Positional tolerancing

The essential requirement is to be able to define the limits for location of actual features, e.g. axes, points, median surfaces and nominally plane surfaces, relative to each other or in relation to one or more datum.

To accurately achieve this aim, it is essential that the primary constituents, theoretically exact dimensions, tolerance zones, and datums are utilized. The tolerance zone is symmetrically disposed about its theoretically exact location.

Utilizing these primary constituents ensures positional tolerances do not accumulate when dimensions are arranged in a chain, as would be the case if the feature pattern location were to be specified by coordinate tolerances.

Note: The practice of locating groups of features by positional tolerancing and their pattern location by coordinate tolerances, is no longer recommended by BS 8888 and BS EN ISO 5458.

Figure 23.1 illustrates the advantage of specifying a circular tolerance zone to a feature located by positional tolerancing. Note that the shaded tolerance area represents an increase of more than 57%.

Tolerance Bs8888 Drawing

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