HEIDENHAIN IK 5494-2D - Manual (Page 395)

Appendix
C
Tolerances
Appendix C:
Tolerances
Concentricity tolerance
The mathematical definition of concentricity is explained in detail in the ASME Y14.5M-1994 standard
and involves “the midpoints of opposing elements” in the determination of actual concentricity. This is
not practical in a discrete point measuring system, so the QC5000 software uses the center of the feature
(determined by the best fit) to estimate the concentricity.
Reference Features
When a reference feature is called for in a tolerance definition, the reference feature will nearly always
refer to a datum feature such as a skew line or datum circle. The field is required except for the
MMC / LMC tolerance case described next.
Reference feature called for in MMC or LMC circle tolerance
If the reference feature is a datum circle, and if that datum circle has an MMC or LMC tolerance applied,
then a bonus tolerance may be derived from the reference feature and applied to the feature as a bonus if
so specified in the drawing. This is an optional field.
Projected zone
The user may optionally call out an projected tolerance zone when defining a tolerance on the true position
of an axis. This allows the user to determine, for example if the limited measured portion of a parallel
line remains parallel at some point outside the measurable portion. The fields are optional. The projected
zone is specified by dimensions in the appropriate axis. projecting the parallelism zone of a horizontal line
would require specifying the X position of the start and end of the zone.