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Acommontypeoffixture,usedinmaterialstensiletesting

Fixture(tool)FromWikipedia,thefreeencyclopedia

Afixtureisaworkholdingorsupportdeviceusedinthemanufacturingindustry.[1][2]Fixturesareusedtosecurelylocate(positioninaspecificlocationororientation)andsupportthework,ensuringthatallpartsproducedusingthefixturewillmaintainconformityandinterchangeability.Usingafixtureimprovestheeconomyofproductionbyallowingsmoothoperationandquicktransitionfromparttopart,reducingtherequirementforskilledlaborbysimplifyinghowworkpiecesaremounted,andincreasingconformityacrossaproductionrun.[2]

Afixturediffersfromajiginthatwhenafixtureisused,thetoolmustmoverelativetotheworkpieceajigmovesthepiecewhilethetoolremainsstationary.[3]

Contents

1Purpose2Design

2.1Location2.2Support

3Typesoffixtures3.1Millingfixtures3.2Drillingfixtures

4Seealso5Notes6References

Purpose

Afixture'sprimarypurposeistocreateasecuremountingpointforaworkpiece,allowingforsupportduringoperationandincreasedaccuracy,precision,reliability,andinterchangeabilityinthefinishedparts.Italsoservestoreduceworkingtimebyallowingquicksetup,andbysmoothingthetransitionfromparttopart.[3]Itfrequentlyreducesthecomplexityofaprocess,allowingforunskilledworkerstoperformitandeffectivelytransferringtheskillofthetoolmakertotheunskilledworker.[2]Fixturesalsoallowforahigherdegreeofoperatorsafetybyreducingtheconcentrationandeffortrequiredtoholdapiecesteady.[3]

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Thesemodularfixturecomponentsmaybebuiltintovariousarrangementstoaccommodatedifferentworkpieces

Acommonbenchvisetheleftjawistheimmovablesurface,andtherightjawisthemovableclamp

Economicallyspeakingthemostvaluablefunctionofafixtureistoreducelaborcosts.Withoutafixture,operatingamachineorprocessmayrequiretwoormoreoperatorsusingafixturecaneliminateoneoftheoperatorsbysecuringtheworkpiece.[4]

Design

Fixturesmustalwaysbedesignedwitheconomicsinmindthepurposeofthesedevicesistoreducecosts,andsotheymustbedesignedinsuchawaythatthecostreductionoutweighsthecostofimplementingthefixture.Itisusuallybetter,fromaneconomicstandpoint,forafixturetoresultinasmallcostreductionforaprocessinconstantuse,thanforalargecostreductionforaprocessusedonlyoccasionally.[4]

Mostfixtureshaveasolidcomponent,affixedtothefloorortothebodyofthemachineandconsideredimmovablerelativetothemotionofthemachiningbit,andoneormoremovablecomponentsknownasclamps.Theseclamps(whichmaybeoperatedbymanydifferentmechanicalmeans)allowworkpiecestobeeasilyplacedinthemachineorremoved,andyetstaysecureduringoperation.Manyarealsoadjustable,allowingforworkpiecesofdifferentsizestobeusedfordifferentoperations.Fixturesmustbedesignedsuchthatthepressureormotionofthemachiningoperation(usuallyknownasthefeed)isdirectedprimarilyagainstthesolidcomponentofthefixture.Thisreducesthelikelihoodthatthefixturewillfail,interruptingtheoperationandpotentiallycausingdamagetoinfrastructure,components,oroperators.[5]

Fixturesmayalsobedesignedforverygeneralorsimpleuses.Thesemultiusefixturestendtobeverysimplethemselves,oftenrelyingontheprecisionandingenuityoftheoperator,aswellassurfacesandcomponentsalreadypresentintheworkshop,toprovidethesamebenefitsofaspeciallydesignedfixture.Examplesincludeworkshopvises,adjustableclamps,andimproviseddevicessuchasweightsandfurniture.

Eachcomponentofafixtureisdesignedforoneoftwopurposes:locationorsupport.

Location

Locatingcomponentsensurethegeometricalstabilityoftheworkpiece.Theymakesurethattheworkpiecerestsinthecorrectpositionandorientationfortheoperationbyaddressingandimpedingallthedegreesoffreedomtheworkpiecepossesses.[6]

Forlocatingworkpieces,fixturesemploypins(orbuttons),clamps,andsurfaces.Thesecomponentsensurethattheworkpieceispositionedcorrectly,andremainsinthesamepositionthroughouttheoperation.Surfacesprovidesupportforthepiece,pinsallowforpreciselocationatlowsurfaceareaexpense,and

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clampsallowfortheworkpiecetoberemovedoritspositionadjusted.Locatingpiecestendtobedesignedandbuilttoverytightspecifications.[7]

Support

Indesigningthelocatingpartsofafixture,onlythedirectionofforcesappliedbytheoperationareconsidered,andnottheirmagnitude.Locatingpartstechnicallysupporttheworkpiece,butdonottakeintoaccountthestrengthofforcesappliedbytheprocessandsoareusuallyinadequatetoactuallysecuretheworkpieceduringoperation.Forthispurpose,supportcomponentsareused.[8]

Tosecureworkpiecesandpreventmotionduringoperation,supportcomponentsprimarilyusetwotechniques:positivestopsandfriction.Apositivestopisanyimmovablecomponent(suchasasolidsurfaceorpin)that,byitsplacement,physicallyimpedesthemotionoftheworkpiece.Supportcomponentsaremorelikelytobeadjustablethanlocatingcomponents,andnormallydonotpresstightlyontheworkpieceorprovideabsolutelocation.[9]

Supportcomponentsusuallybearthebruntoftheforcesdeliveredduringtheoperation.Toreducethechancesoffailure,supportcomponentsareusuallynotalsodesignedasclamps.

Typesoffixtures

Fixturesareusuallyclassifiedaccordingtothemachineforwhichtheyweredesigned.Themostcommontwoaremillingfixturesanddrillfixtures.[10]

Millingfixtures

Millingoperationstendtoinvolvelarge,straightcutsthatproducelotsofchipsandinvolvevaryingforce.Locatingandsupportingareasmustusuallybelargeandverysturdyinordertoaccommodatemillingoperationsstrongclampsarealsoarequirement.Duetothevibrationofthemachine,positivestopsarepreferredoverfrictionforsecuringtheworkpiece.Forhighvolumeautomatedprocesses,millingfixturesusuallyinvolvehydraulicorpneumaticclamps.[11][12]

Drillingfixtures

Drillingfixturescoverawiderrangeofdifferentdesignsandproceduresthanmillingfixtures.Thoughworkholdingfordrillsismoreoftenprovidedbyjigs,fixturesarealsousedfordrillingoperations.

Twocommonelementsofdrillingfixturesaretheholeandbushing.Holesareoftendesignedintodrillingfixtures,toallowspaceforthedrillbititselftocontinuethroughtheworkpiecewithoutdamagingthefixtureordrill,ortoguidethedrillbittotheappropriatepointontheworkpiece.Bushingsaresimplebearingsleevesinsertedintotheseholestoprotectthemandguidethedrillbit.[13]

Becausedrillstendtoapplyforceinonlyonedirection,supportcomponentsfordrillingfixturesmaybesimpler.Ifthedrillisalignedpointingdown,thesamesupportcomponentsmaycompensatefortheforcesofboththedrillandgravityatonce.However,thoughmonodirectional,theforceappliedbydrillstendsto

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beconcentratedonaverysmallarea.Drillingfixturesmustbedesignedcarefullytopreventtheworkpiecefrombendingundertheforceofthedrill.[14]

Seealso

Degreesoffreedom(engineering)Kinematiccoupling

Notes

1. ^Colvin&Haas1938,p.1.

2. ^abcHenriksen1973,p.1.

3. ^abcColvin&Haas1938,p.3.

4. ^abColvin&Haas1938,p.4.5. ^Colvin&Haas1938,p.5.6. ^Henriksen1973,p.18.7. ^Henriksen1973,Ch.4.8. ^Henriksen1973,p.19.9. ^Henriksen1973,Ch.12.

10. ^Henriksen1973,p.3.11. ^Henriksen1973,Ch.19.12. ^Colvin&Haas1938,Ch.11.13. ^Colvin&Haas1938,Ch.4.14. ^Colvin&Haas1938,Ch.10.

References

Colvin,FredH.Haas,LucianL.(1938).JigsandFixtures:AReferenceBook.NewYorkandLondon:McGrawHillBookCompany.Henriksen,ErikK.(1973).JigandFixtureDesignManual.NewYork,N.Y.:IndustrialPressInc.

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