机械类英文文献+翻译).pdf

机械工业出版社2004年3月第1版

20.9MACHINABILITY
Themachinabilityofamaterialusuallydefinedintermsoffourfactors:
1、Surfacefinishandintegrityofthemachinedpart;
2、Toollifeobtained;
3、Forceandpowerrequirements;
4、Chipcontrol.
Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlow
forceAndpowerrequirements.Asforchipcontrol,longandthin(stringy)curedchips,
ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecoming
entangledinthecuttingzone.
Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablish
relationshipsthatquantitativelydefinethemachinabilityofamaterial.In
manufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobe
themostimportantfactorsinmachinability.Althoughnotusedmuchanymore,
approximatemachinabilityratingsareavailableintheexamplebelow.
20.9.1MachinabilityOfSteels
Becausesteelsareamongthemostimportantengineeringmaterials(asnotedin
Chapter5),theirmachinabilityhasbeenstudiedextensively.Themachinabilityof
steelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainso-called
free-machiningsteels.
ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganese
sulfideinclusions(second-phaseparticles),whichactasstressraisersintheprimary
shearzone.Asaresult,thechipsproducedbreakupeasilyandaresmall;this
improvesmachinability.Thesize,shape,distribution,andconcentrationofthese
inclusionssignificantlyinfluencemachinability.Elementssuchastelluriumand
selenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersin
resulfurizedsteels.
Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causing
increasedhardness.Hardersteelsresultinbetterchipformationandsurfacefinish.
Notethatsoftsteelscanbedifficulttomachine,withbuilt-upedgeformationand
poorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformation
ofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.
LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipof
manganesesulfideinclusions.Innon-resulfurizedgradesofsteel,leadtakesthe
for