Why must a turning axle be slightly smaller than its hole?

A wheel needs room to turn around an axle or with an axle inside a support. If a shaft and its hole have exactly the same drawn width, the real pieces may fit too tightly. Makers deliberately allow a small difference called clearance where free movement is needed.

Real parts are never made to one perfectly exact size. A drawing can therefore specify a permitted range, called a tolerance. For a simple teaching example, a shaft might be allowed to measure from 9.8 to 9.9 millimeters, while its hole may measure from 10.0 to 10.1 millimeters. Even the largest permitted shaft is smaller than the smallest permitted hole.

The smallest difference in this example is 0.1 millimeter. The largest is 0.3 millimeter. Designers must decide whether both are suitable. Too little clearance may make movement difficult; too much may let the part wobble. A tolerance is therefore a carefully chosen allowance, not permission for careless work.

Other joints need a tight fit rather than free movement. Their size ranges are chosen differently. Shape, surface finish, temperature and material also affect how real parts fit. A long bent shaft can cause trouble even if one width measurement looks correct.

Makers measure finished parts to check the important sizes. By designing acceptable ranges before production, they can make many pieces that work together. The useful achievement is not making everything exactly the same. It is controlling differences well enough that the intended movement still works.