Interchangeable Parts - Selective Assembly

Selective Assembly

Interchangeability relies on parts' dimensions falling within the tolerance range. The most common mode of assembly is to design and manufacture such that, as long as each part that reaches assembly is within tolerance, the mating of parts can be totally random. This has value for all the reasons already discussed earlier.

There is another mode of assembly, called selective assembly, which gives up some of the randomness capability in trade-off for other value. There are two main areas of application that benefit economically from selective assembly: when tolerance ranges are so tight that they cannot quite be held reliably (making the total randomness unavailable); and when tolerance ranges can be reliably held, but the fit and finish of the final assembly is being maximized by voluntarily giving up some of the randomness (which makes it available but not ideally desirable). In either case the principle of selective assembly is the same: parts are selected for mating, rather than being mated at random. As the parts are inspected, they are graded out into separate bins based on what end of the range they fall in (or violate). Falling within the high or low end of a range is usually called being "light" or "heavy"; violating the high or low end of a range is usually called being oversize or undersize. Examples are given below.

French and Vierck provide a one-paragraph description of selective assembly that aptly summarizes the concept.

One might ask, if parts must be selected for mating, then what makes selective assembly any different from the oldest craft methods? But there is in fact a significant difference. Selective assembly merely grades the parts into several ranges; within each range, there is still random interchangeability. This is quite different from the older method of fitting by a craftsman, where each mated set of parts is specifically filed to fit each part with a specific, unique counterpart.