Location Tolerances

Abstract

This chapter describes how to use location tolerances to specify an allowable location error. The location tolerances that should be used, in function of the feature of the workpiece that has to be located, are in particular presented. The effects of the material condition requirement and the correct choice of the modifiers for position tolerances (axis and surface interpretation) are illustrated. Particular emphasis is given to the location of the patterns, as introduced with the new rules of the ISO 5458:2018 standard (multiple indicator pattern specification and simultaneous requirement). When geometrical position tolerances are applied, the value of the tolerance is calculated from the mating conditions (fixed and floating fastener conditions). The ASME standards specify, without any shadow of doubt, that the position tolerance symbol should only be utilised for a “feature of size”, while the ISO standards allow it to be used to position a planar surface. The ISO standards are defined as “CMM Friendly”, that is, the preferred control system is the coordinate measurement machine, while the ASME standards are based on functional gauges that represent a physical representation of the tolerance zone. A special functional application of location tolerances is to control concentricity and symmetry. These controls have been removed from the new ASME standard.