Tolerance is the permitted range of error for a part dimension.
Because machining always produces some slight error, drawings specify an allowable range within which the part passes.
That range is the tolerance.
A Closer Look
For example, if a dimension is specified as "diameter 10 mm ±0.01," any part between 9.99 mm and 10.01 mm passes.
The smaller (tighter) the tolerance, the higher the machining precision required.
Tolerances are set according to the function of the part.
The more exactly parts must fit together, the tighter the tolerance.
Through program control and in-machine measurement, a CNC automatic lathe can mass-produce tight-tolerance parts consistently.
Separate from dimensional tolerances, there are also geometric tolerances such as roundness, which express the accuracy of form.