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1、专业英语教材机械与土木工程学院Unit1EngineeringMaterialsInthedesignandmanufactureofaproduct,itisessentialthatthematerialandtheprocessbeunderstood.Materialdifferwidelyinphysicalproperties,machinabilitycharacteristics,methodsofforming,andpossibleservicelife.Thedesignersh
2、ouldconsiderthesefactsinselectinganeconomicalmaterialandaprocessthatisbestsuitedtotheproduct.Engineeringmaterialsareoftwobasictypes:metallicornonmetallic.Nonmetallicmaterialsarefurtherclassifiedasorganicorinorganicsubstances.Sincethereisaninfinitenumber
3、ofnonmetallicmaterialsaswellaspureandalloyedmetals,considerablestudyisnecessarytochoosetheappropriateone.Fewindustrialmaterialsexistaselementsinnature.Forexample,thenaturalcompoundsofmetals,suchasoxides,sulfides,orcarbonates,mustundergoaseparatingorrefi
4、ningoperationbeforetheycanbefurtherprocessed・Onceseparated,theymusthaveanatomicstructurethatisstableatordinarytemperaturesoveraprolongedperioclInmetalworking,ironisthemostimportantnaturalelement.Ironhaslittleindustrialuseinitspurestate,butwhencombinedwi
5、thotherelementsintovariousalloysitbecomestheleadingengineeringmetal.Thenonferrousmetals,includingcopper,tin,zinc,nickel,magnesium,aluminum,lead,andothersallplayanimportantpartinoureconomy;eachhasspecificpropertiesanduses.Manufacturingrequirestoolsandmac
6、hinesthatcanproduceeconomicallyandaccurately.Economydependsontheproperselectionofthemachineorprocessthatwillgiveasatisfactoryfinishedproduct,itsoptimumoperation,andmaximumperformanceoflaborandsupportfacilities-Theselectionisinfluencedbythequantityofitem
7、stobeproduced.Usuallythereisonemachinebestsuitedforacertainoutput.Insmall-lotorjobshopmanufacturing,general-purposemachinessuchasthelathe,drillpress,andmillingmachinemayprovetobethebestbecausetheyareadaptable,havelowerinitialcost,requirelessmaintenance,
8、andpossesstheflexibilitytomeetchangingconditions.However,aspecial-purposemachineshouldbeconsideredforlargequantitiesofastandardizedproduct.Amachinebuiltforonetypeofworkoroperation,suchasthegrindingofapistonorthesurfacingofacylind
