Lecture 3
5
Lecture 3 SWAGING In this process, the diameter of a rod or a tube is reduced by forcing it into a confining die. A set of reciprocation dies provides radial blows to cause the metal to flow inward and acquire the form of the die cavity. The die movements may be of in – and – out type or rotary. The latter type is obtained with the help of a set of rollers in a cage, in a similar action as in a roller bearing. The workpiece is held stationary and the dies rotate, the dies strike the workpiece at a rate as high as 10 20 strokes per second. Screwdriver blades and soldering iron tips are typical examples of swaged products. Fig 3.1 shows these and other products made by swaging. Fig 3.1 Typical parts made by swaging. In tube swaging, the tube thickness and / or internal dia of tube can be controlled with the use of internal mandrels. For small – diameter tubing, a thin rod can be used as a mandrel; even internally shaped tubes can be swaged by using shaped mandrels. Fig 3.2 shows the process.
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Transcript of Lecture 3
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Lecture3
SWAGING
Inthisprocess,thediameterofarodoratubeisreducedbyforcingitintoaconfiningdie.Asetofreciprocationdiesprovidesradialblowstocausethemetaltoflowinwardandacquiretheformofthediecavity.Thediemovementsmaybeofinandouttypeorrotary.Thelattertypeisobtainedwiththehelpofasetofrollersinacage,inasimilaractionasinarollerbearing.Theworkpieceisheldstationaryandthediesrotate,thediesstriketheworkpieceatarateashighas1020strokespersecond.
Screwdriverbladesandsolderingirontipsaretypicalexamplesofswagedproducts.Fig3.1showstheseandotherproductsmadebyswaging.
Fig3.1Typicalpartsmadebyswaging.
Intubeswaging,thetubethicknessand/orinternaldiaoftubecanbecontrolledwiththeuseofinternalmandrels.Forsmalldiametertubing,athinrodcanbeusedasamandreleveninternallyshapedtubescanbeswagedbyusingshapedmandrels.Fig3.2showstheprocess.
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Fig3.2(a)Swagingoftubeswithoutamandrel.Wallthicknessismoreinthediegap.(b)Swagingwithamandrel.Thefinalwallthicknessofthetubedependsonthemandreldiameter.
(c)Examplesofcrosssectionsoftubesproducedbyswagingonshapedmandrels.
Theprocessisquiteversatile.Themaximumdiameterofworkpiecethatcanbeswagedislimitedtoabout150mmworkpiecesassmallas0.5mmdiameterhavebeenswaged.Theproduction ratecanbeashighas30partsperminutedependingupon thecomplexityof thepartshapeand theparthandlingmeansadopted.
Thepartsproducedbyswaginghavetoleranceintherange0.05mmto0.5mmandimprovedmechanicalproperties.Useoflubricantshelpsinobtainingbetterworksurfacefinishandlongerdielife.Materials,suchastungstenandmolybdenumaregenerallyswagedatelevatedtemperaturesastheyhavelowductilityatroomtemperature.Hotswagingisalsousedtoformlongorsteeptapers,andforlargereductions.
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Swagingisanoisyoperation.Thelevelofnoisecanbe,however,reducedbypropermountingofthemachineorbytheuseofenclosure.
WIREDRAWING
Wiredrawingisprimarilythesameasbardrawingexceptthatitinvolvessmallerdiametermaterialthatcanbecoiled.ItisgenerallyperformedasacontinuousoperationondrawbenchliketheoneshowninFig3.3
Fig3.3Wiredrawingonacontinuousdrawblock.Therotatingdrawblockprovidesacontinuouspullontheincomingwire.
Largecoilofhotrolledmaterialofnearly10mmdiameteristakenandsubjectedtopreparationtreatmentbeforetheactualdrawingprocess.Thepreparationtreatmentforsteelwireconsistsof:
Cleaning.Thismaybedonebyacidpickling,rinsing,anddrying.Or,itmaybedonebymechanicalflexing.
Neutralization.Anyremainingacidontherawmaterialisneutralizedbyimmersingitinalimebath.Thecorrosionprotectedmaterialisalsogivenathinlayeroflubricant.
Tobeginthedrawingprocess,oneendofcoil isreducedincrosssectionuptosomelengthandfedthroughthedrawingdie,andgripped.AwiredrawingdieisgenerallymadeoftungstencarbideandhastheconfigurationshowninFig3.4fordrawingveryfinewire,diamonddieispreferred.
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Fig3.4Crosssectionthroughatypicalcarbidewiredrawingdie.
Smalldiameterwireisgenerallydrawnontandommachineswhichconsistsofaseriesofdies,eachheldinawatercooleddieblock.Eachdiereducesthecrosssectionbyasmallamountsoastoavoidexcessivestraininthewire.Intermediateannealingofmaterialbetweendifferentstatesofwiremayalsobedone,ifrequired.
Wiredrawingterms:
WhereDo,Df,LoandLfaretheoriginalandfinaldiameterandlength.AoandAfareoriginalandfinalcrosssectionalarea.
For a single cold drawing pass, the percent area reduction that can be donedepends uponmany factors. These include the type ofmaterial, its size, initialmetallurgicalcondition,thefinalsizeandmechanicalpropertiesdesired,diedesignandlubricationefficiency.Thepercentofareareductionperpasscanrangefromnearzeroto50%.
Diepull
Theforcerequiredtopullthestockthroughthedie(underfrictionlessconditions)canbecomputedasfollows.
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WhereF=diepull,i.e.theforcerequiredtopullthestockthroughthedie
Yavg=averagetruestressofthematerialinthediegap
Ao,Af=originalandfinalareasofcrosssectionofmaterial.
Alternatively,thefollowingexpressioncanbeused
F=ct(AoAf)
wherecisaconstantwhosevalueisintherange1.5to3.0dependingupontheareareduction,(lowervalueforhigher%reduction),andtistensilestrengthofmaterialbeforedrawing.
Thepullforcedeterminesthemachinecapacityneeded.
TUBEDRAWING
Thediameterandwallthicknessoftubesthathavebeenproducedbyextrusionorotherprocessescanbereducedbytubedrawingprocess.Theprocessoftubedrawing(Fig3.5)issimilartowireorroddrawingexceptthatitusuallyrequiresamandreloftherequisitediametertoformtheinternalhole.
Tubesaslargeas0.3mindiametercanbedrawn.
Fig3.5
DrawingEquipment
Drawingequipmentcanbeofseveraldesigns.ThesedesignscanbeclassifiedintotwobasictypesDrawbench,andBullblock.Adrawbench(Fig3.5)usesasingledieandthepullingforceissuppliedbyachaindriveorbyhydraulicmeans.Drawbenchisusedforsinglelengthdrawingofrodortubewithdiametergreaterthan20mm.Lengthcanbeasmuchas30m.Thedrawingspeedattainableonadrawbenchrangesfrom5m/minto50m/min.Drawbenchesareavailablehavingcapacitiestoprovidepullforceofupto1MN.
Bullblockorrotatingdrum(Fig3.3)isusedfordrawingrodsorwiresofverylonglength.
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