注塑模具毕业设计文献翻译.doc

Dimensional Tolerances and Surface Roughness The manufacture of machine parts is founded on the engineering drawing. Everyone engaged in manufacturing has a direct or indirect interest in understanding the meaning of the drawings on which the entire production process is established. The engineer in industry is constantly fated with the fact that no two machine parts can ever be made exactly the same. He learns that the small variations that occur in repetitive production must be considered in the design so that the tolerances placed on the dimensions will restrict the variations to acceptable limits. The tolerances provide 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 components are made smaller. Designers are constantly admonished to use the widest tolerances possible. Situations may arise, however, in which the relationship between the various tolerances required for proper functioning has not been fully explored. Under such conditions the designer is tempted to specify part tolerances that are unduly tight in the hope that no difficulty will arise at the time of assembly. This 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 product is to achieve its intended purpose and yet be economical to produce. The 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 processes and machine capabilities is of great assistance in helping to determine the tolerances in the most effective manner. 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 drawing can be cause of much confusion and expense. When specifying the tolerances, the designer must keep in mind that the drawing must contain all requisite information if the designer's intent is to be fully realized. The drawing must therefore give complete information and at the same time be as simple as possible. The detail of drawing must be capable of being universally understood. The drawing must have one and only one meaning to everyone who will use it - the design, purchasing, tool design, production, inspection, assembly, and servicing departments. Tolerances may be placed on the drawing in a number of different ways. In the unilateral 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 that 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 purpose 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 material. For a shaft or external dimension, the fundamental dimension is the largest value permitted, and all the variation, 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) 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 an external dimension and the largest for an internal dimension. The tolerances thus provide parts containing larger amounts of material. Maximum material tolerances have a production advantage. For art external dimension, should the worker aim at the fundamental or largest value but form something small, the parts may be rework to bring them within acceptable limits. A worker keeping the mean dimension in mind would have smaller margins for any errors. These terms do, however, provide convenient expressions for denoting the different methods for specifying the tolerances on drawings. Dimensional variations in manufacturing are unavoidable despite all efforts to keep production conditions as constant as possible. The reasons for the variation in a chosen 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 causes. A small modification in the process can cause variations in a dimension. A slight change in the properties of the raw material can cause a dimension to vary. Tools will wear and must be reset. Changes may occur in the speed, the lubricant, the temperature, the operator, and other conditions. A systematic search will generally bringsuch muses to light and steps can then be taken to have them eliminated. Chance causes. Chance causes, 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 though all conditions have been held as constant as possible. When the variations due to assignable causes have been located and removed one by 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. Today's technology requires that parts be specified with increasingly exact dimensions. Many parts 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 interchangeability is called tolerancing. Each dimension is allowed a certain degree of variation within a specified zone, or tolerance. For example, a part's dimension might be expressed as 20 ± 0.50, which allows a tolerance (variation in size) of 1.00 mm. A tolerance should be as large as possible without interfering with the function of the part to 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-or-minus tolerancing is used, it is applied to a theoretical dimension called the 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 limit directions from the basic dimension (larger and smaller) is bilateral. Tolerances may also be given in limit form, with dimensions representing the largest and smallest sizes for a feature. Some tolerancing terminology and definitions are given below. Tolerance: the difference between the limits prescribed for a single feature. Basic size: the theoretical size, form which limits or deviations are calculated. Deviation: the difference between the hole or shaft size and the basic size. Upper deviation: the difference between the maximum permissible size of a part and 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 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 parts. Interference fit: results in an interference between the two assembled parts-the shaft is larger than the hole, 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 parts-the shaft may be either smaller or larger than the hole and still be within the prescribed tolerances. Selective assembly: a method 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 tolerances 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 bemuse a hole can be enlarged by machining but not reduced in size. The basic shaft system is applicable .when shafts are available in highly precise standard sizes. The largest diameter of the shaft is the basic diameter for applying tolerances and allowances. The largest shaft size is used as the basic diameter because shafts can be machined to smaller size but not enlarged. International tolerance (IT) grade: a series of tolerances that vary with basic size to provide a uniform level of accuracy within a given grade. There are I8 IT grades: IT01,IT0, IT1 . IT16. Tolerance symbols: notes giving the specifications of tolerances and fits; the basic size is a number, followed by the fundamental deviation letter and the IT number, which combined give the tolerance zone; uppercase letters indicate the fundamental deviations for holes, andlowercase letters indicate fundamental deviations for shafts. Because the surface texture (or surface finish) of a part affects its function, it must be precisely specified. 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 microinehes(uin). Waviness: a widely spaced variation that exceeds the roughness width cutoff measured in inches or millimeters; 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 widely 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.尺寸与表面粗糙度工程图样是制造机器零件的依据。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画对这样一个事实，即任何两个机器零件都不能制造得完全相同。他知道在没汁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汁要求，就需要对设计进行修改。 工程图样中模糊不清楚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)来说都应该是惟一的。 公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、切削温度、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20±05，其公差(尺寸变动量)为100mm。 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，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到的差值。 实际尺寸：加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：两个装配在起配件之间留有间隙的配合。 过盈配合：两个装配在起的零件之间有过盈的配合一一轴大于孔，需要用力或压力进行配合，具有类似于将两个零件焊接在起的效果。 过渡配合：在两个装配在一起的零件之间或者存在着过盈，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(IT)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：ITOI，ITO，IT1，ITl6。 公差符号：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进行测量。 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。缺陷的影响通常在粗糙度的高度测量中被忽略。