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1、 Dimensional Tolerance The manufacture of machine parts is founded on the engineering drawingEveryone engaged in manufacturing has a direct or indirect interest in understanding the meaning of the drawings on which the entire production process is establishedThe engineer in industry is constantly fa
2、ced with the fact that no two machine Darts can ever be made exactly the same.He learns that the small variations that occur in repetitive production must he considered in the design 80 that the tolerances placed on the dimensions will restrict the variations to acceptable limits .The tolerances pro
3、vide zones in which the outline of finished part must lie. Proper tolerancing practice ensures that the finished product functions in its intended manner and operates for its expected life. A designer is well aware that the cost of a finished product can increase rapidly as the tolerances on the com
4、ponents are made smallerDesigners are constantly admonished to us the widest tolerances possible. Situations may arise，however，in which the relationship between the various tolerances required for proper functioning has not been fully exploredUnder such conditions the designer is tempted to specify
5、part tolerances that ale unduly tight in the hope that no difficulty will arise at the time of assemblyThis is obviously an expensive substitute for a more thorough analysis of the tolerancing situation The allocation of proper production tolerances is therefore a most important task if the finished
6、 product is to achieve its intended purpose and yet be economical t0 produceThe size of the tolerances，as specified by the designer，depends on the many conditions pertaining to the design as well as on past experience with similar products if such experience is available.A knowledge of shop processe
7、s and machine capabilities is of great assistance in helping to determine the tolerances in the most effective maimer. A revision of the design may be called for if the tolerances are too small to be maintained by the equipment available for producing the dimension. Ambiguities in engineering drawin
8、g can be cause of much confusion and expense. When specifying the tolerances，the designer must keep in mind that the drawing must contain a11 requisite information if the designers intent is to be fully realizedThe drawing must therefore give complete information and at the same time be as simpe as
9、possibleThe detail of drawing must be capable of being universally understoodThe drawing must have one and only one meaning to everyone who will use itthe design，purchasing，tool design,production，inspection，assembly，and servicing departments. Tolerances may be placed on the drawing in a number of di
10、fferent ways. In theunilateral system one tolerance is zero and all the variation of the dimension is given by the other tolerance. In bilateral dimensioning a mean dimension is used with plus and minus variations extending each way from the mean dimension. Unilateral tolerancing has the advantage t
11、hat a tolerance revision can be made with the least disturbance to the remaining dimensions. In the bilateral system a change in the tolerances also involves a change in at least one of the mean dimensions .Tolerances can be easily changed back and forth between unilateral and bilateral for the purp
12、ose of making calculations. A part is said to be at the maximum material condition (MMC) when the dimensions are all at the limits that will give a part containing the maximum amount of materialFor a shaft or external dimension，the fundamental dimension is the largest value permitted,and all the var
13、iation，as permitted by the tolerance，serves to reduce the dimension. For a hole or internal dimension，the fundamental dimension is the smallest value permitted,and the variation as given by the tolerance serves to make the dimension larger. A part is said to be at the least material condition (LMC)
14、when the dimensions are all at the limits that give a part with the smallest amount of material .For LMC the fundamental value is the smallest for&it external dimension and the largest for an internal dimension .The tolerances thus provide parts containing larger amounts of material. Maximum materia
15、l tolerances have a production advantage. For an external dimension，should the worker aim at the fundamental or largest value but form something smallThe parts may be rework to bring them within acceptable limitsA worker keeping the mean dimension in mind would have smaller margins for any errorsThe
16、se terms do,however, provide convenient expressions for denoting the different methods for specifying the tolerances on drawings Dimensional variations in manufacturing are unavoidable despite a11 efforts to keep production conditions as constant as possibleThe reasons for the variation in a chosen
17、dimension on parts all made by the same process are of interest .The reasons can usually be grouped into two general classes：assignable cause and chance causes Assignable cacs A small modification in the process can cause variations in adimension . A slight change in the properties of the raw materi
18、al can cause a dimension to varyTools will wear and must be resetChanges may occur in the speedthe lubricantThe temperature，the operator，and other conditions. A systematic search will generally bring such causes to light and steps can then be taken to have them eliminated. Chance cause. Chance cause
19、s,on the other hand, occur at random and are due to vague and unknown forces which can neither be traced nor rectified . They are inherent in the process and occur even thought all conditions have been held as constant possible. When the variations due to assignable causes have been located and remo
20、ved one be one, the desired state of stability or control is attained. If the variations due to chance causes are too great, it is usually necessary to move the operation to more accurate equipment rather than spend more effort in trying to improve the process. Todays technology requires that parts
21、be specified with increasingly exact dimensions. Many parks made by different companies at widely separated locations must be interchangeable，which requires precise size specifications and production. The technique of dimensioning parts within a required range of variation to ensure interchangeabili
22、ty is called tolerancing. Each dimension is allowed a certain degree of variation within a specified zone，or tolerance For example，a partS dimension might be expressed as 200 50，which allows a tolerance(variation in size)of 100 mm. A tolerance should be as large as possible without interfering with
23、the function of the part t0 minimize production COSTS Manufacturing costs increase as tolerances become smaller There are three methods of specifying tolerances on dimensions：Unilateral，bilateral，and limit forms. When plus orminus tolerancing is sed, it is applied to a theoretical dimension called t
24、he basic dimension . When dimensions can vary in only one direction from the basic dimension (either larger or smaller) tolerancing is unilateral. Tolerancing that permits variation in both directions from the basic dimension(1arger and smaller)is bilateral. Tolerances may also be given in limit for
25、m，with dimensions representing the largest and smallest sines for a featurce. Some tolerancing terminology and definitions are given below. Tolerance：the difference between the limits prescribed for a single feature. Basic size：the theoretical sizeForm which 1imits or deviations are calculated. Devi
26、ation：the difference between the hole or shaft size and the basic size. Upper deviation：the difference between the maximum permissible size of a part mad its basic size. Lower deviation：the difference between the minimum permissible size of a part and its basic size. Actual size：the measured size of
27、 the finished part Fit：the tightness between two assembled parts The three types of fit are：clearance，interference and transition. Clearance fit：the clearance between two assembled mating partsInterference fit：results in an interference between tee two assembled parts-the shaft is larger than the ho
28、le，requiring a force or press fit，an effect similar to welding the two parts. Transition fits：may result in either an interference or a clearance between the assembled partsthe shaft may be either smaller or larger than the hole and still be within the prescribed tolerances Selective assembly：a meth
29、od of selecting and assembling parts by trial and error and by hand，allowing parts to be made with greater tolerances at less cost as a compromise between a high manufacturing accuracy and ease of assembly The basic hole system utilizes the smallest hole size as the basic diameter for calculating to
30、lerances and allowances. The basic hole system is efficient when standard drills，reamers，and machine tools are available to give precise hole sizes The smallest hole size is the basic diameter because a hole can be enlarged by machining but not reduced in size The basic shaft system is applicable wh
31、en shafts are available in highly precise standard sizes. The largest diameter of the shaft is the basic diameter for applying tolerances and allowancesThe largest shaft size is used as the basic diameter because shafts Can be machined to smaller size but not enlarged. International tolerance(IT)gra
32、de：a series of tolerances that vary with basic size to provide a uniform level of accuracy within a given gradeThere are 18 IT grades：IT01， ITO，ITl，ITl6. Tolerance symbols：alphanumeric symbols giving the specifications of tolerances and fits； the basic size is a number，followed by the fundamental de
33、viation letter and the IT number，which combined give the tolerance zone；uppercase letters indicate the fundamental deviations for holes，and lowercase letters indicate fundamental deviations for shafts Because the surface texture(or surface finish)of a part affects its function，it must be precisely s
34、pecified Surface texture is the variation in a surface，including roughness，waviness，lay and flaws Roughness：the finest of the irregularities in the surface caused by the manufacturing process used to smooth the surface Roughness height is measured in micrometers(um) or microinches(uin) Waviness：a wi
35、dely spaced variation that exceeds the roughness width cutoff measured in inches 01millimeters；roughness may be regarded as a surface variation superimposed on a wavy surface Lay：the direction of the surface pattern caused by the production method used. Flaws：defects occurring infrequently or at wid
36、ely varying intervals On a surface， including cracks，blow holes，checks，scratches，and the like；the effect of flaws is usually omitted in roughness height measurements. 尺寸公差 工程图样是制造机械零件的依据。因此，从事制造业的人员都要正确理解应用于整个生产过程的图样的含义。 在企业中工作的工程师总要面对这样一个事实，即任何两个机器零件都不能制造得完全相同。他知道在设计中必须考虑在重复性生产中所产生的微小尺寸差异，在图样上标注合适的
37、公差，将尺寸的变化限制在允许的范围内。加工后的零件的外形轮廓必须位于公差规定的区域内。采用适当的公差可以保证产品在功能和使用寿命方面都能达到预期的目标。 每位设计人员都非常清楚，如果零件都以较小的公差来加工制造，则产品的成本就会迅速增加。因此，工程师们不断地得到劝告，要采用尽可能大的公差。然而，有时可能出现对功能要求所需要的各种公差之间的相互关系没有进行充分研究的情况。在这种情况下，为了保证零件在装配时不发生问题，设计人员通常不恰当地将公差规定得过于严格。相对于认真、透彻地对公差进行分析来说，这显然是一个价格昂贵的替代方式。 要使产品以较低的价格被生产出来并且能够满足设计的要求，规定适当的加工
38、公差是最为重要的工作。公差的大小是由设计人员所确定的，它取决于许多与设计有关的条件以及过去在设计类似产品时所获得的经验(如果有这方面经验的话)。如果所规定的公差太小，以至于采用现有的加工设备加工工件的这个尺寸时无法达到设计要求，就需要对设计进行修改。 工程图样中模糊不清的地方会引起很多混乱和经济损失。在拟定公差时，设计人员必须充分认识到，要完全达到其设计目的，图样上必须包含所有必要的信息。因而，图样上必须给出全部信息，并且尽可能地简单明了。图样中的每个部分内容都应该能被大家所理解。图样中所表示的含义对于所有使用它的人员(设计，采购，刀具设计，生产，检验，装配和维修部门)来说都应该是唯一的。 公
39、差在图样上可以采用不同的标注方式。在单向制中，一个极限偏差是零，另一个极限偏差就是尺寸允许的全部变动量。在双向制中尺寸标注中，采用平均尺寸和在其正负两个方向上的变动量来表示。 当所有的尺寸都处在允许零件含有的材料量为最多的极限状态时，就称这个零件处于最大实体状态(MMC)。对于一根轴或者一个外形尺寸，它的基本尺寸为最大极限尺寸，它在公差范围内变动时，只能使尺寸减小。对于一个孔或者内部尺寸，它的基本尺寸为最小极限尺寸，在公差范围内的变动，只能使尺寸增大。 当所有的尺寸都处在允许零件含有的材料量为最小的极限状态时，就称这个零件处于最小实体状态(LMC)。按LMC标注公差时，对于外形尺寸，它的基本尺
40、寸为最小极限尺寸；对于内孔尺寸，它的基本尺寸为最大极限尺寸。在公差范围内的尺寸变动，会使零件包含的材料量增加。按最大实体尺寸标注公差对生产有利。对于一个外形尺寸，工人按照其基本尺寸或最大极限尺寸进行加工，如果其去除量过小，还可以通过重新加工，使工件尺寸在允许的范围内。一个工人按平均尺寸进行加工时，加工偏差只能在小范围内变动。不管怎样，上述概念为以不同方式在零件图样上标注公差提供了方便的表达形式。 在机械制造中，虽然尽可能保持稳定的生产条件，但是加工后获得的尺寸仍然不可避免地出现误差。在完全相同的制造过程中，按某一指定尺寸加工一批零件，加工后所得到的尺寸却并不完全相同。产生这种现象的原因值得人们
41、研究。一般将产生这种现象的原因分两大类，系统原因和随机原因。 系统原因 生产过程中某些因素的微小变动可以引起尺寸变化。原材料性能的微小变化可以引起尺寸变化。刀具受到磨损并且需要重新安装。速度、润滑剂、刀削温度、操作人员以及其他条件都会发生变化。通过系统地分析研究，一般可以找出这些原因并可采取相应的步骤来消除它。 随机原因 另一方面随机原因的出现是具有偶然性的。它们是由一些既无法确定又不能控制的力所造成的。它们是生产过程的固有误差，即使尽可能地保持所有条件完全一致，它们仍然不可避免地存在。 当依次检查由系统原因造成的误差，并且将其逐一排除后，即可达到理想的稳定状态或控制状态。如果随机原因对尺寸变
42、化的影响过大，一般来说采用更精密的加工设备，要比花费更多精力来改变生产过程更为有效。 现代技术对零件尺寸精度的要求越来越严格。而且，目前许多零件是由散布在各地的不同厂家生产的，因此必须对这些零件的尺寸和生产做出严格的规定，以保证它们具有互换性。 给零件标注尺寸使其在一个规定的区间内变动，以保证它们具有互换性的技术称为公差技术。允许每个尺寸在规定范围内具有一定的变动量，称为公差。例如，一个零件的尺寸可以被表示成200 5，其公差(尺寸变动量)为1.00mm。 在不影响零件性能的情况下，应当给尺寸尽可能大的公差，以把生产成本降至最低。制造成本会随着公差的降低而升高。 有三种表示尺寸公差的方式：单向
43、，双向和极限方式。当采用正负公差时，就将公差加到被称为基本尺寸的理论尺寸上去。当只允许尺寸有向基本尺寸的单一方向(或者变大，或者变小)的变动时，就是单向公差。在尺寸可以在基本尺寸的两个方向(变大或者变小)都可以变动时，公差就是双向的。公差也可以用极限形式给出，表示零件外形的最大和最小尺寸。 一些与公差有关的术语和定义如下所述。 公差：为某个尺寸所规定的上限与下限之间的差值。 基本尺寸：理论尺寸，是计算极限尺寸和偏差的起始尺寸。 偏差：孔的尺寸或者轴的尺寸减去基本尺寸所得的差值。 上偏差：零件最大极限尺寸减去其基本尺寸所得的差值。 下偏差：零件最小极限尺寸减去其基本尺寸所得的差值。 实际尺寸：加
44、工后零件的实测尺寸。 配合：两个装配在一起的零件之间的松紧程度。可以把配合分为三类：间隙配合，过盈配合，过渡配合。 间隙配合：两个装配在一起配件之间留有间隙的配合。 过盈配合：两个装配在一起的零件之间有过盈的配合轴大于孔，需要用力或压力进行配合，具有类似于将两个零件焊接在一起的效果。 过渡配合：在两个装配在一起的零件之问或者存在着过盈，或者存在着间隙的配合轴可以小于或大于孔，但仍在规定的公差内。 选择装配：通过手工试配来选择并装配零件的方法。通过这种方法，可以装配在较低的成本下制造出来的公差较大的零件，它可作为高的制造精度和易于装配的零件之间的一种折中方法。 基孔制：采用最小的孔的尺寸作为计算
45、公差和加工余量基本尺寸。当采用标准的钻头、铰刀和机床对孔进行精加工时，基孔制系统是非常有效的。采用最小的孔的尺寸作为基本尺寸是因为孔的尺寸可以通过机械加工变大，但不能减小。 当轴可以按照非常的高精度的标准尺寸提供时，采用基轴制是适用的。计算公差和余量时，采用轴的最大直径作为基本尺寸。这是因为轴可以通过加工变成较小的尺寸，但其尺寸不能增大。 国际公差()等级：一系列随基本尺寸变化，且在规定等级内提供均匀精度的公差。共有18个IT等级：IT01，IT0，IT1，ITl6。 公差符号：字母数字符号给出了公差与配合的技术要求，基本尺寸是一个数字，后面跟着表示基本偏差的字母和表示IT等级的数字。它们共同
46、决定公差带的大小和位置。大写字母代表孔的基本偏差，小写字母代表轴的基本偏差。 由于表面形貌(即表面光洁程度)会影响零件的性能，因此对其大小必须精确地加以规定，表面形貌是表面上的差异，包括粗糙度，波度，加工纹理方向和缺陷。 粗糙度：由用来使工件表面光滑的加工工艺所造成的最细微的表面不平度。表面粗糙度的高度采用微米或微英寸作测量的单位。 波度：是超过粗糙度宽度界限的大间隔偏差，采用英寸或毫米测量。可将粗糙度看做叠加在波度表面上的表面不平度。 加工纹理方向：由所采用的加工方法所产生的表面刀痕图案的方向。 缺陷：不经常出现或者在很大区间内才会出现的表面瑕疵，其包括裂纹、气孔、微细裂纹，划痕等。缺陷的影
47、响通常在粗糙度的高度测量中被忽略。五分钟搞定5000字毕业论文外文翻译，你想要的工具都在这里!在科研过程中阅读翻译外文文献是一个非常重要的环节，许多领域高水平的文献都是外文文献，借鉴一些外文文献翻译的经验是非常必要的。由于特殊原因我翻译外文文献的机会比较多，慢慢地就发现了外文文献翻译过程中的三大利器：Google“翻译”频道、金山词霸（完整版本）和CNKI“翻译助手。具体操作过程如下： 1.先打开金山词霸自动取词功能，然后阅读文献； 2.遇到无法理解的长句时，可以交给Google处理，处理后的结果猛一看，不堪入目，可是经过大脑的再处理后句子的意思基本就明了了； 3.如果通过Google仍然无法理解，感觉就是不同，那肯定是对其中某个“常用单词”理解有误，因为某些单词看似很简单，但是在文献中有特殊的意思，这时就可以通过CNKI的“翻译助手”来查询相关单词的意思，由于CNKI的单词意思都是来源与大量的文献，所以它的吻合率很高。 另外，在翻译过程中最好以“段落”或者“长句”作为翻译的基本单位，这样才不会造成“只见树木，不见森林”的误导。四大工具： 1、Google翻译： google，众所周知，谷歌里面的英文文献和资料还算是比较详实的。我利用它是这样的。一方面可以用它查询英文