The Symmetry Feature Control Frame (FCF) dimension has been modified to allow an increased variety of input features and datums than you may find available using legacy symmetry dimensions. These changes in turn give you the ability to more easily create symmetry dimensions that apply to a broader range of dimensioning problems. The GD&T Symmetry dialog box has not been modified in any way. The changes were made only to the internal verification of considered features and datums and the internal tolerance analysis.
You need to use datums to construct a datum reference frame alignment which PC-DMIS will then use to evaluate the symmetry points. You can specify a single primary datum or a primary and secondary datum to impose a perpendicularity constraint.
If you specify a single primary datum, it becomes the locating datum and defines the nominal (0 deviation) position of the symmetry dimension.
If you specify two datums, it imposes a perpendicularity constraint on the secondary datum. In this case, the secondary datum becomes the locating datum and defines the nominal (0 deviation) position of the symmetry dimension. The datum reference frame alignment solves the secondary datum in a constrained orientation to the primary (as is done with Position evaluations). If the locating datum is a plane, it may be specified as a compound datum referencing two planar datums, for example, A-B. In this case, the mid-plane of A and B is used as the plane datum. Datums are always specified at RFS.
See the Description column in the table below for specific datum information for the different symmetry input features.
Feature Types for Symmetry FCF Dimensions
PC-DMIS considers any of the following feature types as valid input features for a Symmetry FCF dimension:
Case
1: Two Planes
These planes must be nominally parallel. They create a mid-plane from which
PC-DMIS determines
the symmetry dimension of the two planes.
Case
2: A Constructed Mid-Plane
Since you have already constructed a mid-plane, the processing and datum
information is the same as Two Planes above.
Case
3: Two Lines
These lines must be nominally parallel. They create a mid-line from which
PC-DMIS determines
the symmetry dimension of the two lines.
Case
4: A Constructed Mid-Line
With an already-constructed mid-line, the internal processing follows what
is described in Two Lines above.
Case
5: Two Points
The FCF uses these two points internally to construct a mid-point that
nominally lies within the planar tolerance zone about the locating datum.
The reported tolerance value is two times the maximum deviation of the
mid-point from the locating datum.
Case
6: A Constructed Mid-Point
In this case, you have already constructed a mid-point. The processing
shall be the same as for Two Points above.
Case
7: A Point Set
PC-DMIS assumes
that the points making up the set alternate on opposite sides of the locating
datum, such that the first point and second point have their mid-point
inside the planar tolerance zone. This is the same analysis as for the
legacy symmetry
dimension using a point set.
Case
8: Two Point Sets
PC-DMIS assumes
that both point sets are the same size, that the points in each set reside
on opposite sides of the locating datum, and that the points within each
set are opposite one another.
Case
9: A Measured or Auto Circle, Cone, or Cylinder
PC-DMIS calculates
a "circle set" (or sets) from the input feature and then evaluates
for symmetry the centroids of the sets.
Case
10: A Constructed Circle, Cone, or Cylinder
PC-DMIS checks
a constructed cone or cylinder feature's endpoints or a constructed circle's
centroid for symmetry.