Chapter 23 – Positional tolerancing

Publisher Summary True-position dimensioning defines the exact location on a component of features such as holes, slots, and keyways and also differentiates between ideal and other toleranced dimensions. True-position dimensions are always shown as boxed on engineering drawings and they are never individually toleranced, but must be accompanied by a positional or zone tolerance for the feature to which they are applied. The positional tolerance is the permitted deviation of a feature from a true position. The positional-tolerance zone defines the region which contains the extreme limits of position and which can be rectangular, circular, or cylindrical. The practice of locating groups of features by positional tolerancing and their pattern location by coordinate tolerances are no longer recommended by BS 8888 and BS EN ISO 5458. The method of positional tolerancing has several advantages. The interpretation is easier, as true boxed dimensions fix the exact positions of details and there are no cumulative tolerances. It permits the use of functional gauges to match the mating part, and it can ensure interchangeability without resorting to small position tolerances, required by the coordinate tolerancing system.