生产成本降至最低。制造成本会 随着公差的降低而升高。
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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 both directions from the basic dimension (larger and smaller) is bilateral.
有三种表示尺寸公差的方式：单向，双向
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.
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 术语和定义如下所述。
Tolerance: the difference between the limits prescribed for a single feature.
Basic size: the theoretical size, from
which limits or deviations are calculated.
公差：为某个单一要素所规定的上限与下 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。
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Deviation: the difference between the hole or shaft size and the basic size.
偏差：孔的尺寸或者轴的尺寸减去基本 尺寸所得的差值。
Upper deviation: the difference
between the maximum permissible size of a part and its basic size.
上偏差：零件最大极限尺寸减去其基本 尺寸所得的差值。
Lower deviation: the difference
between the minimum permissible size of a part and its basic size.
下偏差：零件最小极限尺寸减去其基本 尺寸所得的差值。
Actual size: the measured size of the finished part.
实际尺寸：加工后零件的实测尺寸。
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Fit: the tightness between two
assembled parts. The three types of fit are: clearance, interference and transition.
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t类：间隙配合，过 盈配合，过渡配合。
Clearance fit: the clearance between two assembled mating parts.
间隙配合：两个装配在一起配件之间留有 间隙的配合。
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.
过盈配合：两个装配在一起的零件之间有 过盈的配合——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 在一起的效果。
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.
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。
Selective assembly: a method of
selecting and assembling parts by trial and error[1] 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.
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。
间隙配合、过盈配合和过渡配合
0 +
-基本尺寸
孔
轴
孔
轴
孔 轴
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The basic hole system [2] 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 because a hole can be enlarged by machining but not reduced in size.
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。
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