《港口机械与电气专业》-Lesson 18 Dimensions and Tolerances1

TextContentsdimension 尺寸tolerance 公差drawing 制图productiondepartment 生产部门variation 偏差

figure 数字nominalsize 公称尺寸closetolerance 紧公差allowance 容差

Notes:1.Indimensioningadrawing,thenumbersplacedinthedimensionlinesrepresentdimensionthatareonlyapproximateanddonotrepresentanydegreeofaccuracy.在标注图纸中的尺寸时,尺寸线中标的数字只代表近似尺寸,不表示任何程度的精度。2.Ashaftmighthaveanominalsizeof63.5mm,butforpracticalreasonsthisfigurecouldnotbemaintainedinmanufacturingwithoutgreatcost.轴的公称尺寸可能为63.5毫米,但是由于实践中(一些)因素,在制造过程中不消耗巨资,不可能达到这个数值。

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Haveyoudrawnsomedrawingsinyourcollegeoruniversity?Yes,mostofushavedrawnsome.Doyourememberthereare2kindsofdrawingswhicharedetaildrawingandassemblydrawing?Generallyspeaking,weallknowtheimportanceoftheassemblyanddetaildrawings,becausetheyconveytheimportantinformationtotheworkers.Beforethedrawingsaresenttotheshop,thedesignerisoftencalledupontocheckthedrawings.Whyshouldthedrawingsbecarefullychecked?Canyouguess?Yes,thedimensionofthedrawingsshouldbedoneclearly.Itshouldbedoneinamannerthatwillbemostconvenientandunderstandabletotheproductiondepartments.

Thencanyouguesswhytolerancesmustbeplacedonthedimensionsofdrawings?Yes,weusethemtolimitthepermissiblevariationsinsizebecauseitisimpossibletomanufacturepartexactlytoagivendimension.Indimensioningadrawing,thenumbersplacedinthedimensionlinesrepresentdimensionthatareonlyapproximateanddonotrepresentanydegreeofaccuracy.1Tospecifyadegreeofaccuracy,itisnecessarytoaddtolerancefigurestothedimension.Atoleranceistheamountofvariationpermittedinthepart.

Wecantakeashaftasanexample.Ashaftmighthaveanominalsizeof63.5mm,butforpracticalreasonsthisFigurecouldnotbemaintainedinmanufacturingwithoutgreatcost.2So,acertaintolerancewouldbeaddedand,ifavariationof±0.08mmcouldbepermitted,thedimensionwouldbestated63.5mm±0.08mm.Somedimensionsaregivenclosetolerances,whichmeanthepartmustfitproperlywithsomeotherpart.Generallyspeaking,theminimumcostofproductionandassemblywillmaximizeprofit.Thecostofapartgoesupasthetoleranceisdecreased.Ifaparthasseveralormoresurfacestobemachined,thecostcanbeexcessivewhenlittledeviationisallowedfromthenominalsize.

Thereisanotherconcept,whichissometimesconfusedwithtolerance.Itiscalledallowance.Itistheminimumclearancespaceintendedbetweenmatingpartsandrepresentstheconditionoftightestpermissiblefit.Wecanunderstandthisdefinitionfromthefollowingexample.Ifashaft,size1.498+0.000mm,istofitaholeofsize1.500+0.003mm,theminimumsizeholeis1.500mmandthemaximumsizeshaftis1.498mm.Thustheallowanceis0.002mmandthemaximumclearanceis0.008mmasbasedontheminimumshaftsizeandmaximumholedimension.

Ⅰ.AnswerthefollowingquestionsinEnglish.1.What’satolerance?

2.Whytolerancesmustbeplacedonthedimensionsofdrawings?

3.What’stheallowance?

Ⅱ.TranslatethefollowingEnglishsentencesintoChinesesentences.1.Canyouguesswhytolerancesmustbeplacedonthedimensionsofdrawings?Yes,weusethemtolimitthepermissiblevariationsinsizebecauseitisimpossibletomanufacturepartexactlytoagivendimension.

2.Wecantakeashaftasanexample.Ashaftmighthaveanominalsizeof63.5mm,butforpracticalreasonsthisFigurecouldnotbemaintainedinmanufacturingwithoutgreatcost.So,acertaintolerancewouldbeaddedand,ifavariationof±0.08mmcouldbepermitted,thedimensionwouldbestated63.5mm±0.08mm.

3.Ifashaft,size1.498+0.000mm,istofitaholeofsize1.500+0.003mm,theminimumsizeholeis1.500mmandthemaximumsizeshaftis1.498mm.Thustheallowanceis0.002mmandthemaximumclearanceis0.008mmasbasedontheminimumshaftsizeandmaximumholedimension.

Ⅲ.ReciteWhyshouldthedrawingsbecarefullychecked?Canyouguess?Yes,thedimensionofthedrawingsshouldbedoneclearly.Itshouldbedoneinamannerthatwillbemostconvenientandunderstandabletotheproductiondepartments.Thencanyouguesswhytolerancesmustbeplacedonthedimensionsofdrawings?Yes,weusethemtolimitthepermissiblevariationsinsizebecauseitisimpossibletomanufacturepartexactlytoagivendimension.Indimensioningadrawing,thenumbersplacedinthedimensionlinesrepresentdimens