It expresses the numerical value of a length in a particular unit on the part. The basic
size of a part is its nominal dimension from where all variations are generally made.
It is determined by the part designer from its functional requirements to meet the specified
objective. The other term used with respect to a part is the nominal size.
The nominal size is the size of the part specified in the drawing as a matter of convenience. It is used primarily for the purpose of identification of a component and is never used in the precision measurement of parts.
A rigid attitude towards the maintenance of a basic size of the part may increase the
manufacturing cost and a little variation in dimension is accepted resulting in a size, which
is different from the basic size of the part. This is called the actual size.
The actual size of a dimension or part is its measured size. An actual size of a ready part will, therefore, always deviate from one specified in the drawing, i.e. from the nominal or basic size of the part. Whereas the difference between the basic size and actual size must not exceed from a certain limit otherwise it will interfere with the interchangeability of mating components during assembly or sub assembly of parts.
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