fabrication.docx

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