Machine parts are manufactured so they are interchangeable. In other words, each part of a machine or mechanism is made to a certain size and shape so it will fit into any other machine or mechanism of the same type. To make the part interchangeable, each inpidual part must be made to a size that will fit the mating（配对） part in the correct way. It is not only impossible, but also impractical（不实际的） to make many parts to an exact（精确的） size. This is because machines are not perfect（完美的）, and the tools become worn. A slight variation（变化） from the exact size is always allowed. The amount of this variation depends on the kind of part being manufactured. For example, a part might be made of 6 in. long with a variation allowed of 0.003 (three-thousands) in. above and below this size. Therefore, the part could be 5.997 to 6.003 in. and still be correct size. These are known as the limits（尺寸极限）. The difference（差别） between upper and lower limits is called the tolerance（公差）.
A tolerance is the total permissible variation（允许偏差） in the size of a part.
The basic size（基本尺寸） is that size from which limits of size are derived（得到） by the application of allowance and tolerances.
Sometimes the limit is allowed in only one direction. This is known as unilateral tolerance（单向公差）.
Unilateral tolerance is a system of dimensioning where the tolerance (that is variation) is shown in only one direction from the nominal size.
Unilateral tolerance allows the changing of tolerance on a hole or shaft without seriously affecting the fit.
When the tolerance is in both direction from the basic size, it is known as a bilateral tolerance （双向公差）(plus and minus).
Bilateral tolerance is a system of dimensioning where the tolerance (that is variation) is split and is shown on either side of the nominal size.
Limit dimensioning is a system of dimensioning where only the maximum and minimum dimensions are shown. Thus, tolerance is the difference between these two dimensions.
Vocabulary
1.interchangeable a. 可交换的；可拆卸的；通用的
2.impractical a. 不实用的；不切实际的
3.worn v. wear的过去式；a.磨损的
4.derive v. 从…得到[出]；推导出
5.slight a. 轻的；很小的
6.the exact size 准确的尺寸
7.upper and lower limits 上、下极限尺寸
8.tolerance n. 公差；容差
9.variation n. 变化；变量；偏[误]差
10.unilateral n. 单向[边，方面]的
11.allowance n. 允许（量）；许可（限度）
12.seriously ad. 严格地；严重地
13.affect vt. 影响；对…起作用
14.fit v. 装配； 配合； 安装
15.split v. 分开；剖分
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