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gpphJ-news 发布时间: 2020-07-12 11:18:14

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Anotherdifferencebetweensolidandexpandingdies,whichmaybepointedout,isinthefirmnesswithwhichthecuttingedgesareheld.Withasoliddie,theedgesorteethbeingallcombinedinonesolidpiece,arefirmlyheldinafixedposition;whilewithexpandingdiestheirpositionhastobemaintainedbymechanicaldeviceswhichareliabletoyieldunderthepressurewhicharisesincutting.Theresultis,thattheprecisionwithwhichascrewingmachinewithmovabledieswillact,isdependentuponthestrengthofthe'abutment'behindthedies,whichshouldbeahardunyieldingsurfacewithasmuchareaaspossible.

(1.)Canthedrivingpowerbeemployeddirectlytoshiftthebeltsofaplaningmachine?—(2.)Whyareplaningmachinesgenerallyconstructedwitharunningcarriageinsteadofrunningtools?—(3.)Whatobjectionexistsinemployingatrainofspurwheelstodriveaplaningmachinecarriage?—(4.)Whatisgainedbyshiftingthebeltsofaplaningmachinedifferentially?—(5.)Whatproducesthescreechingofbeltssocommonwithplaningmachines?—(6.)Whatconditionsfavourtheshiftingofplaningmachinebelts?

Thisbringsusbacktotheoldproposition,thatforstructureswhichdonotinvolvemotion,mathematicaldatawillfurnishdimensions;butthesamerulewillnotapplyinmachinery.Tofollowtheproportionsforshaftsthatwouldbefurnishedbypuremathematicaldatawouldinnearlyallcasesleadtoerror.Experiencehasdemonstratedthatforordinarycases,wherepoweristransmittedandappliedwithtolerableregularity,ashaftthreeinchesindiameter,makingonehundredandfiftyrevolutionsaminute,itsbearingsthreetofourdiametersinlength,andplacedtenfeetapart,willsafelytransmitfiftyhorse-power.(1.)Whyareshapingmachinesbetteradaptedthanplaningmachinesforplaningslots,key-ways,andsoon?—(2.)Whatobjectsaregainedbyaquickreturnmotionofthecutterbarofshapingmachines?

Text-books,suchasrelatetoconstruction,consistgenerallyofexamples,drawings,andexplanationsofmachines,gearing,tools,andsoon;suchexamplesareofusetoalearner,nodoubt,butinmostcaseshecanexaminethemachinesthemselves,andonenteringashopisbroughtatonceincontactnotonlywiththemachinesbutalsowiththeiroperation.Examplesanddrawingsrelatetohowmachinesareconstructed,butwhenalearnercomestotheactualoperationofmachines,anewandmoreinterestingproblemisreachedinthereasonswhytheyaresoconstructed.

Marineandrailwayengineeringhaveengrossedthebesttalentintheworld;investigationandresearchhasbeenexpendeduponthesesubjectsinadegreecommensuratewiththeirimportance,anditwouldbehardtosuggestasinglewantinthemanyabletext-booksthathavebeenprepareduponthesubjects.Marineandrailwayengineeringaresciencesthatmay,inasense,beseparatedfromtheordinaryconstructivearts,andstudiedattheendofacourseinmechanicalengineering,butarehardly[61]propersubjectsforanapprenticetotakeupatthebeginning.

Foundingandcastingrelatetoformingpartsofmachinerybypouringmeltedmetalintomoulds,theforceofgravityalonebeingsufficienttopressorshapeitintoevencomplicatedforms.Asaprocessforshapingsuchmetalasisnotinjuredbythehighdegreeofheatrequiredinmelting,mouldingisthecheapestandmostexpeditiousofallmeans,evenforformsofregularoutline,whiletheimportanceofmouldinginproducingirregularformsissuchthatwithoutthisprocessthewholesystemofmachineconstructionwouldhavetobechanged.Foundingoperationsaredividedintotwoclasses,knowntechnicallyasgreensandmoulding,andloamordrysandmoulding;thefirst,whenpatternsorduplicatesareusedtoformthemoulds,andthesecond,whenthemouldsarebuiltbyhandwithouttheaidofcompletepatterns.Foundinginvolvesaknowledgeofmixingandmeltingmetalssuchasareusedinmachineconstruction,thepreparingandsettingofcoresfortheinternaldisplacementofthemetal,coolingandshrinkingstrains,chills,andmanyotherthingsthataremoreorlessspecial,andcanonlybelearnedandunderstoodfromactualobservationandpractice.

Inpractice,thetruthofthispropositionisfullydemonstratedbytheexcessinthenumberoflathesandboringtoolscomparedwiththoseforplaning.

Thevalvegearingofautomaticsteam-hammerstofillthetwoconditionsofallowingadeadoranelasticblow,furnishesoneofthemostinterestingexamplesofmechanicalcombination.

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网友评论

  • 雨花郡主aeksi

     1.Aconceptionofcertainfunctionsinamachine,andsomedefiniteobjectwhichitistoaccomplish.

  • 雪灵芝9jwi7

  • 如萱紫儿CPdt3

    Inmakingdesignsitisbesttoemploynoreferencesexceptsuchasarecarriedinthememory.Themorefamiliarapersoniswithmachineryofanyclass,themoreablehemaybetopreparedesigns,butnotbymeasuringandreferringtootherpeople'splans.Dimensionsandarrangementfromexamplesare,bysuchacourse,unconsciouslycarriedintoanewdrawing,evenbythemostskilled;besides,itisbynomeansadignifiedmattertocollectotherpeople'splans,andbyalittlecombinationandmodificationproducenewdesigns.Itmaybeaneasyplantoacquireacertainkindofproficiency,butwillmostcertainlyhinderanengineerfromeverrisingtothedignityofanoriginaldesigner.

  • 但法国THX0T

     

  • 大笨蛋VTIhP

    v

  • 扒皮成acYJG

    Directactionandreactionareequal;ifaforceisexertedinanydirectiontheremustbeanequalforceactingintheoppositedirection;amachinemustabsorbitsownstrains.

     2.Drawingsintrueelevationorinsectionarebaseduponflatplanes,andgivedimensionsparalleltotheplanesinwhichtheviewsaretaken.

  • 草莓果ZJiS0

    Thefirstoftheseoperationsincludesallthreadingprocessesperformedonlathes,whetherwithasingletool,bydiescarriedpositivelybysliderests,orbymilling.

     

  • 绿卡纸QV1PI

    Anengineermaydirectandcontrolworkwithoutaknowledgeofpracticalfitting,butsuchcontrolismerelyacommercialone,andcannotofcourseextendtomechanicaldetailswhicharegenerallythevitalpart;theobediencethatmaythusbeenforcedincontrollingothersisnottobeconfoundedwiththerespectwhichasuperiorknowledgeofworkcommands.

     Thesteam-engineisthemostimportant,andinEnglandandAmericabestknownamongmotiveagents.Theimportanceofsteamcontrastedwithothersourcesofmotive-powerisduenotsomuchtoadiminishedcostofpowerobtainedinthisway,butforthereasonthattheamountofpowerproducedcanbedeterminedatwill,andinmostcaseswithoutreferencetolocalconditions;themachinerycanwithfuelandwaterbetransportedfromplacetoplace,asinthecaseoflocomotiveswhichnotonlysupplypowerfortheirowntransit,butmovebesidesvastloadsofmerchandise,ortravel.

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