The machine shop is the shop on which devolves the requirement of finishing work to specified dimensions within very small limits of allowable error.

The discerning of small differences in physical measurements is dependent upon the degree of refinement of measuring instruments, upon variations of temperature, and upon the delicacy of the sense of touch.

All products turned out by the machine shop do not require the same degree of exactness in finished dimensions, and it would be a needless expense to finish all work with the same high degree of precision required for some grades of work. Most of the refined finishing to a particular dimension is done not closer than 1/1000 of an inch. A drawing of a piece of work which is to be finished with especial care for making a close fit must state limits of error allowable and on which side of the given dimension this is allowed.

A wheel hub may be forced or driven tightly on its shaft. This is a driving or forcing fit. If the hole in the hub is bored slightly smaller than the diameter of the shaft, the hub may be expanded by heat until it fits over the shaft. On cooling it grips the shaft and makes a shrinkage fit.* A shaft may fit more or less closely in the bearing in which it revolves. If the fit is not too tight to prevent free motion, it is called a working or sliding fit.

Forcing fits are made by hydraulic forcing presses, or by the pull of heavy bolts.

The degree of refinement necessary in a working fit for any moving part of a machine depends upon the size and degree of refinement needed in the machine. The main bearing of a marine engine is usually adjusted to about .012 inch. The main spindle of a machine lathe is about .002" smaller than the bearing in which it revolves, and a very accurate grinding machine is adjusted as close as the metal of a carefully ground shaft and bearing can come together without gripping.