新華時評:決戰貧困,雙手創造美好明天

nGXo4-news 发布时间: 2020-07-10 11:55:10

“正 规 分 分 彩app”InregardtotheuseoftheTsquareandsetsquares,nousefulrulescanbegivenexcepttoobserveothers,andexperimentuntilconvenientcustomsareattained.Abeginnershouldbecarefulofadoptingunusualplans,andaboveallthings,ofmakingimportantdiscoveriesastonewplansofusinginstruments,assumingthatcommonpracticeisallwrong,andthatitisleftforhimtodevelopthetrueandproperwayofdrawing.Thisisakindofdiscoverywhichisveryapttointrudeitselfatthebeginningofanapprentice'scourseinmanymattersbesidesdrawing,andoftenleadshimtodoandsaymanythingswhichhewillafterwardswishtorecall.

Thefavourablereceptionwhichthearticleson"ShopManipulation"metwithduringtheirserialpublication,andvariousrequestsfortheirreproductionintheformofabook,hasledtothepresentedition.

Inthemanufactureofmachines,thereareusuallysomanysizesandmodifications,thatdrawingsshouldassistanddetermineinalargedegreethecompletenessofclassificationandrecord.Takingthemanufactureofmachinetools,forexample:wecannotwellsay,eachtimetheyaretobespokenof,athirty-sixinchlathewithoutscrewandgearing,athirty-twoinchlathewithscrewandgearing,aforty-inchlathetriplegearedordoublegeared,withatwentyorthirtyfootframe,andsoon.Toavoidthisitisnecessarytoassumesymbolsformachinesofdifferentclasses,consistinggenerallyofthelettersofthealphabet,qualifiedbyasinglenumberasanexponenttodesignatecapacityordifferentmodifications.Assuming,inthecaseofenginelathes,Atobethesymbolforlathesofallsizes,thenthoseofdifferentcapacityandmodificationcanberepresentedinthedrawingsandrecordsasA1,A2,A3,A4,andsoon,requiringbuttwocharacterstoindicatealatheofanykind.Thesesymbolsshouldbemarkedinlargeplainlettersontheleft-handlowercornerofsheets,sothatthemanager,workman,oranyoneelse,canseeataglancewhatthe[86]drawingsrelateto.Thissymbolshouldrunthroughthetime-book,costaccount,salesrecord,andbethetechnicalnameformachinestowhichitapplies;inthiswaymachineswillalwaysbespokenofintheworksbythenameoftheirsymbol.

PREFACE.

[37]

Referringfirsttothesavingeffectedbycombiningseveraloperationsinonemachine,thereisperhapsnotoneconstructorintwentythateverstopstoconsiderwhatisreallygained,andperhapsnotonepurchaserinahundredthatdoesthesamething.Theimpressionis,thatwhenonemachineperformstwooperationsitsavesasecondmachine.AremarkableexampleofthisexistsinthemanufactureofcombinationmachinesinEuropeforworkingwood,whereitiscommontofindcomplicated[69]machinesthatwillperformalltheoperationsofajoiner'sshop,butasaruleonlyonethingatatime,andusuallyinaninconvenientmanner,eachoperationbeinghamperedandinterferedwithbyanother;andinchangingfromonekindofworktoanothertheadjustmentsandchangesgenerallyequalandsometimesexceedtheworktobedone.Whatisstrangerstillis,thatsuchmachinesarepurchased,whentheircostoftenequalsthatofseparatemachinestoperformthesamework.

Thereis,however,aprincipleinhydraulicmachinerythatmustbetakenintoaccount,incomparingitwithpositivelygearedmechanism,whichoftenleadstolossofpowerthatinmanycaseswilloverbalanceanygainderivedfromthepeculiaractionofhydraulicapparatus.Ialludetothelossofpowerincidenttodealingwithaninelasticmedium,wheretheamountofforceexpendedisconstant,regardlessoftheresistanceoffered.Ahydrauliccrane,forinstance,consumespowerinproportiontoitsmovements,andnotastheamountofdutyperformed;ittakesthesamequantityofwatertofillthecylindersofsuchcranes,whetherthewaterexertmuchorlittleforceinmovingthepistons.Thedifferencebetweenemployingelasticmediumslikeairandsteam,andaninelasticmediumlikewater,fortransmittingforceinperformingirregularduty,hasbeenalreadyalludedto,andformsaveryinterestingstudyforastudentinmechanics,leading,asitdoes,tothesolutionofmanyproblemsconcerningtheuseandeffectofpower.

Movementsandstrainsmaybecalledtwooftheleadingconditionsuponwhichdesignsformachinesarebased:movementsdeterminegeneraldimensions,andstrainsdeterminetheproportionsandsizesofparticularparts.Movementandstraintogetherdeterminethenatureandareaofbearingsorbearingsurfaces.

Nextinorderarestrains.Asthecuttingactionisthesourceofstrains,andastheresistanceofferedbythecuttingtoolsisasthelengthorwidthoftheedges,itwillbefoundinthepresentcasethatwhileotherconditionsthusfarhavepointedtosmallproportions,thereisnowanewonewhichcallsforlargeproportions.Indisplacingthemetalbetweenteethofthree-quartersofaninchpitch,thecuttingedgeortheamountofsurfaceacteduponisequaltoawidthofoneinchandahalf.Itistrue,thedisplacementmaybesmallateachcut,butthestrainisrathertobebaseduponthebreadthoftheactingedgethantheactualdisplacementofmetal,andwefindherestrainsequaltotheaveragedutyofalargeplaningmachine.Thisstrainradiatesfromthecuttingpointasfromacentre,fallingonthesupportsoftheworkwithatendencytoforceitfromtheframing.Betweentherackandthecrank-shaftbearing,throughthemediumofthetool,cutterbar,connection,andcrankpin,andinvariousdirectionsanddegrees,thisstrainmaybefollowedbymeansofasimplediagram.Besidesthiscuttingstrain,therearenoneofimportance;thetensionofthebelt,thesidethrustinbearings,thestrainfromtheangularthrustofthecrank,andtheendthrustofthetool,althoughnottobelostsightof,neednothavemuchtodowithproblemsofstrength,proportion,andarrangement.

Mechanicalengineering,likeeveryotherbusinesspursuit,isdirectedtotheaccumulationofwealth;andastheattainmentofanypurposeismoresurelyachievedbykeepingthatpurposecontinuallyinview,therewillbenoharm,andperhapsconsiderablegainderivedbyanapprenticeconsideringatthebeginningthemainobjecttowhichhiseffortswillbedirectedafterlearninghisprofessionortrade.Sofarasanabstractprincipleofmotives,thesubjectisofcourseunfittoconsiderinconnectionwithengineeringoperations,orshopmanipulation;butbusinessobjectshaveapracticalapplicationtobefollowedthroughoutthewholesystemofindustrialpursuits,andareaspropertobeconsideredinconnectionwithmachine-manufacturingasmechanicalprinciples,orthefunctionsandoperationofmachines.

Thereadermaynoticehoweverythingpertainingtopatternsandmouldingresolvesitselfintoamatterofjudgmentonthepartofworkmen,andhowdifficultitwouldbetoapplygeneralrules.

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网友评论

  • 雨花郡主gGDuT

    Thegeneralprinciplesofhammer-action,sofarasalreadyexplained,applyaswelltohammersoperatedbydirectsteam;andalearner,informingaconceptionofsteam-hammers,mustnotfallintothecommonerrorofregardingthemasmachinesdistinctfromotherhammers,orasoperatinguponnewprinciples.Asteam-hammerisnothingmorethanthecommonhammerdrivenbyanewmedium,ahammerreceivingpowerthroughtheagencyofsteaminsteadofbelts,shafts,andcranks.Thesteam-hammerinitsmostimprovedformissoperfectlyadaptedtofillthedifferentconditionsrequiredinpower-hammering,thatthereseemsnothinglefttobedesired.

     

  • 雪灵芝KpZe3

  • 如萱紫儿mQ9cR

  • 但法国BBuK9

     Itmaybeassumedthatthesameconditionsapplytothestandardsofacommonplaningmachine,butthecaseisdifferent;theuprightframingiseasilymadestrongenoughbyincreasingitsdepth;butthestrainuponrunningjointsisasthedistancefromthematwhichaforceisapplied,ortoemployatechnicalphrase,astheamountofoverhang.Withamovingplatenthelargerandheavierapiecetobeplaned,themore[132]firmlyaplatenishelddown;andasthecrosssectionofpiecesusuallyincreaseswiththeirdepth,theresultisthataplaningmachineproperlyconstructedwillactnearlyaswellonthickasthinpieces.

  • 大笨蛋Afsr3

    v

  • 扒皮成KE96C

     

  • 草莓果uagtZ

    Forotherkindsofworktherearedrawingsfurnished,andtheplansaredictatedbytheengineeringdepartmentofmachinery-buildingestablishments,butpattern-makersmaketheirownplansforconstructingtheirwork,andhaveeventoreproducethedrawingsofthefitting-shoptoworkfrom.Nearlyeverythingpertainingtopatternsislefttobedecidedbythepattern-maker,who,fromthesamedrawings,andthroughtheexerciseofhisjudgmentalone,maymakepatternsthataredurableandexpensive,ortemporaryandcheap,astheprobableextentoftheirusemaydetermine.

     Thecontentsofthepresentwork,excepttheIntroductionandthechapteronGauges,consistmainlyinarevisionofaseriesofarticlespublishedin"Engineering"andtheJournaloftheFranklinInstitute,undertheheadof"ThePrinciplesofShopManipulation,"during1873and1874.

  • 绿卡纸lHymo

    Judgedupontheoreticalgrounds,andleavingoutthemechanicalconditionsofoperation,itwouldatoncebeconcededthataproperplanwouldbetomovethelightestbody;thatis,ifthetoolsandtheirattachmentswereheavierthanthematerialtobeactedupon,thenthematerialshouldbemovedforthecuttingaction,andviceversa.Butinpracticethereareotherconditionstobeconsideredmoreimportantthanaquestionoftherelativeweightofreciprocatingparts;anditmustberememberedthatinsolvinganyproblempertainingtomachineaction,theconditionsofoperationaretobeconsideredfirstandhaveprecedenceoverproblemsofstrain,arrangement,oreventhegeneralprinciplesofconstruction;thatis,theconditionsofoperatingmustformabasefromwhichproportions,arrangements,andsoon,mustbededuced.Astandardplaningmachine,suchasisemployedformostkindsofwork,isarrangedwitharunningplatenorcarriageuponwhichthematerialisfastenedandtraversedbeneaththecuttingtools.[130]Theuniformityofarrangementanddesigninmachinesofthiskindinallcountrieswherevertheyaremade,mustleadtotheconclusionthattherearesubstantialreasonsforemployingrunningplatensinsteadofgivingacuttingmovementtothetools.

     Itisnotalwaysaneasymattertohituponanobjecttowhichdesignsmaybedirected;andalthoughatfirstthoughtitmayseemthatanymachine,orpartofamachine,iscapableofimprovement,itwillbefoundnoeasymattertodetectexisting[153]faultsortoconceiveplansfortheirremedy.

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