Fits and Clearances

For ease of manufacture and replacement, it is essential that the components of similar mechanisms should be interchangeable. For this reason, tolerance and allowance limits are imposed on the manufacturing procedures.

The limits on dimensions ensure if any two mating parts are manufactured to the tolerances and allowances, stated on the drawing. Then they will assemble without the need of further hand working or machining to achieve the required standard of fit. Because shafts are much easier than holes to machine small diameters for engineering purposes and is usually the ‘hole-based’ system. The holes are created to a certain tolerance and the sizes of the shafts are altered to provide the required class of fit between the two items.