Limits The maximum and minimum dimensions for a given feature are known as the 'limits'. For example, 20 ± 0.1.
The upper and lower limits of size are 20.1 mm and 19.9 mm respectively.
Tolerance The algebraic difference between the upper
and lower limit of size is known as the 'tolerance'. In the example above, the tolerance is 0.2 mm. The tolerance is the amount of variation permitted.
Nominal dimension Limits and tolerances are based on 'nominal dimensions' which are target dimensions. In practice there is no such thing as a nominal dimension, since no part can be manufactured to a theoretical exact size.
The limits referred to above can be set in two ways:
(a) unilateral limits - limits set wholly above or below the nominal size;
(b) bilateral limits - limits set partly above and partly below the nominal size.
Geometrical tolerance These tolerances specify the maximum error of a component's geometrical characteristic, over its whole dimensioned length or surface. Defining a zone in which the feature may lie does this.
Tolerance zone A tolerance zone is the space in which any deviation of the feature must be contained.
the space between two concentric circles;
the space between two equidistant lines or two parallel straight lines;
the space within a cylinder;
the space between two coaxial cylinders;
the space between two equidistant surfaces or two parallel planes;
the space within a sphere.
The tolerance applies to the whole extent of the considered feature unless otherwise specified.
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