TABLE OF CONTENTS It is not possible to work to an exact size nor is it possible to measure to an exact size. Therefore dimensions are given limits of size. Providing the dimensions of a part lie within the limits of size set by the designer, then the part will function correctly. Similarly the dimensions of gauges and measuring equipment are given limits of size. As a general rule, the limits of size allocated to gauges and measuring instruments are approximately 10 times more accurate than the dimensions they are intended to check (gauges) or measure (measuring instruments).
The upper and lower sizes of a dimension are called the limits and the difference in size between the limits is called the tolerance. The terms associated with limits and fits can be summarized as follows:
Nominal size: This is the dimension by which a feature is identified for convenience. For example, a slot whose actual width is 25.15 mm would be known as the 25-mm wide slot.
Basic size: This is the exact functional size from which the limits are derived by application of the necessary allowance and tolerances. The basic size and the nominal size are often the same.
Actual size: The measured size corrected to what it would be at 20 C.
Limits: These are the high and low values of size between which the size of a component feature may lie. For example, if the lower limit of a hole...
