Dimension and Tolerance
尺寸和公差
In dimensioning a drawing, the numbers placed in the dimension lines represent dimension that are only approximate and do not represent any degree of accuracy unless so stated by the designer.
在图纸标注尺寸时,除非设计者有意标明,注在尺寸线上的数字代表的尺寸仅仅是近似的,并不代表任何精度等级。
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.
为了详细标明精度等级,有必要在尺寸上增加公差数字。公差是零件允许的变动量或给定尺寸允许的总变动。
A shaft might have a nominal size of 2.5in.(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).
一根轴可能的名义尺寸为2.5in.(63.5mm),但由于实际原因不用大成本是不能在制造中保持这个数字的,因此要增加确定的公差。如果允许有±0.003in.(±0.08mm)的变化,则此尺寸可表达为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.
具有紧密公差的尺寸表示该零件必须恰当地与某些其它零件配合。所采用的制造工艺和使利润最大化的最小生产及装配成本都要求给定公差以保持所需允差。
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.
一般而言,零件的成本随着公差的减小而上升。如果一个零件有若干或较多表面要机加工且几乎不允许偏离名义尺寸,则成本会超过正常合理的界限。