fabrication.docx
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fabrication
Fabrication(metal)
Fabricationasanindustrialtermreferstobuildingmetalstructuresbycutting,bending,andassembling.Thecuttingpartoffabricationisviasawing,shearing,orchiseling(allwithmanualandpoweredvariants)andviaCNCcutters(usingalaser,plasmatorch,orwaterjet).Thebendingisviahammering(manualorpowered)orviapressbrakesandsimilartools.Theassembling(joiningofthepieces)isviawelding,bindingwithadhesives,riveting,threadedfasteners,orevenyetmorebendingintheformofacrimpedseam.Structuralsteelandsheetmetalaretheusualstartingmaterialsforfabrication,alongwiththeweldingwire,flux,andfastenersthatwilljointhecutpieces.Aswithothermanufacturingprocesses,bothhumanlaborandautomationarecommonlyused.Theproductresultingfrom(theprocessof)fabricationmaybecalledafabrication.Shopsthatspecializeinthistypeofmetalworkarecalledfabshops.Theendproductsofothercommontypesofmetalworking,suchasmachining,metalstamping,forging,andcasting,maybesimilarinshapeandfunction,butthoseprocessesarenotclassifiedasfabrication.
Fabricationcomprisesoroverlapswithvariousmetalworkingspecialties:
∙Fabricationshopsandmachineshopshaveoverlappingcapabilities,butfabricationshopsgenerallyconcentrateonmetalpreparationandassemblyasdescribedabove.Bycomparison,machineshopsalsocutmetal,buttheyaremoreconcernedwiththemachiningofpartsonmachinetools.Firmsthatencompassbothfabworkandmachiningarealsocommon.
∙Blacksmithinghasalwaysinvolvedfabrication,althoughitwasnotalwayscalledbythatname.
∙Theproductsproducedbywelders,whichareoftenreferredtoasweldments,areanexampleoffabrication.
∙Boilermakersoriginallyspecializedinboilers,leadingtotheirtrade'sname,butthetermasusedtodayhasabroadermeaning.
∙Similarly,millwrightsoriginallyspecializedinsettingupgrainmillsandsawmills,buttodaytheymaybecalleduponforabroadrangeoffabricationwork.
∙Ironworkers,alsoknownassteelerectors,alsoengageinfabrication.Oftenthefabricationsforstructuralworkbeginasprefabricatedsegmentsinafabshop,thenaremovedtothesitebytruck,rail,orbarge,andfinallyareinstalledbyerectors.
Metalfabrication
Metalfabricationisavalueaddedprocessthatinvolvestheconstructionofmachinesandstructuresfromvariousrawmaterials.Afabshopwillbidonajob,usuallybasedontheengineeringdrawings,andifawardedthecontractwillbuildtheproduct.
Fabricationshopsareemployedbycontractors,OEM'sandVAR's.Typicalprojectsinclude;looseparts,structuralframesforbuildingsandheavyequipment,andhandrailingsandstairsforbuildings.
Engineering
Thefabricatormayemployorcontractoutsteeldetailerstoprepareshopdrawings,ifnotprovidedbythecustomer,whichthefabricatingshopwilluseformanufacturing.ManufacturingengineerswillprogramCNCmachinesasneeded.
Rawmaterials
Standardrawmaterialsusedbymetalfabricatorsare;
∙platemetal
∙formedandexpandedmetal
otubestock,CDSM
osquarestock
osectionalmetals(Ibeams,Wbeams,C-channel...)
∙weldingwire
∙hardware
∙castings
∙fittings
Cuttingandburning
Therawmaterialhastobecuttosize.Thisisdonewithavarietyoftools.
ThemostcommonwaytocutmaterialisbyShearing(metalworking);
Specialbandsawsdesignedforcuttingmetalhavehardenedbladesandafeedmechanismforevencutting.Abrasivecut-offsaws,alsoknownaschopsaws,aresimilartomitersawsbutwithasteelcuttingabrasivedisk.Cuttingtorchescancutverylargesectionsofsteelwithlittleeffort.
BurntablesareCNCcuttingtorches,usuallynaturalgaspowered.Plasmaandlasercuttingtables,andWaterjetcutters,arealsocommon.Platesteelisloadedonatableandthepartsarecutoutasprogrammed.Thesupporttableismadeofagridofbarsthatcanbereplaced.SomeveryexpensiveburntablesalsoincludeCNCpunchcapability,withacarouselofdifferentpunchesandtaps.Fabricationofstructuralsteelbyplasmaandlasercuttingintroducesrobotstomovethecuttingheadinthreedimensionsaroundthematerialtobecut.
Forming
Hydraulicbrakepresseswithv-diesarethemostcommonmethodofformingmetal.Thecutplateisplacedinthepressandav-shapeddieispressedapredetermineddistancetobendtheplatetothedesiredangle.Wingbrakesandhandpoweredbrakesaresometimesused.
Tubebendingmachineshavespeciallyshapeddiesandmandrelstobendtubularsectionswithoutkinkingthem.
Rollingmachinesareusedtoformplatesteelintoaroundsection.
EnglishWheelorWheelingMachinesareusedtoformcomplexdoublecurvatureshapesusingsheetmetal.
Machining
Mainarticle:
machining
Fabshopswillgenerallyhavealimitedmachiningcapabilityincluding;metallathes,mills,magneticbaseddrillsalongwithotherportablemetalworkingtools.
Welding
Mainarticle:
welding
Weldingisthemainfocusofsteelfabrication.Theformedandmachinedpartswillbeassembledandtackweldedintoplacethenre-checkedforaccuracy.Afixturemaybeusedtolocatepartsforweldingifmultipleweldmentshavebeenordered.
Thewelderthencompletesweldingpertheengineeringdrawings,ifweldingisdetailed,orperhisownjudgmentifnoweldingdetailsareprovided.
Specialprecautionsmaybeneededtopreventwarpingoftheweldmentduetoheat.Thesemayincludere-designingtheweldmenttouselessweld,weldinginastaggeredfashion,usingastoutfixture,coveringtheweldmentinsandduringcooling,andstraighteningoperationsafterwelding.
StraighteningofwarpedsteelweldmentsisdonewithanOxy-acetylenetorchandissomewhatofanart.Heatisselectivelyappliedtothesteelinaslow,linearsweep.Thesteelwillhaveanetcontraction,uponcooling,inthedirectionofthesweep.Ahighlyskilledweldercanremovesignificantwarpageusingthistechnique.
Steelweldmentsareoccasionallyannealedinalowtemperatureoventorelieveresidualstresses.
[edit]Finalassembly
Aftertheweldmenthascooleditisgenerallysandblasted,primedandpainted.Anyadditionalmanufacturingspecifiedbythecustomeristhencompleted.Thefinishedproductistheninspectedandshipped.
Specialties
Manyfabshopshavespecialtyprocesseswhichtheydeveloporinvestin,basedontheircustomersneedsandtheirexpertise:
∙brazing
∙casting
∙chipping
∙drawing
∙extrusion
∙forging
∙heattreatment
∙hydroforming
∙ovensoldering
∙plasticfabrication
∙powdercoating
∙powdermetallurgy
∙punching
∙shearing
∙spinning
∙Englishwheeling
∙welding
Andhigher-levelspecializationssuchas:
∙electrical
∙hydraulics
∙prototyping/machinedesign/technicaldrawing
∙sub-contractmanufacturing
Brazing
Brazingpractice
Thisarticle'sintroductionsectionmaynotadequatelysummarizeitscontents.TocomplywithWikipedia'sleadsectionguidelines,pleaseconsiderexpandingtheleadtoprovideanaccessibleoverviewofthearticle'skeypoints.(August2010)
Brazingisametal-joiningprocesswherebyafillermetalisheatedaboveanddistributedbetweentwoormoreclose-fittingpartsbycapillaryaction.Thefillermetalisbroughtslightlyaboveitsmelting(liquidus)temperaturewhileprotectedbyasuitableatmosphere,usuallyaflux.Itthenflowsoverthebasemetal(knownaswetting)andisthencooledtojointheworkpiecestogether.[1]Itissimilartosoldering,exceptthetemperaturesusedtomeltthefillermetalisabove450 °C(842 °F),or,astraditionallydefinedintheUnitedStates,above800 °F(427 °C).
∙
Fundamentals
Inordertoobtainhigh-qualitybrazedjoints,partsmustbecloselyfitted,andthebasemetalsmustbeexceptionallycleanandfreeofoxides.Inmostcases,jointclearancesof0.03to0.08mm(0.0012to0.0031in)arerecommendedforthebestcapillaryactionandjointstrength.[2]However,insomebrazingoperationsitisnotuncommontohavejointclearancesaround0.6 mm(0.024in).Cleanlinessofthebrazingsurfacesisalsoofvitalimportance,asanycontaminationcancausepoorwetting.Thetwomainmethodsforcleaningparts,priortobrazingarechemicalcleaning,andabrasiveormechanicalcleaning.Inthecaseofmechanicalcleaning,itisofvitalimportancetomaintainthepropersurfaceroughnessaswettingonaroughsurfaceoccursmuchmorereadilythanonasmoothsurfaceofthesamegeometry.[2]
Anotherconsiderationthatcannotbeover-lookedistheeffectoftemperatureandtimeonthequalityofbrazedjoints.Asthetemperatureofthebrazealloyisincreased,thealloyingandwettingactionofthefillermetalincreasesaswell.Ingeneral,thebrazingtemperatureselectedmustbeabovethemeltingpointofthefillermetal.However,thereareseveralfactorsthatinfluencethejointdesigner'stemperatureselection.Thebesttemperatureisusuallyselectedsoasto:
(1)bethelowestpossiblebrazetemperature,
(2)minimizeanyheateffectsontheassembly,(3)keepfillermetal/basemetalinteractionstoaminimum,and(4)maximizethelifeofanyfixturesorjigsused.[2]Insomecases,ahighertemperaturemaybeselectedtoallowforotherfactorsinthedesign(e.g.toallowuseofadifferentfillermetal,ortocontrolmetallurgicaleffects,ortosufficientlyremovesurfacecontamination).Theeffectoftimeonthebrazedjointprimarilyaffectstheextenttowhichtheaforementionedeffectsarepresent;however,ingeneralmostproductionprocessesareselectedtominimizebrazingtimeandtheassociatedcosts.Thisisnotalwaysthecase,however,sinceinsomenon-productionsettings,timeandcostaresecondaryt