## Creating a Symmetry Feature Control Frame Dimension

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 now considers any of the following feature types as valid input features for a Symmetry FCF dimension:

Valid Input Features | Description

1. Two Planes

These planes must be nominally parallel and will create a mid-plane from which PC-DMIS will determine the symmetry dimension of the two planes.

The two planes must be nominally parallel. The FCF will use them internally to construct a mid-plane. This mid-plane must nominally lie within the tolerance zone about the locating datum.

PC-DMIS checks the “corner points” of the mid-plane to verify that they lie within the a planar tolerance zone of two times the maximum deviation of the “corner points” from the locating datum.

These “corner points” are calculated by projecting the actual probe hits of each of the two planes onto the mid-plane.

1. - The maximum distance of the projected points from the centroid of the mid-plane in the X direction (of the datum reference frame) determines the “length” of the mid-plane.

   - The maximum distance of the projected points from the centroid of the mid-plane in the Y direction (of the datum reference frame) determines the “width” of the mid-plane.

The “length” and “width” calculate the corner points relative to the centroid of the mid-plane. If there are no actual probe hits to project (for example, the two planes are themselves constructed planes), then only the centroid of the mid-plane is checked to lie within the tolerance zone.

Datum Information

One Datum - This is a plane nominally coplanar with the constructed mid-plane. The vector of the datum plane defines the direction of measurement.

Two Datums - First datum: This is a plane nominally coplanar with the constructed mid-plane. Second datum: This is either a plane nominally coplanar with the constructed mid-plane or an axis type feature whose axis nominally lies in the constructed mid-plane. If a plane, the vector of the secondary datum plane determines the direction of measurement. If an axis type feature, "double-crossing" the vector of the axis datum with the mid-plane vector determines the direction of measurement.

2. A Constructed Mid-Plane

Since you have already constructed a mid-plane so the processing and datum information is the same as case 1 above.

3. Two Lines

These lines must be nominally parallel and will create a mid-line from which PC-DMIS will determine the symmetry dimension of the two lines.

The two lines must be nominally parallel. The FCF will use them internally to construct a mid-line. This mid-line must nominally lie within the tolerance zone about the locating datum.

PC-DMIS checks the “endpoints” of the mid-line to verify that they lie within the planar tolerance zone of two times the maximum deviation of the “endpoints” from the locating datum.

These “endpoints” are calculated by projecting the actual probe hits of the two lines onto the mid-line. The projected points farthest away from the centroid of the mid-line determine the “endpoints”. If there are no actual probe hits to project (for example, the two lines are themselves constructed lines), then only the centroid of the mid-line is checked to lie within the tolerance zone.

Datum Information

One Datum - This datum is either a plane, nominally perpendicular to the plane of the two lines, that contains the mid-line, or an axis type feature whose axis nominally lies in the plane that is perpendicular to the plane of the two lines and contains the mid-line. If a plane, the vector of the datum plane determines the direction of measurement. If an axis type feature, the direction of measurement is perpendicular to the axis datum in the current workplane.

Two Datums - First Datum: A plane nominally perpendicular to the mid-line. Second Datum: This is either a plane feature or an axis feature which satisfies the same requirements as a single primary datum. If a plane, the vector of the plane determines the direction of measurement. If an axis type feature, the direction of measurement is perpendicular to the axis datum in the current workplane.

4. A Constructed Mid-Line

With an already constructed mid-line, the internal processing follows what is described in case 3 above.

Datum Information

One Datum - This datum is either a datum plane which nominally contains the mid-line or an axis type feature whose axis nominally is parallel to the mid-line.

Two Datums - Primary Datum: This is a plane nominally perpendicular to the mid-line. Secondary Datum: This is either a plane or an axis feature which satisfies the same requirements as a single primary datum. The direction of measurement is determined the same way as for case 2 above.

5. Two Points

The FCF will use 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.

Datum Information

One Datum: You can select either a plane or an axis type feature for your datum. If a plane, it should be nominally perpendicular to the line connecting the two points and containing the mid-point. The vector of the datum plane determines the direction of measurement. If an axis feature, it should line in the plane perpendicular to the line connecting the two points and the mid-point. The direction of measurement is perpendicular to the axis datum in the current workplane.

6. A Constructed Mid-Point

In this case, the user has already constructed a mid-point and the processing shall be the same as for case 5 above.

Datum Information

One Datum: You can specify either a plane that nominally contains the mid-point or an axis type feature. PC-DMIS determines the direction of measurement in the way as case 4 above.

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.

Datum Information

One Datum: You can specify a plane or an axis type feature. The analysis in this case is the same as for the legacy symmetry dimension.

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.

PC-DMIS will compute mid-points from the points inside the two sets in this fashion: mid-point n is calculated from point n of the first set and point n of the second set and so on for each point. The resulting mid-points should be inside the planar tolerance zone about the locating datum. This is the same analysis as case 7 above except that the points are taken from two sets.

Datum Information

One Datum: You can specify a plane or an axis type feature for the primary datum. The analysis in this is the same as described in case 7 above.

9. A Measured or Auto Circle, Cone, or Cylinder

PC-DMIS will calculate a "circle set" (or sets) from the input feature and then evaluates for symmetry the centroids of the sets.

Probe points within 1mm of a plane that is perpendicular to the feature axis, belong to the same “circle set”. PC-DMIS solves the points as a 2D circle in a plane perpendicular to the feature axis. The centroids of all the “circle sets” are evaluated for symmetry to see if they all lie within the tolerance zone about the datum. The tolerance zone may be cylindrical or planar depending on the datums used. The reported tolerance value is two times the maximum deviation of the centroids of the “circle sets” from the locating datum.

Datum Information

One Datum - You can select a plane that nominally contains the considered feature's axis or an axis type feature nominally coincident with the axis of the considered feature. If a plane, the tolerance zone is planar and the vector of the plane determines the direction of measurement. If an axis type feature, the tolerance zone is cylindrical.

Two Datums - Primary Datum: This is a plane that nominally perpendicular to the considered feature. Secondary Datum: You can select either a plane nominally containing the considered feature’s axis or an axis type feature nominally coincident with the axis of the considered feature. If you choose a plane, the tolerance zone is planar and the vector of the datum plane determines the direction of measurement. If you choose an axis type feature, the tolerance zone is cylindrical.

10. A Constructed Circle, Cone, or Cylinder

PC-DMIS will check a constructed cone or cylinder feature's endpoints or a constructed circle's centroid for symmetry.

The endpoints must all lie within the tolerance zone about the locating datum. The tolerance zone may be cylindrical or planar depending on the datums used. The reported tolerance value shall be two times the maximum deviation of the endpoints from the locating datum.

Datum Information

This is the same as case 9 above.