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 limiting 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.
When 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. This 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.