英语翻译Drawings should be carefully checked to see that the dim

英语翻译
Drawings should be carefully checked to see that the dimensioning is done in a manner that will be most convenient and understandable to the production departments.It is obvious that a drawing should be made in such a way that it has one and only one interpretation .
Tolerances must be placed on the dimensions of a drawing to limit the permissible variations in size because it is impossible to manufacture a part exactly to a given dimension .Although small tolerances give higher quality work and a better operating mechanism,the cost of manufacture increases rapidly as the tolerances are reduced,as indicated by the typical curve of Fig 14.1.It is therefore important that the tolerances be specified at the largest values that the operating or functional considerations permit.
Tolerances may be either unilateral or bilateral.In unilateral dimensioning,one tolerance is zero,and all the variations are given by the other tolerance.Inbilateral dimensioning,a mean dimension is used which extends to the midpoint of the tolerance zone with equal plus and minus variations extending each way from this dimension.
The development of production processes for large-volume manufacture
at low cost has been largely dependent upon interchangeability of component parts.Thus the designer must determine both the proper tolerances for the individual parts,and the correct amount of clearance or interference to permit assembly with the mating parts.The manner of placing tolerance on drawings depends somewhat on the kind of product or type of manufacturing process
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 2.5 in.（63.5mm）,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 +0.003in（+0.08mm）could be permitted,the dimension would be stated 2.500+0.003（63.5+0.08mm）
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.

图纸应仔细核对,尺寸应以易于生产部门理解的方式来标注.很显然,图纸应以这种方式来绘制,而且这也是唯一的一种方式.
公差必须标注在图纸尺寸上,以约束尺寸的允许偏差,因为完全按照给出的尺寸来制造工件是不可能的.尽管小的公差会达到高水平的质量及加工精度,但成本也相应大大增加,如14.1图形中的曲线.因此,在考虑加工和性能的同时尽可能将公差确定在最大值.
公差可以是单向的也可以是双向的.在单向尺寸中,一个公差是零,所有的偏差都由其他的公差给出.双向的尺寸,是以一个平均的尺寸为中心点向两个方向以相同的加减数值扩散.

生产工艺的发展,以适应低成本大批量的制造在很大程度上取决于零件的互换性.因此设计者必须给各工件确定合适的公差及游隙或干涉值以使其可以很好地装配在一起.图纸上公差标注方式会因产品类型或制造工艺的不同而有所不同.
确定精度值,需将公差值加在尺寸上.公差是工件允许的偏差值或所给尺寸的全部偏差值总和.轴可能会有2.5 in.（63.5mm）的名义值,但由于一些实际的原因,如没有较高的成本,该数值在制造过程中不能被保持.因此,需要加一个适当的公差值,如果允许 +0.003in（+0.08mm）的偏差,该尺寸就被标注为2.500+0.003（63.5+0.08mm） .
如尺寸所给公差很接近则表示该工件必须与其他工件很好地匹配,都必须给出公差以保证所要求的留量.已有的制造工艺,以最低的成本来生产和装配将达到最大的收益.总的来说,公差减小则工件成本增加.如果一个工件有几个或更多的加工面,名义尺寸有很小的偏差则其成本会很高.