspringbackresultsinacalculatedincreaseinbendangle.Thisincreasemakestherecoveredbendangleexactlywhatwasoriginallyplanned.

Figure:283

Anothermethodforeliminatingspringbackisbyplasticallydeformingthematerialinthebendregion.Localizedcompressiveforcesbetweenthepunchanddieinthatareawillplasticallydeformtheelasticcore,preventingspringback.Thiscanbedonebyapplyingadditionalforcethroughthetipofthepunchaftercompletionofbending.Atechniqueknownasbottoming,orbottomingthepunch.

Figure:284

Stretchformingisametalbendingtechniquethateliminatesmostofthespringbackinabend.Subjectingtheworktotensilestresswhilebendingwillforcetheelasticregiontobeplasticallydeformed.Stretchformingcannotbeperformedforsomecomplexbendsandforverysharpangles.Theamountoftensionmustbecontrolledtoavoidcrackingofthesheetmetal.Stretchformingisaprocessoftenusedintheaircraftbuildingindustry.

Figure:285



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



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



MetalTubeBulgingTubebulgingisasheetmetalmanufacturingprocessinwhichsomepartoftheinternalgeometryofahollowmetaltubeissubjectedtopressure,causingthetubetobulgeoutward.Theareabeingbulgedisusuallyconstrainedwithinadiethatcancontrolitsgeometry.Totallengthofthetubewillbedecreasedbecauseofthewideningofthebulgingarea.Therearedifferentmetalbulgingtechniquesemployedinmanufacturingindustry.

Onemaingroupofprocessesusesanelastomerplug,usuallypolyurethane.Thisplugisplacedwithinthetube.Pressureisappliedtotheelastomercausingittobulge.Expandingoutward,theplugbendsthesheetmetaltube.Uponremovaloftheforce,theelastomerplugreturnstoitsoriginalshapeandcanbeeasilyremoved.Polyurethaneplugsaredurableandwillcreateagoodpressuredistributionoverthesurfaceduringbending.Hydraulicpressuremayalsobeusedtoproducethesamebulgingeffect.Howeverelastomerplugsarecleaner,easytoremoveandrequirelesscomplicatedtooling.Splitdiesareusedtofacilitatetheremovalofthepart.

Figure:287



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



Compressionbendingisatubebendingprocessthathassomesimilaritiestoedgebendingofsheetmetalwithawipingdie.Thetubestockisheldbyforcetoafixedformblock.Awiperlikedieappliesforce,bendingthetubeovertheformblock.

Figure:290

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