第四届中国媒体经营管理论坛

OCfPj-news 发布时间: 2020-07-12 03:24:54

“幸运分分彩计划网”Machinesandtoolsoperatingbypercussiveaction,althoughtheycompriseanumerousclass,andareappliedinnearlyallmechanicaloperations,haveneverreceivedthatamountofattentionintext-bookswhichtheimportanceofthemachinesandtheirextensiveusecallsfor.Suchmachineshavenotevenbeensetoffasaclassandtreatedofseparately,althoughthedistinctionisquiteclearbetweenmachineswithpercussiveaction,andthosewithwhatmaybetermeddirectaction,bothinthemannerofoperatingandinthegeneralplansofconstruction.Thereis,ofcourse,nolackofformul?fordeterminingthemeasureofforce,andcomputingthedynamiceffectofpercussivemachinesactingagainstameasuredorassumedresistance,andsoon;butthisisnotwhatismeant.Therearecertainconditionsintheoperationofmachines,suchasthestrainswhichfalluponsupportingframes,theeffectproduceduponmalleablematerialwhenstruckorpressed,andmoreespeciallyofconditionswhichmayrenderpercussiveorpositiveactingmachinesapplicabletocertainpurposes;butlittleexplanationhasbeengivenwhichisofvaluetopracticalmen.

(1.)Howmaygaugingimplementsaffectthedivisionoflabour?—(2.)Inwhatwaydostandarddimensionsaffectthevalueofmachinery?—(3.)Whycannotcylindricaljointsbefittedbytryingthemtogether?—(4.)Underwhatcircumstancesisitmostimportantthatthepartsofmachineryshouldhavestandarddimensions?—(5.)Whichsenseismostacuteintestingaccuratedimensions?—(6.)Howmayslightvariationsindimensionsbemadeapparenttosight?

Inregardtoshadedelevations,itmaybesaidthatphotographyhassupersededthemforthepurposeofillustratingcompletedmachines,andbutfewestablishmentscaretoincurtheexpenseofink-shadedelevations.Shadedelevationscannotbemadewithvariousdegreesofcare,andinalongerorshortertime;thereisbutonestandardforthem,andthatisthatsuchdrawingsshouldbemadewithgreatcareandskill.Imperfectshadedelevations,althoughtheymaysurpriseandpleasetheunskilled,areexecrableintheeyesofadraughtsmanoranengineer;andasthemakingofshadedelevationscanbeofbutlittleassistancetoanapprenticedraughtsman,itisbettertosavethetimethatmustbespentinordertomakesuchdrawings,andapplythesamestudyandtimetoothermattersofgreaterimportance.Thefirstand,perhaps,themostimportantmatterofallinfoundingengineeringworksisthatofarrangement.Asacommercialconsiderationaffectingthecostofmanipulation,andtheexpenseofhandlingmaterial,thearrangementofanestablishmentmaydetermine,inalargedegree,theprofitsthatmaybeearned,and,asexplainedinapreviousplace,uponthismatterofprofitsdependsthesuccessofsuchworks.

Questionsandanswersconstitutetheprincipalmediumforacquiringtechnicalinformation,andengineeringapprenticesshouldcarefullystudythephilosophyofquestionsandanswers,justashedoestheprinciplesofmachinery.Withouttheartofquestioningbutslowprogresswillbemadeinlearningshopmanipulation.Aproperquestionisonewhichthepersonaskedwillunderstand,andtheanswerbeunderstoodwhenitisgiven;notaneasyrule,butacorrectone.Themainpointistoconsiderquestionsbeforetheyareasked;makethemrelevanttotheworkinhand,andnottoomany.Toaskfrequentquestions,istoconveyanimpressionthattheanswersarenotconsidered,aninferencewhichiscertainlyafairone,ifthequestionsrelatetoasubjectdemandingsomeconsideration.Ifamanisaskedoneminutewhatdiametricalpitchmeans,andthenextminutehowmuchcastironshrinksincooling,heisveryapttobedisgusted,andthinkthesecondquestionnotworthanswering.

Fourth.—Thereisnodirectconnectionbetweenthemovingpartsofthehammerandtheframing,exceptlateralguidesforthehammer-head;thesteambeinginterposedasacushioninthelineofmotion,thisreducestherequiredstrengthandweightoftheframingtoaminimum,andavoidspositivestrainsandconcussion.Keepingthisinview,andreferringtoanordinaryplaningmachinewithwhichthereaderispresumedtobefamiliar,thefocalpointofthecuttingstrainisattheedgeofthetools,andradiatesfromthispointasfromacentretothevariouspartsofthemachineframe,andthroughthejointsfixedandmovablebetweenthetoolsandtheframe;tofollowbackfromthiscuttingpointthroughthemechanismtotheframeproper;firststartingwiththetoolanditssupportsandgoingtothemainframe;thenstartingfromthematerialtobeplaned,andfollowingbackintheotherdirection,untilwereachthepointwherethestrainsareabsorbedbythemainframe,examiningthejointswhichinterveneinthetwocases,therewillappearsomereasonsforrunningcarriages.

Anotherpeculiarfeatureintrip-hammersistherapiditywithwhichcrystallisationtakesplaceintheattachmentsforholdingthedieblockstothehelves,wherenoelasticmediumcanbeinterposedtobreaktheconcussionofthedies.Boltstopassthroughthehelve,althoughmadefromthemostfibrousSwedishiron,willonsomekindsofworknotlastformorethantendays'use,andoftenbreakinasingleday.Thesafestmodeofattachingdieblocks,andtheonemostcommon,istoforgethemsolid,withaneyeorabandtosurroundtheendofthehelve.

Uponenteringtheshop,alearnerwillgenerally,touseashopphrase,"beintroducedtoahammerandchisel;"hewill,perhaps,regardthesehandtoolswithakindofcontempt.Seeingotheroperationscarriedonbypower,andthemachinesinchargeofskilledmen,heislikelytoesteemchippingandfilingasofbutlittleimportanceandmainlyintendedforkeepingapprenticesemployed.Butlongafter,whenascoreofyearshasbeenaddedtohisexperience,thehammer,chisel,andfile,willremainthemostcrucialtestofhishandskill,andafterlearningtomanipulatepowertoolsofallkindsinthemostthoroughmanner,afewblowswithachippinghammer,orahalf-dozenstrokeswithafile,willnotonlybeamoredifficulttestofskill,butonemostlikelytobemetwith.

CHAPTERIII.ENGINEERINGASACALLING.

Steam-engines.

Irregularityofcoolingmaybetheresultofunequalconductingpowerindifferentpartsofamouldorcores,oritmaybe[98]fromthevaryingdimensionsofthecastings,whichcontainheatastheirthickness,andgiveitoffinthesameratio.Asarule,thedragorbottomsideofacastingcoolsfirst,especiallyifamouldrestsontheground,andthereisnotmuchsandbetweenthecastingsandtheearth;thisisacommoncauseofunequalcooling,especiallyinlargeflatpieces.Airbeingabadconductorofheat,andthesandusuallythinonthecopeortopside,theresultisthatthetopofmouldsremainquitehot,whileatthebottomtheearth,beingagoodconductor,carriesofftheheatandcoolsthatsidefirst,sothattheiron'sets'firstonthebottom,afterwardscoolingandcontractingonthetop,sothatcastingsarewarpedandleftwithinherentstrains.

Thenameofhavingbeenanapprenticetoafamousfirmmaysometimeshaveaninfluenceinenablinganengineertoformadvantageouscommercialconnections,butgenerallyanapprenticeshipisofvalueonlyasithasfurnishedsubstantialknowledgeandskill;foreveryonemustsoonerorlatercomedowntothesolidbasisoftheiractualabilitiesandacquirements.Theengineeringinterestisbyfartoopracticaltorecogniseashadowinsteadoftruesubstance,andthereisbutlittlechanceofdeceptioninacallingwhichdealsmainlywithfacts,figures,andpositivedemonstration.

J.R.

Iamnotawarethatthederivationofourstandardmeasureshasbeen,inanhistoricalway,astheforegoingremarkswillindicate,norisitthepurposeheretofollowsuchhistory.Areader,whoseattentionisdirectedtothesubject,willfindnotroubleintracingthematterfromothersources.Thepresentobjectistoshowwhatawonderfulseriesofconnectionscanbetracedfromsosimpleatoolasameasuringgauge,andhowabstruse,infact,aremanyapparentlysimplethings,oftenregardedasnotworthathoughtbeyondtheirpracticalapplication.

文章推荐:

脸书5000万用户信息遭窃 创始人扎克伯格"道歉"

“责任中国”人民网2012年度评选揭晓仪式:十大责任公民获奖者龚大锬

阿尼帕·阿力马洪(维吾尔族):让民族团结代代相传

要不要给美欧关系准备更多“呼吸机”

后世博的中国影响:人才、传播与合作论坛

当心!8类药会升高血糖,血糖超13.9别运动

小米赴港IPO之前 手机出货量已居全球第四

网友评论

  • 雨花郡主5MNGQ

     

  • 雪灵芝e9lM0

  • 如萱紫儿5oZmE

    Alearnerwhogoesfromgearingandshaftstosteamandhydraulics,frommachinetoolstocranesandhoistingmachinery,willnotaccomplishmuch.Thebestwayistoselectatfirstaneasysubject,onethatadmitsofagreatrangeofmodification,andifpossible,onethathasnotassumedastandardformofconstruction.Bearingsandsupportsforshaftsandspindles,[158]isagoodsubjecttobeginwith.

  • 但法国jgPmQ

    Tomakethisplainer,thelawsofforces,theproportionofparts,strengthofmaterial,andsoon,aresubjectsofgeneralknowledgethatmaybeacquiredfrombooks,andunderstoodwithouttheaidofanacquaintancewiththetechnicalconditionsofeitherthemodeofconstructingorthemannerofoperatingmachines;buthowtoconstructproperpatternsforcastings,orhowthepartsofmachineryshouldbemoulded,forged,orfitted,isspecialknowledge,andmusthavereferencetoparticularcases.Theproportionsofpulleys,bearings,screws,orotherregulardetailsofmachinery,maybelearnedfromgeneralrulesandprinciples,butthehandskillthatentersintothemanufactureofthesearticlescannotbelearnedexceptbyobservationandexperience.Thegeneraldesign,orthedispositionofmetalinmachine-framing,canbetoagreatextentfoundeduponrulesandconstantsthathavegeneralapplication;but,asinthecaseofwheels,theplansofmouldingsuchmachineframesarenotgovernedbyconstantrulesorperformedinauniformmanner.Patternsofdifferentkindsmaybeemployed;mouldsmaybemadeinvariousways,andatagreaterandlessexpense;themetalcanbemixedtoproduceahardorasoftcasting,astrongoraweakone;theconditionsunderwhichthemetalispouredmaygovernthesoundnessorshrinkage,—thingsthataredeterminedbyspecialinsteadofgeneralconditions.

     

  • 大笨蛋0QIIf

    v

  • 扒皮成6zKUC

     Thefollowingpropositionswillplacethissubjectofaimsandobjectsbeforethereaderinthesenseintended:—

  • 草莓果xCX5o

     Aleadingprincipleinmachineryoftransmissionthatmorethananyotherfurnishesdataforstrengthandproperproportionsis,thatthestressuponthemachinery,whateveritmaybe,isinverseasthespeedatwhichitmoves.Forexample,abelttwoincheswide,movingonethousandfeetaminute,willtheoreticallyperformthesameworkthatonetenincheswidewilldo,movingataspeedoftwohundredfeetaminute;orashaftmakingtwohundredrevolutionsaminutewilltransmitfourtimesasmuchpowerasashaftmakingbutfiftyrevolutionsinthesametime,thetorsionalstrainbeingthesameinbothcases.

  • 绿卡纸6hb9I

    Thisarticlehasbeenintroduced,notonlytogiveatrueunderstandingoftheeffectandvalueofmachinecombination,buttocautionagainstacommonerrorofconfoundingmachinecombinationwithinvention.

     Forahalfcenturypastnoconsiderablechangehasbeenmadeinenginelathes,atleastnonewprincipleofoperationhasbeenadded,butmanyimprovementshavebeenmadeintheiradaptationandcapacityforspecialkindsofwork.Improvementshavebeenmadeinthefacilitiesforchangingwheelsinscrewcuttingandfeeding,byfrictionalstartinggearforthecarriages,anindependentfeedmovementforturning,arrangementstoadjusttools,crossfeedingandsoon,addingsomething,nodoubt,totheefficiencyoflathes;buttheimprovementsnamedhavebeenmainlydirectedtosupplantingtheskilloflathemen.

标签列表