Machine parts are manufactured so they are interchangeable. In other words, each pert 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 individual part must be made to a size that will fit the mating part in the correct way.

Limits. Limits are the maximum and minimum values prescribed for a specific dimension. For instance, if the nominal diameter of a hole is 1 in., and the limits of size are expressed as 0.999/1.001 then any work not over 1.001 in. nor under 0.999 in. will be accepted.

Tolerance. The difference between upper and lower limits is called the tolerance. With a maximum limit of 1.001 in. and a minimum limit of 0.999 in., the tolerance would be 0.002 in. Tolerances may be shown on drawings by several different methods.

Allowance. Allowance applies particularly to fits, as in the case or a hole and a shaft. It is the intentional difference between maximum limits of mating parts to accomplish the desired fit. The machining allowance is the layer of metal removed in machining a surface. Besides "design tolerance", there are also "operational tolerances. " An operational tolerance is the specified machining accuracy of a surface in an intermediate operation followed by one or several more operations for machining the same surface. The machining allowances depend upon the method employed for making the blank. These methods vary for various scales of production.

Fits. In machine construction, many of the parts have close and important relation to one another, and a certain amount of hand-fitting is essential. Fits can be classed as sliding fits, running fits, forced fits and shrink fits.