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1z9Os-news 发布时间: 2020-07-09 04:12:53

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Tomasterthevariousdetailsofmachinemanufacture,includingdraughting,founding,forging,andfitting,isofitselfaworkequaltomostprofessionalpursuits,tosaynothingofmanualskill;andwhenwecometoaddmachinefunctionsandtheirapplication,generatingandtransmittingpower,withotherthingsthatwillnecessarilybeincludedinpractice,thetaskassumesproportionsthatmakesitappearahopelessone.Besides,theworkofkeepingprogresswiththemechanicartscallsforacontinualaccretionofknowledge;anditisnosmalllabourtokeepinformedofthecontinualchangesandimprovementsthataregoingoninallpartsoftheworld,whichmayatanytimemodifyandchangebothmachinesandprocesses.Butfewmen,evenunderthemostfavourableconditions,havebeenabletoqualifythemselvesascompetentmechanicalengineerssoonerthanatfortyyearsofage.

Thismatterofshrinkageisindeedtheonlyconditioninpattern-makingwhichisgovernedbyanythingnearaconstantrule,andevenshrinkagerequiressometimestobevariedtosuitspecialcases.Forsmallpatternswhosedimensionsdonotexceedonefootinanydirection,rappingwillgenerallymakeupforshrinkage,andnoallowanceisrequiredinthepatterns,butpattern-makersaresopartialtotheruleofshrinkage,astheonlyconstantoneintheirwork,thattheyareaversetoadmittingexceptions,andusuallykeeptotheshrinkruleforallpieces,whetherlargeorsmall.

CHAPTERXV.PNEUMATICMACHINERYFORTRANSMITTINGPOWER.

Theexpenseofpatternsshouldbedividedamongandchargedtothemachinesforwhichthepatternsareemployed,buttherecanbenoconstantrulesforassessingordividingthiscost.Apatternmaybeemployedbutonce,oritmaybeusedforyears;itiscontinuallyliabletobesupersededbychangesandimprovementsthatcannotbepredictedbeforehand;andinpreparingpatterns,thequestioncontinuallyarisesofhowmuchoughttobeexpendedonthem—amatterthatshouldbedeterminedbetweentheengineerandthepattern-maker,butisgenerallylefttothepattern-makeralone,forthereasonthatbutfewmechanicalengineersunderstandpattern-makingsowellastodictateplansofconstruction.

Proceedingfromthesepremises,thefirstthingshouldbetoexaminetheactionofexistingvalvegear,todeterminewherethiswantofthedead-strokefunctioncanbestbesupplied,andtogaintheaidofsuchsuggestionsasexistingmechanismmayoffer,alsotoseehowfartheappliancesinusemaybecomeapartofanynewarrangement.

Millingtoolslargeenoughtoadmitofdetachablecuttersbeingemployed,arenotsoexpensivetomaintainassolidtools.Edgemovementcansometimesbemultipliedinthisway,soastogreatlyexceedwhatasingletoolwillperform.Beginningatthetoolthereis,first,aclampedjointbetweenthetoolandtheswingblock;second,amovablepivotedjointbetweentheblockandshoepiece;third,aclampedjointbetweentheshoepieceandthefrontsaddle;fourth,amovingjoint[131]wherethefrontsaddleisgibedtotheswingorquadrantplate;fifth,aclampjointbetweenthequadrantplateandthemainsaddle;sixth,amovingjointbetweenthemainsaddleandthecrosshead;seventh,aclampjointbetweenthecrossheadandstandards;andeighth,boltedjointsbetweenthestandardsandthemainframe;makinginalleightdistinctjointsbetweenthetoolandtheframeproper,threemoving,fourclamped,andoneboltedjoint.

Nothingcanbemoreunreasonablethantoexpectanapprenticeengineertobeginbyaninductivecourseinlearningandreasoning[3]aboutmechanics.Evenifthemindwerecapableofsuchacourse,whichcannotbeassumedinsointricateandextensiveasubjectasmechanics,therewouldbeawantofinterestandanabsenceofapparentpurposewhichwouldhinderorpreventprogress.Anyrationalviewofthematter,togetherwithasmanyfactsascanbecited,willallpointtotheconclusionthatapprenticesmustlearndeductively,andthatsomepracticeshouldaccompanyorprecedetheoreticalstudies.Howdullandobjectlessitseemstoayoungmanwhenhetoilsthrough"thesumofthesquaresofthebaseandperpendicularofaright-angletriangle,"withoutknowingapurposetowhichthisproblemistobeapplied;hegenerallywonderswhysuchpuzzlingtheoremswereeverinvented,andwhattheycanhavetodowiththepracticalaffairsoflife.Butifthesamelearnerweretohappenuponabuildersquaringafoundationbymeansoftherule"six,eight,andten,"andshouldinthisoperationdetecttheapplicationofthattiresomeproblemof"thesumofthesquares,"hewouldatonceawaketoanewinterestinthematter;whatwasbeforetediousandwithoutobject,wouldnowappearusefulandinteresting.Thesubjectwouldbecomefascinating,andthelearnerwouldgoonwithanewzealtotraceouttheconnectionbetweenpracticeandotherproblemsofthekind.Nothinginspiresalearnersomuchascontactwithpractice;thenaturaltendency,asbeforesaid,istoproceeddeductively.

Inregardtoshadedelevations,itmaybesaidthatphotographyhassupersededthemforthepurposeofillustratingcompletedmachines,andbutfewestablishmentscaretoincurtheexpenseofink-shadedelevations.Shadedelevationscannotbemadewithvariousdegreesofcare,andinalongerorshortertime;thereisbutonestandardforthem,andthatisthatsuchdrawingsshouldbemadewithgreatcareandskill.Imperfectshadedelevations,althoughtheymaysurpriseandpleasetheunskilled,areexecrableintheeyesofadraughtsmanoranengineer;andasthemakingofshadedelevationscanbeofbutlittleassistancetoanapprenticedraughtsman,itisbettertosavethetimethatmustbespentinordertomakesuchdrawings,andapplythesamestudyandtimetoothermattersofgreaterimportance.

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网友评论

  • 雨花郡主nMr8h

    Machinerycanbedividedintofourclasses,eachconstitutingadivisionthatisveryclearlydefinedbyfunctionsperformed,asfollows:—

     3.Twoelevationstakenatrightanglestoeachother,fixallpoints,andgivealldimensionsofpartsthathavetheiraxisparalleltotheplanesonwhichtheviewsaretaken;butwhenamachineiscomplex,orwhenseveralpartslieinthesameplane,threeandsometimesfourviewsarerequiredtodisplayallthepartsinacomprehensivemanner.

  • 雪灵芝MkWM6

    Ifitwerenotforsecuringventforgas,mouldscouldbemadefromplasticmaterialsoastoproducefinecastingswithclearsharpoutlines.

  • 如萱紫儿SeBxQ

    Coresareemployedmainlyforwhatmaybetermedthedisplacementofmetalinmoulds.Thereisnoclearlineofdistinctionbetweencoresandmoulds,asfoundingisnowconducted;coresmaybeofgreensand,andmadetosurroundtheexteriorofapiece,aswellastomakeperforationsortoformrecesseswithinit.Theterm'core,'initstechnicalsense,meansdriedmoulds,asdistinguishedfromgreensand.Wheelsorothercastingsaresaidtobecastincoreswhenthemouldsaremadeinpiecesanddried.Supportingandventingcores,andtheirexpansion,areconditionstowhichespecialattention[95]iscalled.Whenacoreissurroundedwithhotmetal,itgivesoff,becauseofmoistureandtheburningofthe'wash,'alargeamountofgaswhichmusthavefreemeansofescape.Inthearrangementofcores,therefore,attentionmustbehadtosomemeansofventing,whichisgenerallyattainedbyallowingthemtoprojectthroughthesidesofthemouldandcommunicatewiththeairoutside.

  • 但法国ifpdk

    Grinding,nodoubt,iftracedtotheprinciplesthatlieatthebottom,isnothingmorethanacuttingprocess,inwhichtheedgesemployedareharderthananymaterialthatcanbemadeintocutters,theedgesfirmlysupportedbybeingimbeddedintoamassastheparticlesofsandareingrindstones,ortheparticlesofemeryinemerywheels.

     

  • 大笨蛋EI2CL

    v

  • 扒皮成DDiNn

    Seventh.—Draught,thebevelorinclinationonthesidesofpatternstoallowthemtobewithdrawnfromthemouldswithoutdraggingorbreakingthesand.

     

  • 草莓果kRpGj

     CHAPTERXXIV.TRIP-HAMMERS.

  • 绿卡纸J3e3f

     Oneofthepeculiaritiesoftrip-hammersasmachinesisthemechanicaldifficultiesinconnectingthemwiththedrivingpower,especiallyincaseswherethereareanumberofhammerstobedrivenfromoneshaft.

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