曲柄板课程设计

摘要机械产品的生产过程是从原材料开始到成品出厂的全部劳动过程。它包括了毛坯的制造，零件的机械加工和热处理，机器的装配、检验、测试、和涂装等过程。机械加工工艺是实现产品设计，保证产品质量，节约能源，降低消耗的重要手段，是生产中最活泼的因素。它既是构思和想法，又是实在的方法和手段，并落实在由工件、机床、夹具所构成的工艺系统中。在制定工艺过程中，要确定各工序的安装工位和该工序需要的工步，加工该工序所需的机床，进给量，切削深度，主轴转速和切削速度，该工序的夹具，刀具及量具，还有走刀次数和走刀长度，最后计算该工序的根本时间，辅助时间和工作地效劳时间。夹具是机床上装夹工件的一种装置，使工件相对于机床和刀具有一个正确的位置，并在加工过程中保持该位置不变。在使用夹具装夹工件时，工件相对于刀具及机床的位置精度由夹具保证，不受工人技术水平的影响，使一批工件的加工精度趋于一致。使用夹具装夹工件方便、快速，工件不需要划线找正，可显著地减少辅助工时，工件在夹具中装夹后提高了工件的刚性，可加大切削用量；可使用多件、多工位装夹工件的夹具，并可采用高效夹紧机构，进一步提高劳动生产率。关键词：工艺，工序，工步，加工余量，定位方案，夹紧力AbstractMechanicalproductionprocessfromrawmaterialtofinishedproductallthelaborprocess.Itincludesablankmanufacturing,partsmachiningandheattreatment,machineassembly,inspection,testing,andpaintingandotherprocesses.Processisthemostactivefactorofproduction.Machiningprocessistoachieveproductdesign,productquality,saveenergyandreduceconsumptionoftheimportantmeansisthemostactivefactorofproduction.Itisbothaconceptandideas,butalsorealwaysandmeans,andtoimplementintheworkpiece,machinetools,fixturesposedbytheprocesssystem.Inthedevelopmentprocess,wemustdeterminetheinstallationprocessstationandtheprocessneedsfurtherwork,processingoftheprocessoflocomotivesandmachinetools,feedrate,cuttingdepth,spindlespeedandcuttingspeed,theprocessofthefixture,toolandgauge,therearethecuttingtimesandtoolpathlength,thefinalcalculationofthebasictimeoftheprocess,supporttimeandefforttoservice.Machinetoolclampingfixtureisadevicetotheworkpiecerelativetothemachinetoolandcuttingtoolworkpiecehaveacorrectposition,andduringprocessingtomaintainthesameposition.Usingthefixtureworkpiececlampingoftheworkpiecerelativetothelocationofmachinetoolsandprecisionguaranteedbythefixture,withouttechnicalskills,whichmakesthenumberofworkpiecemachiningprecisionofline．Workpiececlampingfixtureeasytouse,fast,crossedtheworkpiecedoesnotneedtofindis,cansignificantlyreducetheauxiliaryworkinghours,afterclampingtheworkpieceinthefixturetoimprovetherigidityoftheworkpiececanbeincreasedcutting;canusemultiplepieces,multiplebitworkpiececlampingfixture,clampingbodiescanbeusedefficientlytofurtherimprovelaborproductivityKeywords：Technology,theprocess,processstep,allowances，clampingforce目录TOC\o"1-2"\h\u13169摘要 ②适用范围螺旋夹紧机构结构简单,制造方便夹紧行程不受限制且夹紧可靠，所以在手动夹紧装置中广泛使用。偏心夹紧机构偏心夹紧机构是指用偏心件直接或间接与其他原件组合来使用的实心夹紧工件的机构。适用范围：圆偏心轮夹紧后，自锁性能较差，只适用于切削力负荷较小又无很大振动的场合，又因结构尺寸不能太大，为满足自锁条件，夹紧行程受到限制，所以对夹紧面的尺寸公差要求严格。〔4〕铰链夹紧机构铰链夹紧机构是由铰链杠杆组合而成的一种增力结构，其结构简单，增力倍数大，摩擦损失小，但无自锁性能。常与动力装置连用，故在机械化装置中广泛应用。〔5〕联动夹紧机构利用一个原始作用力实现多件或单件的多点多向同时夹紧的机构称为联动夹紧机构。定心夹紧机构定心夹紧机构是一种能够同时实现定位和夹紧的特殊夹紧机构。它的定位原件也是夹紧原件，它将工件定位并夹紧以后，能使其定位面的中心或对称中心固定在规定的位置。定心夹紧机构可以分为刚性定心夹紧机构和弹性定心夹紧机构。刚性定心夹紧机构定心精度不高，但夹紧行程大，常用在粗加工，弹性定心夹紧机构定位精度高，但夹紧行程小，常用与精加工。分度装置分度装置常用在铣床或钻床的转开工作台或其它必须分度的装置上。分度装置一般由对定销和分度盘两个主要局部组成。其中之一需要装在夹具需要分度转动的部位上，另一个装在夹具需要固定的部位上。设计分度时最主要的原那么是：〔1〕保证必要的分度精度，产生分度误差的原因很多，主要原因是分度盘与分度销之间存在间隙，分度销与固定套之间的间隙，分度套筒在分度盘上的位置不够精确，以及分度套内外两圆柱之间的偏心差。〔2〕保证分度的方便可靠，加工批量较大的工件时，常用机械化自动化的分度，批量较小时常用手动分度。〔3〕保证分度销强度足够强，为保证分度销足够强强度，在受力较大的情况下，往往使分度销只起分度作用，而防止承受任何外力。分度装置可分为回转分度装置和直线分度装置。回转分度装置，是指在工件一次装夹中通过夹具的某局部带开工作转动一定的角度，完成多工位加工的分度装置。回转分度装置主要用于工件外表圆周分度孔或槽的加工。直线分度装置是指工件一次装夹中，通过夹具的某局部带开工件直线移动一定距离完成多工位加工的分度装置，它是对直线方向上的尺寸进行分度的装置。夹具体夹具体是夹具的根底件，他将夹具上的各种装置和原件连接成一个整体，并通过它将夹具安装到机床上。它的结构形状尺寸及大小，取决于加工工件的特点尺寸大小，各种原件的结构和布局，夹具机床的连接方式，切削力及重力等大小的影响。1.夹具毛坯的类型〔1〕铸造夹具体铸造夹具体工艺性好，可铸成各种复杂形体。它具有较好的抗压强度，刚度和抗震性，切削性好，但生产周期长，需时效处理，以消除内应力。常用材料灰铸铁，要求强度高时用铸钢，要求重量轻时用铸铝。目前铸造夹具体的应用最广。〔2〕焊接夹具体焊接夹具体是采用钢板型材焊接而成，这类结构制造方便，生产周期短，本钱低。易减轻重量。但焊接过程中的热变形和剩余应力较大，易变形，为此，焊接后须进行退火处理，以保证夹具尺寸的稳定性。它适用于新产品适制，临时急用及结构简单的夹具体。〔3〕锻造夹具体锻造夹具体适用于结构简单，尺寸不大，要求强度和刚度较大的夹具体。它能承受较大的冲击载荷，锻造后酌情采用调质，正火或回火处理。〔4〕装配夹具体装配夹具体是选用标准毛胚件或零件及个别非标准件通过销钉，螺钉连接组装而成。为保证夹具体不会产生不允许的变形或振动，夹具体应有足够的强度或刚度。2.夹具体应满足的要求〔1〕应有足够的强度和刚度。在加工过程中，夹具体要承受较大的切削力和夹紧力。为保证夹具体不会产生不允许的变形或振动，夹具体应有足够的驱动和刚度。〔2)力求结构简单和装卸方便。在保证强度和刚度的前提下。尽可能体积小重量轻。〔3)结构工艺性好，夹具体应便于制造、装配、检验和维修。例如：铸造夹具体上安装各种元件的外表应铸造出3—5mm凸台。〔4)排除，切屑要方便。〔5〕夹具体在机床上安装稳定可靠。〔6〕有适当的精度和尺寸稳定性。夹具体上的重要外表，如安装定位元件的外表，夹具体的安装基面，应有适当的尺寸和形状精度，它们之间还应有适当的位置精度。3.2专用夹具的设计及原理分析为保证曲柄板使用要求，需加工曲柄板上五个的孔和同轴线上五个的沉孔且相互夹角从而设计立轴回转分度夹具，保证工件的加工精度和加工质量。定位基准和定位元件的设计与分析根据曲柄板的形状和加工技术要求及精度。工件定位用了平面定位，内孔定位及外圆柱定位。当曲柄板底面与设置成三角形布置的三个固定支撑接触为平面定位，限制工件的,,三个自由度，工件H8孔与分度盘上H7的外圆柱面接触行成圆柱孔定位，限制了工件,两个自由度，当工件上h6外圆柱面与活动V形块接触形成外圆柱定位，限制了一个自由度。如图3-1所示:图3-1曲柄板定位示意图综上分析：H8孔，端面和h6外圆在分度盘f7外圆，三个支撑钉和工件以活动V形块上定位，限制了工件的6个自由度，实现了曲柄板的完全定位。分度装置的设计与分析根据要求要在立式钻床上加工曲柄板上通孔5—mm和同轴线上沉孔5mm深3.5mm，且相互夹角均匀分布由可知，为满足加工要求应选用回转分度装置，切选用轴向分度形式。即对定销的运动方向与分度盘的回转轴线平行。在轴向分度形式中最常用的对定机构。其特点是结构简单，制造容易，当对定机构有污物或碎屑粘付时对定销插入分度套时能将污物推出，并不影响对定元件的接触。主要用于中等精度分度的装置中。对定销与分度套之间常采用H7/g6的配合。其结构如以下图：1—手柄2—夹具体3—分度盘4—分度盘套筒5—对定销6—弹簧图3—2枪栓式圆柱销对定机构分度原理分析：对定销5装在夹具体2上，并借助弹簧的作用插入分度盘相应的孔中，以确定工件与钻套间的相对位置。分度盘的孔座数与工件被加工孔数相等，且成4X=均匀分布。分度时抬起手柄1拔出对定销5，转动分度盘3，当下一个分度孔与对定销对准时，对定销在弹簧作用下，插入分度孔即可钻第二个孔，以此类推。钻模板与导引元件的设计钻模板的设计钻模板是供安装钻套用的。要求具有一定的强度和刚度，以防止由于变形而影响钻套的位置精度和导向精度。常用的有以下几种类型：〔1〕固定式钻模板这种钻模版是直接固定在夹具体上而不能移动的，因此用固定式钻模版加工孔时所获得较高的位置精度，但有时对装卸工件不甚方便。〔2〕铰链式钻模板钻模板与夹具体为铰链连接，使用铰链式钻模板装卸工件方便，但铰链处必然有间隙，因而加工孔的位置精度比固定式钻模板的精度低。〔3〕可卸式钻模板当装卸工件必须将钻模板取下时，那么采用可卸式钻模板，使用这种钻模板时，装卸钻模板较费时费力，钻模的位置精度较低，一般多在使用其它类型钻模板不便于装夹工件时使用。〔4〕悬挂式钻模板在立式钻床上多用多轴转动头加工平行孔系时，所用钻模板常通过两导柱直接悬挂在传动箱上，并随机床主轴往复移动，这种钻模版为悬挂式钻模板。根据曲柄板几何形状尺寸，加工特点及定位基准，为方便装卸工件应选用铰链式钻模板。2.导引元件设计钻套在钻模中的作用是保证被加工孔的位置精度，引导刀具防止其在加工过程中发生偏斜，提高刀具的刚性，防止加工时的振动。常用的标准钻模有以下几种类型：〔1〕固定钻套固定钻套安装在钻模板或夹具体中，其配合为H7/h6或H7/r6。固定钻套结构简单，钻孔精度高，适用于单一钻孔工序和小批量生产。〔2〕可换钻套当工件为单一钻孔工艺，大批量生产时为便于更换磨损的钻套，选用可换钻套。〔3〕快换钻套当工件需钻，扩，铰多工艺加工时，能快速更换不同孔径的钻套，应选用快换钻套。更换钻套时将钻套缺口转至螺钉处，即可取出钻套。削边的方向应考虑刀具的旋向，以免钻套自动脱出。根据加工5—通孔与5—沉孔的要求可以先钻再扩，后用平头钻三次加工，也可以用钻—扩复合刀具一次加工获得。本次设计利用复合刀具一次加工。但综合考虑采用快换钻套。钻模板的结构如图3—3所示：1—圆柱销2—钻模板3—支撑钉4—螺钉5—快换钻套6—菱形螺母图3—3铰链式钻模板夹具方案与夹具体的设计1.夹具体设计由可设计夹具体，材料为HT200，因为夹具体为铸件，应进行人工失效处理，具体加工技术要求及结构见夹具体零件图。2.夹具方案根据曲柄板形状特点并由知应选择轴向方向为夹紧的方向，曲柄板上端面及底面为被夹紧的两面。并选择夹紧机构为螺旋加紧机构。装夹时，拧紧螺母，通过开口垫圈，将工件端面牢固的压在定位原件上，活动V形块通过弹簧的作用对工件起到一定的辅助夹紧作用。其加紧结构如图2—4所示1—分度盘2—支撑钉3—开口垫圈4—螺母5—双头螺柱图3—4螺旋夹紧机构结构图由以上各主要原件的设计，将各局部连成以整体，够成钻模板的装配图〔见装配图〕。3.定位误差分析该夹具定位基准与设计基准均为螺栓杆轴线，基准不重合误差为零。基准位置误差取决于Φ30H8孔和Φ30f7配合间隙，可确定最大配合间隙为=0.033+0.041=0.074mm,所以定位误差0.074mm。工件尺寸80mm，公差取12级，查表IT12=0.3，所以加工误差=0.3mm，定位误差</3,此定位方案可行。4.夹紧力分析夹紧力是满足定位可靠，夹具可行的因素。夹紧力不能太小，否那么加工时产生位移破坏定位，但也不能太大，否那么工件易变形本次加工孔径较小，所需夹紧力也较小，所设计的夹紧机构能满足加工要求。结论此次毕业设计的内容主要有两个方面：曲柄板的加工工艺设计和夹具的设计。主要目的：在于通过对曲柄板加工工艺的设计，熟悉和掌握零件从毛坯材料选择到成品过程，培养实际生产中产品设计能力；另外通过对夹具的设计，熟悉和掌握夹具的根本知识和设计原那么。通过本次的毕业设计，使我能够对书本的知识做进一步的了解与学习，工艺的设计，能使我们更加了解毛坯材料的选择，材料的热处理，加工方法的选择，加工余量确实定，更加熟悉了工艺规程和先进的加工工艺，学习到了工艺的制定过程，工艺卡片的制作，工艺手册的查询等方面的知识，先制定合理的加工工序，再选用适宜的机床，查工艺手册定适宜的毛坯，再查进给、切削速度、转速，计算实际转速和时间完成工艺卡片；通过专用夹具的设计，使我们掌握了专用夹具的设计步骤及过程，六点定位原理，通过设计定位元件来实现设计标准，通过夹紧力的计算选用夹紧装置，对毛坯夹紧的原那么，最终完成零件的加工到达设计的要求。脚踏实地，认真严谨，实事求是的学习态度，不怕困难、坚持不懈、吃苦耐劳的精神是我在这次设计中最大的收益。我相信通过毕业设计不仅提升了对我大学期间所学根本专业知识综合运用也锻炼了我们实际解决工程实际的能力，而且让我们对所学的知识有了更加深刻的理解，对我们以后的学习、工作有了更大的帮助，为我们留下了珍贵的经验。致谢历经了近两个月的不懈努力，设计及论文已经完成。从最初的茫然，到慢慢的进入状态，再到对思路逐渐的清晰，，紧张而又充实的毕业设计终于落下了帷幕。本论文是在指导老师程显仑老师的悉心指导下完成的。老师渊博的专业知识，严谨的治学态度，精益求精的工作作风，诲人不倦的高尚师德，严以律己、宽以待人的崇高风范，朴实无华、平易近人的人格魅力对我影响深远。不仅是我树立了远大的学术目标、掌握了根本的研究方法，还使我明白了许多待人接物与人处事的道路。本论文从选题到完成，每一步都是在程老师的指导下完成了，倾注了老师大量的心血，在此更要感谢程老师，是在他的细心指导和关心，我才能够顺利的完成毕业设计及论文。还有在我的学业和设计中无不倾注着老师们辛勤的汗水和心血，在此我要向所有的老师致以最衷心的感谢和深深的敬意，同时感谢我的同学和朋友，在我写论文的过程中给予我了很多素材，还在论文的撰写和排版灯过程中提供热情的帮助。由于我专业知识水平有限，所写论文难免有缺乏之处，恳请各位老师和学友批评和指正！参考文献[1]李晓沛、张琳娜、赵凤霞主编.简明公差标准应用手册【M】.上海：上海科学技术出版社.2005.[2]胡荆生主编.公差配合与技术测量根底【M】.北京：中国劳动社会保障出版社.2006.[3]中国机械工程学会编.形状和位置公差【M】.北京：中国方案出版社.2004.[4]于骏一主编．典型零件制造工艺【M】．北京：机械工业出版社，1989.[5]莫雨松、李硕根主编．互换性与技术测量【M】．北京：中国计量出版社，1988.[6]马贤智主编．机械加工余量与公差手册【M】．北京：中国标准出版社，1994.[7]上海金属切削技术协会主编．金属切削手册【M】．上海科学技术出版社，1984.[8]余光国、马俊、张兴发主编．机床夹具设计【M】．重庆：重庆大学出版社，1995.[9]薛源顺主编.机床夹具设计.北京：机械工业出版社【M】.1992[10]东北重型机械学院等．机床夹具设计手册【M】．上海：上海科学技术出版社，1979[11]陈宏钧主编．机械零件切削加工工艺与技术标准实用手册【M】．1996[12]邓文英、宋立宏.金属工艺学【M】.北京:高等教育出版社.2008[13]王启平.机床夹具设计【M】.哈尔滨：哈尔滨工业大学出版社.1988[14]王先逵主编.机械加工工艺手册【M】.北京：机械工业出版社.2007附录英文资料JigsandFixturesTheworkpiecemustbelocatedrelativetothecuttingtool,andbesecuredinthatposition.Aftertheworkpiecehasbeenmarkedout,itisstillnecessarytopositionitwithrespecttothemachinemovements,andtoclamitinthatpositionbeforemachiningisstarted.Whenseveralidenticalworkpiecearetobeproduced,theneedtomarkouteachpartiseliminatedbytheusedofjigsandfixture,butifacastingorforgingisinvolved,atrialworkpieceismarkedout,toensurethattheworkpiececanbeproducedfromit,andtobeensurethatribs,core,ect.Havenotbecomemisplaced.Jigsandfixturesarealikeinthattheybothincorporatedevicestoensurethattheworkpieceiscorrectlylocatedandclamped,buttheydifferinthatjigsincorporatemeansoftoolguidingduringtheactualcuttingoperation,andfixturesdonot.Inpractice,theonlycuttingtoolsthatcanbeguidedwhileactuallycuttingaredrills,reamers,andsimilarcutters,andsojigsareassociatedwithdrillingoperations,andfixtureswithalloperations.Fixturemayincorporatemeansofsettingthecuttingtoolsrelativetothelocationsystem.Theadvantagesofjigsandfixturescanbesummarizedasfollows:1.Markingoutandothermeasuringandsettingoutmethodsareeliminated.2.Unskilledworkersmayproceedconfidentlyandquicklyintheknowledgethattheworkpiececanbepositioncorrectly,andtoolsguidedorset.3.Theassemblyofpartsisfacilitated,sinceallcomponentswillbeidenticalwithinsmalllimits,and‘trying’andfilingofworkiseliminated.4.Thepartswillbeinterchangeable,andtheproductissoldoverawidearea,theproblemofpartswillbesimpliyed.Boltholesoftenhave1.5mmoreven3.0mmclearanceforthebolt,andthereadermaydoubtthenecessityofmakingprecisionjigsforsuchwork.Itmustberememberedthatthejigs,oncemade,willbeusedonmanycomponents,andtheextracostofanaccuratelymadejigisspreadoveralargeoutput.Forthermore,itissurprisinghowsmallerrorsaccumulateinamechanismduringitsassembly.Whenaclearanceisspecified,itisbettertoensureitsobservance,ratherthantoallowcarelessmarkingoutandmachiningtoencroachuponit.Thelocationofworkpiece.Aworkpieceinspacehassixdegreeoffreedom.ConsiderthesefreedomwithrespecttothethreemutuallyperpendicularaxesXX,YY,andZZ.Thebodycanmovealonganyoftheseaxe;itthereforehasthreefreedomoftranslation.Itcanalsorotateaboutanyofthethreeaxes;itthereforehasthreefreedomsofrotation.Thetotalnumberoffreedomsissix.Whenworkpieceislocated,asmanyofthesefreedomaspossiblemustbeeliminated,toensurethattheoperationisperformedwiththerequiredaccuracy.Accuracyisensurebymachiningsuitablelocationfeatureasearlyaspossible,andusingthemforalllocation,unlessotherconsiderationsmeanthatotherlocationfeaturesmustbeused.Ifnecessary,thenewlocationfeaturesmustbemachinedasaresultoflocationfromtheformerlocationfeature.Successfulfixturedesignsbeginwithalogicalandsystematicplan.Withacompleteanalysisofthefixture'sfunctionalrequirements,veryfewdesignproblemsoccur.Whentheydo,chancesaresomedesignrequirementswereforgottenorunderestimated.Theworkpiece,processing,tooling,andavailablemachinetoolsmayaffecttheextentofplanningneeded.Preliminaryanalysismaytakefromafewhoursuptoseveraldaysformorecomplicatedfixturedesigns.Fixturedesignisafive-stepproblem-solvingprocess.Thefollowingisadetailedanalysisofeachstep.1.DefineRequirementsToinitiatethefixture-designprocess,clearlystatetheproblemtobesolvedorneedstobemet.Statetheserequirementsasbroadlyaspossible,butspecificallyenoughtodefinethescopeofthedesignproject.Thedesignershouldasksomebasicquestions:Isthenewtoolingrequiredforfirst-timeproductionortoimproveexistingproduction?Ifimprovinganexistingjob,isthegoalgreateraccuracy,fastercycletimes,orboth?Isthetoolingintendedforonepartoranentirefamilyofparts?Thetoolingdesignermustdeterminehowmuchfreedomandinputthereisoneachproject.Ifmanychoicesregardingmachinetools,operations,andcuttingtoolshavealreadybeenmade,thedesigner'srolewillhavearelativelynarrowfocus.2.Gather/AnalyzeInformationCollectallrelevantdataandassembleitforevaluation.Themainsourcesofinformationarethepartprint,processsheets,andmachinespecifications.Makesurethatpartdocumentsandrecordsarecurrent.Forexample,verifythattheshopprintisthecurrentrevision,andtheprocessinginformationisup-to-date.Checkwiththedesigndepartmentforpendingpartrevisions.Animportantpartoftheevaluationprocessisnotetaking.Complete,accuratenotesallowdesignerstorecordimportantinformation.Withthesenotes,theyshouldbeabletofillinallitemsonthe"ChecklistforDesignConsideration"Allideas,thoughts,observations,andanyotherdataaboutthepartorfixturearethenavailableforlaterreference.Itisalwaysbettertohavetoomanyideasaboutaparticulardesignthantoofew.Fourcategoriesofdesignconsiderationsneedtobetakenintoaccountatthistime:workpiecespecifications,operationvariables,availabilityofequipment,andpersonnel.Thesecategories,whileseparatelycoveredhere,areactuallyinterdependent.Eachisanintegralpartoftheevaluationphaseandmustbethoroughlythoughtoutbeforebeginningthefixturedesign.Workpiecespecificationsusuallyarethemostimportantfactorsandhavethelargestinfluenceonthefixture'sfinaldesign.Typically,theseconsiderationsincludethesizeandshapeofthepart,theaccuracyrequired,thepropertiesofthepartmaterial,thelocatingandclampingsurfaces,andthesizeoftherun.Operationvariablesincludethetypeofoperationsrequiredtomakethepart,numberofoperationsperformed,sequenceofoperations,inspectionrequirements,andtimerestrictions.Availabilityofequipmentrequiredtomachine,assemble,andinspectapartoftendetermineswhetherthefixtureisdesignedforasinglepartormultipleparts.Aprocessengineersometimesselectstheequipmenttomachinepartsbeforethetoolingdesignerbeginsthedesign.Thetoolingdesignershouldverifywhatequipmentwillbeusedforeachoperation.Personnelconsiderationsdealwiththeenduser,oroperator,oftheequipment.Fixturedesignersshouldputthemselvesinthemachineoperator'sshoesandconsideralltheoperationalscenariostheycan.Designersshouldconsidernotonlycorrectusageofthefixture,butalsopossibleincorrectusage.Theymustask,"Isthereanywayformetohurtmyselfwhileoperatingthisequipment?"Additionalfactorsusuallyconsideredinthiscategoryareoperatorfatigue,efficiency,economyofmotion,andthespeedoftheoperation.Thedesigneralsomustknowandunderstandthegeneralaspectsofdesignsafetyandallappropriategovernmentandcompanysafetyrulesandcodes.3.DevelopSeveralOptionsThisphaseofthefixture-designprocessrequiresthemostcreativity.Atypicalworkpiececanbelocatedandclampedseveraldifferentways.Thenaturaltendencyistothinkofonesolution,thendevelopandrefineitwhileblockingoutother,perhapsbettersolutions.Adesignershouldbrainstormforseveralgoodtoolingalternatives,notjustchooseonepathrightaway.Duringthisphase,thedesigner'sgoalshouldbeaddingoptions,notdiscardingthem.Intheinterestofeconomy,alternativedesignsshouldbedevelopedonlyfarenoughtomakesuretheyarefeasibleandtodoacostestimate.Thedesignerusuallystartswithatleastthreeoptions:permanent,modular,andgeneral-purposeworkholding.Eachoftheseoptionshasmanyclampingandlocatingoptionsofitsown.Themorestandardlocatingandclampingdevicesthatadesignerisfamiliarwith,themorecreativehecanbe.Areasforlocatingapartincludeflatexteriorsurfaces,cylindricalandcurvedexteriorsurfaces,andinternalfeatures(suchasholesandslots).Thechoiceofstandardlocatingdevicesisquiteextensive.Similarly,therearecountlesswaystoclampapart,usingawidearrayofstandardclampingdevices.Forexample,aworkpiececanbeclampedfromthetop,orbygrippingitsoutsideedgeoraninternalsurface.4.ChoosetheBestOptionThefourthphaseofthetool-designprocessisacost/benefitanalysisofdifferenttoolingoptions.Somebenefits,suchasgreateroperatorcomfortandsafety,aredifficulttoexpressindollarsbutarestillimportant.Otherfactors,suchastoolingdurability,aredifficulttoestimate.Inanalyzingfixturecosts,theemphasisisoncomparingonemethodtoanother,ratherthanfindingexactcosts.Estimatesareacceptable.Sometimesthesemethodscomparebothproposedandexistingfixtures,sothat,wherepossible,actualproductiondatacanbeusedinsteadofestimates.Toevaluatethecostofanyworkholdingalternative,firstestimatetheinitialcostofthefixture.Tomakethisestimate,drawanaccuratesketchofthefixture.Numberandlisteachpartandcomponentofthefixtureindividually.Hereitisimportanttohaveanorderlymethodforoutliningthisinformation.5.ImplementtheDesignThefinalphaseofthefixture-designprocessconsistsofturningthechosendesignapproachintoreality.Finaldetailsaredecided,finaldrawingsaremade,andthetoolingisbuiltandtested.Usestandardcomponents.Theeconomiesofstandardizedpartsapplytotoolingcomponentsaswellastomanufacturedproducts.Standard,readilyavailablecomponentsincludeclamps,locators,supports,studs,nuts,pins,andahostofotherelements.Mostdesignerswouldneverthinkofhavingtheshopmakecapscrews,bolts,ornutsforafixture.Likewise,nostandardtoolingcomponentsshouldbemadein-house.Thefirstruleofeconomicdesignis:Neverbuildanycomponentyoucanbuy.Commerciallyavailabletoolingcomponentsaremanufacturedinlargequantitiesformuchgreatereconomy.Inmostcases,thecostofbuyingacomponentislessthan20%ofthecostofmakingit.Laborisusuallythegreatestcostelementinthebuildingofanyfixture.Standardtoolingcomponentsareonewaytocutlaborcosts.Browsethroughcatalogsandmagazinestofindnewproductsandapplicationideastomakedesignssimplerandlessexpensive.Useprefinishedmaterials.Prefinishedandpreformedmaterialsshouldbeusedwherepossibletolowercostsandsimplifyconstruction.Thesematerialsincludeprecision-groundflatstock,drillrod,structuralsections,casttoolingsections,precasttoolingbodies,toolingplates,andotherstandardpreformedmaterials.Includingthesematerialsinadesignbothreducesthedesigntimeandlowersthelaborcost.Keeptolerancesasliberalaspossible.Themostcost-effectivetoolingtoleranceforalocatorisapproximately30%to50%oftheworkpiece'stolerance.Tightertolerancesnormallyaddextracosttothetoolingwithlittlebenefittotheprocess.Wherenecessary,tightertolerancescanbeused,buttightertolerancesdonotnecessarilyresultinabetterfixture,onlyamoreexpensiveone.Simplifytoolingdetails.Elaboratedesignsoftenaddlittleornothingtothefunctionofthefixture.Moreoften,apowerclampcandothesamejobatafractionofthecost.Keepthefunctionandoperationofafixtureassimpleaspossible.Thelikelihoodofbreakdownsandotherproblemsincreaseswithcomplexdesigns.Theseproblemsmultiplywhenmovingpartsareaddedtothedesign.Misalignment,inaccuracy,wear,andmalfunctionscausedbychipsanddebriscancausemanyproblemsinthebestfixturedesigns.Reducingdesigncomplexityalsoreducesmisunderstandingsbetweenthedesignerandthemachineoperator.Wheneverpossible,afixture'sfunctionandoperationshouldbeobvioustotheoperatorwithoutinstructions.Oncesketchesandthebasicfixturedesignhavebeencompleted,finalengineeringdrawings,alsocalledshopprints,areusedinthetoolroomtobuildthefixture.Theeasiestwaytoreducemanualdrawingtimeisbysimplifyingthedrawing.Wordsorsymbolsshouldbeusedinplaceofdrawndetailswherepractical.Allextraorunnecessaryviews,projections,anddetailsshouldbeeliminatedfromthedrawing.Drawingacompleteclampassembly,forexample,addsverylittletothetotaldesign.Simplyshowingthenoseoftheclamp,drawninitsproperrelationtotheworkpieceandlabeledwithitspartnumber,conveysthesameinformationinafractionofthetime.Fordrawingsthatrequiremoredetail,usetracingtemplatestoreducedrawingtime.Thesetemplatesshowmoststandardcomponentsinseveralviews.Ifnecessary,theymaybeenlargedorreducedonacopiertoanyscaleneededforadrawing.Oncethepropertracingtemplateisselected,simplyslipitunderthedrawingsheetandalignitwiththedrawing.Whenthetemplateisproperlypositioned,tapeitdownandtracethecomponentonthedrawingsheet.Tracingtemplatessavedrawingtimeandimprovethequalityofthedrawing.Computersarerapidlyreplacingdrawingboardsasthepreferredtoolforpreparingengineeringdrawings.Almosteveryareaofdesignisaffectedbythecomputer.Computers,fromlargemainframestomicros,arebecomingstandardequipmentinmanydesigndepartments.Usingastandardlibraryindesigningthefixturedramaticallyreducesdrawingtime.Allcomponentsaredrawntofullscaleinavarietyofviews.Scalingdownisbestdoneinthefinaldrawing,notwhenstoringstandard-componentdrawings.Storingalargefixturebaseat1/4scaledoeslittlegood,becauseallcomponentswillhavetobe1/4scaletofitonit.Foreaseofuse,allcomponentsshouldbestoredatfullscale.Eachcomponentcanbecalledupfromthelibraryandplacedonthedrawingwhereitisrequired.Oncedrawingshavebeenthoroughlychecked,thenextstepisactuallybuildingtheactualfixture.Duringthebuildingstage,thedesignershouldmakesurethetoolroompersonnelknowexactlywhatmustbedonewhenmakingthefixture.Byperiodicallycheckingwiththefixturebuilder,thedesignercanhelpeliminateanypossiblemisunderstandingsandspeedthebuildingprocess.Ifthereareanydifficultieswiththedesign,thedesignerandbuilder,workingtogether,cansolvetheproblemswithaminimumoflosttime.MACHINABILITYThemachinabilityofamaterialusuallydefinedintermsoffourfactors:Surfacefinishandintegrityofthemachinedpart;Toollifeobtained;Forceandpowerrequirements;Chipcontrol.Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceAndpowerrequirements.Asforchipcontrol,longandthin(stringy)curedchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.MachinabilityOfSteelsBecausesteelsareamongthemostimportantengineeringmaterials(asnotedinChapter5),theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainso-calledfree-machiningsteels.ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusions(second-phaseparticles),whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmall;thisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.Hardersteelsresultinbetterchipformationandsurfacefinish.Notethatsoftsteelscanbedifficulttomachine,withbuilt-upedgeformationandpoorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformationofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipofmanganesesulfideinclusions.Innon-resulfurizedgradesofsteel,leadtakestheformofdispersedfineparticles.Leadisinsolubleiniron,copper,andaluminumandtheiralloys.Becauseofitslowshearstrength,therefore,leadactsasasolidlubricantandissmearedoverthetool-chipinterfaceduringcutting.Thisbehaviorhasbeenverifiedbythepresenceofhighconcentrationsofleadonthetool-sidefaceofchipswhenmachiningleadedsteels.Whenthetemperatureissufficientlyhigh-forinstance,athighcuttingspeedsandfeeds—theleadmeltsdirectlyinfrontofthetool,actingasaliquidlubricant.Inadditiontothiseffect,leadlowerstheshearstressintheprimaryshearzone,reducingcuttingforcesandpowerconsumption.Leadcanbeusedineverygradeofsteel,suchas10xx,11xx,12xx,41xx,etc.LeadedsteelsareidentifiedbytheletterLbetweenthesecondandthirdnumerals(forexample,10L45).(Notethatinstainlesssteels,similaruseoftheletterLmeans“lowcarbon,”aconditionthatimprovestheircorrosionresistance.)However,becauseleadisawell-knowntoxinandapollutant,thereareseriousenvironmentalconcernsaboutitsuseinsteels(estimatedat4500tonsofleadconsumptioneveryyearintheproductionofsteels).Consequently,thereisacontinuingtrendtowardeliminatingtheuseofleadinsteels(lead-freesteels).Bismuthandtinarenowbeinginvestigatedaspossiblesubstitutesforleadinsteels.Calcium-DeoxidizedSteels.Animportantdevelopmentiscalcium-deoxidizedsteels,inwhichoxideflakesofcalciumsilicates(CaSo)areformed.Theseflakes,inturn,reducethestrengthofthesecondaryshearzone,decreasingtool-chipinterfaceandwear.Temperatureiscorrespondinglyreduced.Consequently,thesesteelsproducelesscraterwear,especiallyathighcuttingspeeds.StainlessSteels.Austenitic(300series)steelsaregenerallydifficulttomachine.Chattercanbesproblem,necessitatingmachinetoolswithhighstiffness.However,ferriticstainlesssteels(also300series)havegoodmachinability.