Geometrical Tolerancing

  Geometrical Tolerance is the general term applied to the category of tolerances used to control form, profile, orientation, location and runout, (ASME Y14.5M-1994).
True Geometrical Counterpart represents the theoretically perfect boundary (virtual condition or actual mating envelope) or best-fit (tangent) plane of a specified datum feature, (ASME Y14.5M-1994).
The geometrical tolerancing is divided into four types (by both ISO and ASME/ANSI):
  • Form tolerances
  • Orientation tolerances
  • Location or position tolerances
  • Runout tolerances
Geometrical tolerance objective is the boundary of spaces in which the toleranced feature has to be located with regards to the specified datum or datum system, to meet the tolerance specification. These particular tolerances allow to limit either actual feature defects or fitted features, with respect to nominal characteristics, and without considering the features' dimensions.
Geometrical tolerancing is based on three feature types:
  Tolerance features: a toleranced feature is an actual feature (point, line, surface, except for projected tolerance), or a fitted or a constructed feature. If the toleranced feature corresponds to a group, then each component of the group has the same nature, and the toleranced feature is a toleranced feature group.
  Tolerance zone: a tolerance zone is a space (either surface or volume), bounded by one or several nominal features. That space defines the toleranced feature location in order to satisfy the tolerance specification, (see ISO 1101). When the geometrical tolerance applies on a feature group, then one tolerance zone is linked to one feature.
  Datum elements or datum systems.
Even if the Geometrical Tolerance creation is accomplished without any semantic links, we recommend you to specify datum elements and then declare your geometrical tolerancing with references.