1、Unit 6 Dimensional Tolerances and Surface RoughnessContentslNew Words&Expressions lText&TranslationlComplex Sentence AnalysislKey to ExercisesNew Words&Expressions ldimensional tolerance尺寸公差 lsurface roughness 表面粗糙度lcost reduction 成本降低linterchangeability intteindbiliti n.交替性，互换性lsurface finishes 表面处
2、理lparamount prmaunt adj.极为重要的lcommensurate kmenrit adj.同量的，相称的New Words&Expressions lcontemplate kntempleit v.预期lcomprehensive kmprihensiv adj.广泛的，综合的lappreciation pri:iein n.正确评价，鉴识lmaintenance meintinns n.维持，保养lhydraulic haidr:lik adj.水压的，液压的lcylinder silind n.气缸，圆柱状物lpiston pistn n.活塞New Words&Ex
3、pressions lantifriction ntifrikn n.减摩装置，润滑剂lantifriction bearing 滚动轴承lshaft bearing 轴承lstress reversal 应力反向lbearing surface 承压面，支撑面lmounting maunti n.底座，托架Text&TranslationlBecause of the highly competitive nature of most manufacturing businesses,the question of finding ways to reduce cost is ever pr
4、esent.A good starting point for cost reduction is in the design of product.The design engineer should always keep in mind the possible alternatives available to him in making his design.It is often impossible to determine the best alternatives without a careful analysis of the probable production co
5、st.Designing for function,interchangeability,quality,and economy requires a careful study of tolerances,surface finishes,processes,materials,and equipments.Text&TranslationlTo assure sound and economical design from a producibility standpoint,careful consideration of the following general design rul
6、es,both separately and together,is of paramount importance.The order of importance may vary according to design requirements,or factors,but the overall importance always remains the same.Text&Translationl(1)Seek simplicity.Design for maximum simplicity in functional and physical characteristics.l(2)
7、Determine the best production method.Seek the help of a production engineer to design for the most economical production methods.l(3)Analyze materials.Select materials that will lend themselves to low-cost production as well as to design requirements.Text&Translationl(4)Eliminate fixturing and handl
8、ing problems.Design for ease of locating,setting up,and holding parts.l(5)Employ maximum acceptable tolerances and finishes.1Specify surface roughness and accuracy no greater than that which is commensurate with the type of part or mechanism being designed,and the production method or methods contem
9、plated.lTolerances on finish and dimensions play an important role in the final achievement or absence of practical production design.lA comprehensive study of the principles of interchangeability is essential for a thorough understanding and full appreciation of low-cost production techniques.Inter
10、changeability is the key to successful production regardless of quantity.Details of all parts should be surveyed carefully to assure not only inexpensive processing but also rapid and easy assembly and maintenance.2It must be remembered that each production method has a well-established level of pre
11、cision which can be maintained in continuous production without exceeding normal basic cost.Text&TranslationlEconomic manufacturing does not“just happen”.It starts with design and considers practical limits of machine tools,processes,tolerances,and finishes.3Neither dimensional tolerances nor surfac
12、e roughness should be specified to limits of accuracy closer than those which the actual function or design necessitate.This is done to assure the advantages of lowest possible cost and fastest possible production.Text&TranslationlWithout needing to know how to operate a particular machine to attain
13、 the desired degree of surface roughness,there are certain aspects of all these methods which should be understood by the design engineer.Knowledge of such facts as degree of roughness obtained by any operation,and the economics of attaining a smoother surface with each operation,will aid him in dec
14、iding just which surface roughness to specify.Text&TranslationlBecause of its simplicity,the arithmetical average Ra has been adopted international and is widely used.The applications of surface roughness Ra are described in the following paragraphs.l（1）0.2 m.The finish is used for the interior surf
15、ace of hydraulic struts,for hydraulic cylinders,pistons and piston rods for O-ring packings,for journals operating in plain bearing,for cam faces,and for roils of antifriction bearings when loads are normal.l（2）0.4 m.The finish is used for rapidly rotating shaft bearings,for heavily loaded bearings,
16、for roils in bearings of ordinary commercial grades,for hydraulic applications,for static sealing rings,for the bottom of sealing-ring grooves,for journals operating in plain beings,and for extreme tension members.Text&Translationl（3）0.8 m.The finish is normally found on parts subject to stress conc
17、entrations and vibrations,for broached holes,gear teeth,and other precision machined parts.l（4）1.6 m.This finish is suitable for ordinary bearings,for ordinary machine parts where fairly close dimensional tolerances must be held,and for highly stressed parts that are not subject to severe stress rev
18、ersals.l（5）3.2 m.The finish should not be used on sliding surfaces,but can be used for rough bearing surface where loads are light and infrequent,or for moderately stressed machine parts.l（6）6.3 m.The appearance of this finish is not objectionable,and can be used on non-critical component surface,an
19、d for mounting surfaces for brackets,etc.Text&Translationl由于大多数制造业高度竞争的共性，找到降低成本的方法是永恒的主题。在产品设计中降低成本是一个好的开始。设计工程师在他的设计中应始终谨记他可能的选择。没有对产品的大致成本做细致的分析而能做出最好的选择是不可能的。功能、互换性、质量和经济性的设计都要求对公差、表面光洁度、工艺、材料和设备进行仔细研究。Text&Translationl从可加工的观点来看，要保证合理而经济的设计，这两者和两者的结合都极其重要，就要仔细考虑下面的一般设计规则。按照设计要求或者设计要素，其重要性的顺序可以是变
20、化的，但是其总的重要性总是相同的。l（1）追求简洁。设计时要最大限度的在功能上和物理特性上追求简洁。l（2）决定最好的加工方法。向生产工程师寻求帮助以使设计达到最经济的加工方法。Text&Translationl（3）分析材料。选择加工成本低而又满足设计要求的材料。l（4）消除固定与操作问题。设计要容易定位、安装与调试。l（5）采用最大可接受的公差与表面粗糙度。零件的表面粗糙度和尺寸精确度的确定，要和被设计的零件和装置保持相当，同时要匹配于预期的生产方法。l光洁度和尺寸公差在最终效果或者没有实际加工设计时起重要作用。Text&Translationl通过了解和全方位评价廉价的生产技术是充分研究
21、互换性原则的基础。不管数量如何，互换性是成功加工的关键。所以零件的细节都应该被仔细考虑以确保不仅工艺廉价而且容易快速装配，方便维护。这就必须记住每个加工方法都有一个适当的精度等级以保证在不超出正常的基本成本时的连续加工。Text&Translationl 经济制造并非一蹴而就，它总是以设计和机械制造的刀具、工艺、公差和光洁度的实际限制的考虑而开始。尺寸公差和表面粗糙度都不应该被指定到超过实际功能或设计需要的界限。这样做有利于保证到达尽可能最低的成本和尽可能最快的加工。l设计工程师没有必要一定要知道怎样去具体操作某些特殊的机器以获得期望的表面粗糙度，但应了解所有这些方法的某些特定的方面，比如通过
22、操作获取表面粗糙度的知识和对每种操作得到更光滑表面的经济性的知识，这样在指定表面粗糙度时，将有助于设计工程师做出决定。Text&Translationl由于其简洁性，算术平均Ra已经被国际上采纳并被广泛使用。表面粗糙度Ra的应用具体介绍如下所述。l（1）0.2m。这个光洁度被用于液压支杆的内表面、液压缸、液压活塞和活塞杆、O形密封圈、滑动轴承的轴承颈、凸轮面和正常负载下滚动轴承的滚子。l（2）0.4m。这个光洁度被用于高速旋转轴轴承、重载荷轴承、普通商业的轴承滚子、液压应用、静态密封圈、密封其槽底、运转在滑动轴承中的轴颈和极度张紧的构件。Text&Translationl（3）0.8 m。这个
23、光洁度通常出现在有应力集中和震动的零件中，拉孔、轮齿及其他精加工零件。l（4）1.6 m。这个光洁度适用于普通轴承，尺寸公差必须是被保持的正常封闭处的普通机械零件，不受剧烈交变应力的高应力零件。l（5）3.2 m。这个光洁度不应该被用于滑动表面，但能用于负载轻而平稳的粗糙轴承表面，或者用于张紧适度的机械零件。l（6）6.3 m。这个光洁度的出现并不令人讨厌，它能用于非关键零件表面和轴承座的安装面等。Complex Sentence Analysisl1 Specify surface roughness and accuracy no greater than that which is co
24、mmensurate with the type of part or mechanism being designed,and the production method or methods contemplated.在确定零件的表面粗糙度和尺寸精度时，不应该对零件或者机构的设计，以及将要采用的生产方法提出过高的要求。l该句中的that指代 surface roughness and accuracy。l be commensurate with 是“相称,同量”的意思l contemplated 分词作后置定语，修饰前面的production method or methodsCompl
25、ex Sentence Analysisl2It must be remembered that each production method has a well-established level of precision which can be maintained in continuous production without exceeding normal basic cost.这就必须记住每种加工方法都有一个适当的精度等级以维持连续加工而不超出正常的基本成本。lit：是形式主语，that：后面引导的从句是真正的主语lwhich：引导限制性定语从句，修饰先行词levelComp
26、lex Sentence Analysisl3Neither dimensional tolerances nor surface roughness should be specified to limits of accuracy closer than those which the actual function or design necessitate.尺寸公差和表面粗糙度都不应该被指定到超过实际功能或设计需要的界限。Key to Exercisesl.Translate the following phrases into Chinese or English.l1.尺寸公差 l
27、2.surface roughness l3.成本降低l4.seek simplicityl5.互换性原则l6.economic manufacturing l7.滚动轴承 l8.stress concentrationl9.应力反向l10.bearing surface Key to Exercisesl.Mark the following statement with True(T)or False(F)according to the text.l1.Tl2.Tl3.Tl4.Fl5.TKey to Exercisesl.Fill in the blanks with the suita
28、ble words or phrases given prehensivel2.appreciationl3.regardless ofl4.maintenancel5.well-establishedl6.continuousKey to Exercisesl.Translate the Chinese parts given in the brackets into English.l1.the highly competitive nature of most manufacturing businessesl2.the possible alternatives available l3.Tolerances on finish and dimensionsl4.fixturing and handling problemsl5.for moderately stressed machine partslThe end!
