床身支承.docx

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