36. ASSEMBLY

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the mating of parts to give a combined operation.

In previous centuries, and before, fit in assemblies was often not considered, or when it did matter, each piece was custom fitted.

Modern methods of mass production means that some fundamental methods of fitting are necessary.

The three basic methods of fitting are,

- Fitting - One part is made to size, and then a second part is made to fit it.

- Selective Assembly - parts are made to loose general tolerances, and then the results are sorted into tolerance ranges. (e.g. bearings, solar cells, etc)

- Interchangeable Assembly - Parts are made to tighter tolerances, and as a result assembly of randomly selected parts will yield a good fit. This is essential for modern assembly lines.

The basic categories of fits are,

- Clearance - a gap is always present between parts

- Transitional - the parts will have a gap sometimes, other times the parts will touch

- Interference - both parts will always be in full contact