床身支承.docx
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床身支承.docx
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床身支承
ChapterIV.LatheDesign:
TheBedAndItsSupports
Thedesigneroflathes.Themanufacturer'sviewofalathe.Thepropermedium.Causeoffailure.Thevisionarydesigner.Conscientiouseffortstoimproveindesign.Designofthelathebed.Elementarydesign.ProfessorSweet'sobservations.Theparabolicformoflathebeds.Theauthor'sdesign.Formofthetracks.Bedoftheoldchainlathe.TheEnglishmethodofstatinglathecapacity.Methodofincreasingtheswingofthelathe.TheLodge&Shipleylathebed.Uniformthicknessofmetalinbeds.Idealformofbed.Cross-ties,orbars.FourVs.Flatsurfaces.Lathebedsupports.Heightoflathecenters.Woodenlegsforlathebeds.Anearlyformofbraced,castironlegs.Cabinetsorcupboardbases.Oldstylecastironlegsstillinuse.Formofcabinets.Principlesofthedesignofcabinets.Cabinetsforsmalllathes.TheLodge&Shipleycabinet.TheHendey-Nortoncabinet.
Totheexperiencedandconscientiousdesignerofmachinetoolstheconditionisfrequentlyforceduponhimthatitisofteneasier,andusuallyfarmoreagreeable,todesignmachinesashereallybelievestheyshouldbe,thantodesignsuchmachinesaswillmeetthepopularrequirementsofthemarket.Hemaybesurethatacertainplanwouldmakereallyabetterandmoreefficientmachine,yethemust,fromtheoutset,considerthekindofamachinethecustomerswantandwillbuyandpayfor,sincetheyare,ashasbeenoftensaid,"thecourtoffinalresort"inthematter,andmachine-toolbuildersmanufacturemachinestosell,andnotforthepurposeofexploitingindividualopinions,howevergoodtheymaybe,orthefadsandfanciesofdraftsmenwhoaresometimesimbuedwithvisionaryandimpracticalideas.
Themanufacturerhimselfmaybeperfectlysurethatthemachinesheisturningoutarenotthebestadaptedforthepurposesforwhichtheyareused,orthebesthecouldbuildforthemoney.
Hemaysofarhavethecourageofhisconvictionsastobuildforhisownusemachinesquitedifferentfromthosehemanufacturesforhiscustomers.Yetforsalehemustbuildwhathiscustomerswantwithsmallregardforhisownpersonalopinions.
Bythisitisnotmeantthatthebuilderdoesnotusehisjudgmentinamechanicalway,orthathedoesnotendeavortobuildthebestmachinespossible,inasmuchashedoesgivethisveryquestionmuchtime,attention,andstudy.Yethemust,fromtheverynatureofthecase,alwayskeepinmindthequestion,"Whatwillthecustomersthinkofthisnewmachine?
""Willthisdevicebeasuccess,orwillitproveafailure?
"Somemachinesthathavebeenputonthemarketwithfeelingsofmuchtrepidationhaveprovengreatmoney-makers,whileothermachinespossessingmuchmechanicalexcellencehavefallenflatandalargemajorityofthecustomersrefusedtoendorsethem.Theauthorhasseenmanysuchcases,andthishasprobablybeentheexperienceofeverymanwhohasdesignedandbuiltmachinetools.
Thepropermediuminthematterseemstobetokeepascloselyintouchaspossiblewiththepurchasersofmachinery;toascertaintheirneedsandpreferencesascloselyasmaybe;toanticipatetheirwantswhenpossible,butatthesametimewithconservatism;andtoavoidputtingentirelynewdevicesonthemarketuntiltheyhavebeenthoroughlytestedinthehomeshopandbyafewfriendlyshopsoutsideofit.Andbyentirelynewdevicesismeantsubstantiallynewandcompletemachines,asthebuilderwillfrequentlyhavepartsof,orattachmentsto,theregularlineofmachinesthataremadetotheorderofaparticularcustomerandthathefeelsperfectlysureofbeingwellsuitedtotheworkthatitisexpectedtoperform;yetinthesecasesconsiderablecautionisnecessary.
Theonefruitfulsourceofdifficulty,disappointment,andfailuretobemostavoidedintheproductionofnewdevicesisthemaniaoftenmanifestedbydesignerstoproducesomethingabsolutelynew,decidedlynovel,thelikeofwhichnoonehaseverseenordreamedof,andthatwillstartlethemechanicalworld,revolutionizethebusiness,andproveitsauthoraveritableNapoleonofmechanicalscience.
Whenconfrontedwithsuchamanorsuchacondition,thewisercoursewillbetoabandonsuchambitiousattemptstoeclipseallpreviousefforts,getdownoutoftheclouds,designsomethingofpracticalutility,evenifitisnotstrikinglynew;somethingthatpastexperiencegivessomeguaranteeofsuccess;somethingthatwillsurelybringtheproperfinancialreturnandbeacredittotheshop.Itisalwayswelltoremember,whentemptedtogooffonatangentaftersomethingnewandmarvelous,that"agoodadaptationisbetterthanapoororiginal,"andthatwhenSolomonsaidthat"thereisnonewthingunderthesun,"hedidnotcomefarfromthetruth,sincemanyofthethingswethinkarenewmaybefoundinalmosttheidenticalform,thathavebeeninvented,used,anddiscardedyearsago,astherecordsofmanymechanicallibrariesaswellastheUnitedStatesPatentOfficewillfurnishabundantevidence.
Bytheforegoingremarksitisnotintendedtodiscourageoriginality,originalthoughtandresearch,ortheproperambitiontoimprovement,forweoftenproducemoreofrealvaluebytheefforttoevolvemechanicalimprovementsthanbythedesignofentirelynewmachines,andthestudiousandobservingdesignerwillalwaysbeonthealerttodeviseimprovementsuponexistingformsandprocesses.
Theseobservationsandsuggestionsapplywithasmuchforceintheeffortstoimprovethelatheasanyothermachineincommonuse.Beingtheoldestmachineinthemachineshopdoesnotinanyrespectlimitthefieldforimprovementsinit.Neitherdoesitprecludethedesignofentirelynewmachinesthatmayhave,perhaps,verylittleofthecharacteristicsofalathe,althoughwemustnecessarilybeconfinedtotheessentialsheretoforediscussed,namely,abed,uponwhichrestthehead-stock,tail-stock,andcarriage,ortheirequivalents,ifwewouldclaimthatourmachineisalathe.
LatheDesign:
TheBedAndItsSupports.Part2
Withthesepreliminarystatementsrelativetotheconditionsandrequirementsofgoodandsuccessfuldesigning,wemaytakeupthedesigningoflathessomewhatindetailandinquireintothedesignoftheindividualpartsandgroupsofparts,givingsomeoftheideasofmenprominentinthisfield,andaddingsuchcommentsandsuggestionsasseemproperandpertinenttothecaseasthematterisproceededwith.
Incarryingoutthisplanitwillbenaturaltocommencewiththebed,consideringitsuseandpurpose,andtheproperformtofulfiltherequirementsofthisparticularpart.
Thelathebed,consideredinanelementaryway,andinthecaseofalatheofmoderatelength,maybetakenasabeam,supportedateachendandinitsturnsupportingatoneendthehead-stock,attheotherendthetail-stock,andinthecenterthecarriage,asrepresentedinFig.25.
Fig.25.-ElementaryFormofLatheBed.
Thisbeingtheproblem,andasthehead-stockandthetail-stockstanddirectlyoverthelegsorsupports,wemightconsidertheproblemasthatofabeamloadedatthecenter,whichwouldnaturallysuggestthattheundersideofthebedinsteadofbeingstraightshouldbeaparaboliccurve.ThiswouldresultintheformshowninFig.26,whichwould,ifthecarriagewasstationary,conformtotheconditionsoftheproblem.But,whilethecarriageisnotstationary,itislocatedatwhatwouldnormallybetheweakestpointalongthelengthofthebed,namely,apointfarthestfromeithersupport.Sofartheparaboliccurve,then,iscorrect.Butwhilewehavebeenplacingoursupportsattheextremeendofthebedwehavenoconditionofthecasewhichmakesitincumbentuponustodoso.Inotherwords,wemayaddaportiontoeachendofthebed,outsideof,orbeyondthelineofthesesupports,intheformshowninFig.27,showingamodifiedformofthelathebed,theextensionsateachendbeingintheformofabeamsupportedatoneend.Theoretically,then,thiswouldseemtobetheproperformofalathebedinorderthatitmightconformtothenecessaryrequirementsastoformanditsabilitytosustaintheusualweightsandstrainstowhichitwillbesubjected,andatthesametimenotbeofexcessiveweight,whichwouldentailunnecessaryexpense.
Fig.26.-ParabolicFormofLatheBed.
ThisissubstantiallytheviewtakenbyProf.JohnE.Sweetinreferencetomachinebeds.Hesays:
"Noreasoningcanmakeitoutthattheplaceforthesupportofanordinarysizedlathebedatthetail-stockendofthelatheisattheend.Ifplacedaconsiderabledistancefromtheend,andthetail-stockisattheend,itisbettersupportedthanwheninthemiddleofthepresentstyleoflathesandalsobettersupportedatallotherpoints.Atthehead-stockenditisquiteadifferentmatterasthehead-stockisalwaysfixedandisusuallyheavierloaded,exclusiveofitsowngreaterweight.Wherethehead-stockendsupportisacloset,thereisnowaytomakeitlookrightexcepttohavetheclosetthesamewidthasthehead-stockislong.
Fig.27.-ModifiedParabolicFormofLatheBed.
"Inthecaseofaplaningmachinebedupto12or15feetinlengththereisnoreasonforhavingthreepairsofsupports.Unlessthefoundationisabsolutelyunyielding-athingthatismorerarethantheotherkind-thethreeormorepairsofsupportsareespeciallybad,andtoattempttoholdthefoundationtruewithafrailplanerbedisfoolish.ThedistancebetweenthesupportsinFig.28isnogreaterthanin29,andasinnocasewouldthecenteroftheloadinplaningoverhangthesupportsmorethanaslightdistancethestyleshowninFig.28isquiteaswellsupportedastheother;andwhentheironinthelegsandtheworkt
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