机械专业毕业设计外文翻译--极限与误差(编辑修改稿)

机械专业毕业设计外文翻译--极限与误差(编辑修改稿)内容摘要：

ent any degree of accuracy unless so stated by the designer. To specify a degree of accuracy, it is necessary to add tolerance figures to the dimension. Tolerance is the amount of variation permitted in the part or the total variation allowed in a given dimension. A shaft might have a nominal size of in. (), but for practical reasons this figure could not be maintained in manufacturing without great cost. Hence, a certain tolerance would be added and , if a variation of 177。 in.(177。 ) could be permitted, the dimension would be stated 177。 (177。 ). Dimensions given close tolerances mean that the part must fit properly with some other part. Both must be given tolerances in keeping with the allowance desired, the manufacturing processes available, and the minimum cost of production and assembly that will maximize profit. Generally speaking, the cost of a part goes up as the tolerance is decreased. If a part has several or more surfaces to be machined, the cost can be excessive when little deviation is allowed from the nominal size. Allowance, which is sometimes confused with tolerance, has an altogether different meaning. It is the minimum clearance space intended between mating parts and represents the condition of tightest permissible fit. If a shaft, size  , is to fit a hole of size  , the minimum size hole is and the maximum size shaft is . Thus the allowance is and the maximum clearance is as based on the minimum shaft size and maximum hole dimension. Tolerances may be either unilateral or bilateral. Unilateral tolerance means that any variation is made in only one direction from the nominal or basic dimension. Referring to the previous example, the hole is dimensioned  , which represents a unilateral tolerance. If the dimensions were given as 177。 , the tolerance would be bilateral。 that is , it would vary both over and under the nominal dimension. The unilateral system permits changing the tolerance while still retaining the same allowance or type of fit. With the bilateral system, this is not possible without also changing the nominal size dimension of one or both of the two mating parts. In mass production, where mating parts must be interchangeable, unilateral tolerances are customary. To have an interference or fore fit between mating parts, the tolerances must be such as to create a zero or negative allowance. Tolerances Limits and Fits The drawing must be a true and plete statement of the designer’s expressed in such a way that the part is convenient to manufacture. Every dimension necessary to define the product must be stated once and repeated in different views. Dimensions relating to one particular feature, such as the position and size of hole, where possible, appear on the same view. There should be no more dimensions than are absolutely necessary, and no feature should be located by more than one dimension in any direction. It may be necessary occasionally to give an auxiliary dimension for reference, possibly for inspection. When this is so, the dimension should be enclosed in a bracket and marked for reference. Such dimensions are not governed by general tolerances. Dimensions that affect the function of the part should always be specified and not left as the sum or other dimensions. If this is not done, the total permissible variation on that dimension will form the sum or difference of the other dimensions and their tolerance, and this with result in these tolerances having to be made unnecessarily tight. The overall dimension should always appear. All。