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 Fig14.1. It is therefore important that the tolerances be specified at the largest values that the operating or functional considerations permit.
虽然较小公差能得到较高加工质量和较好操作机构.但随着公差的减小制造成本会迅速增加.如图14.1的典型曲线所示。因此公差被定为从操作或功能考虑允许的最大值是重要的。
Tolerances may be either unilateral or bilateral. In unilateral dimensioning, one tolerance is zero, and all the variations are given by the other tolerance. In bilateral 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.
公差既可以是单向的也可以是双向的。单向标注有一公差为零.所有变化都由另一公差给定。而双向标注则采用一平均尺寸.它将公差带中点从该尺寸双向扩展为相等的正负变化范围。
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.
大规模低成本制造生产工艺的发展很大程度取决于组成零件的互换性。因此设计者必须确定单个零件的合适公差以及配合零件装配允许的正确间隙或过盈量。
The manner of placing tolerances on drawings depends somewhat on the kind of product or type of manufacturing process. If the tolerance on a dimension is not specifically stated, the drawing should contain a blanket note which gives the value of the tolerance for such dimensions.
在图纸上标注公差的方法相当程度上依赖于产品的性质或制造工艺的类型。如果尺寸公差没有特别注明.图纸应该包含一个给出这些尺寸公差值的普遍适用注释。
However, some companies do not use blanket notes on the supposition that if each dimension is considered individually, wider tolerances than those called for in the note could probably be specified. In any event it is very important that a drawing be free from ambiguities and be subject only to a single interpretation.
然而有些公司不采用普遍适用注释.假定每个尺寸是单独被考虑的.可能会规定出比注释中要求的更宽的公差。在任何情况下图纸不模棱两可并只服从于单一的解释是十分重要的。