You can specify a flatness tolerance on a per unit area basis if you use the Per Unit check box. This creates a composite feature control frame, where two separate tolerances share the same flatness symbol. The square symbol after the forward slash, shown here, indicates that the unit area is square:

The number that precedes the slash determines the tolerance. The number that follows the square symbol determines the size of the square unit area. Flatness per unit refines the total flatness, and the you must choose a smaller tolerance value than the total tolerance.

(A) If your drawing has this. (B) It means this. (C) A .010 wide tolerance zone. (D) A .003 wide tolerance zone by 1x1 unit area. Each pair of green lines represents the individual unit areas.

Flatness per unit area is calculated by dividing the plane’s point data into subsets that each fall inside a square of the specified size. The flatness of each data subset is determined by calculating the minimum distance between two parallel planes that contain all of the measured points in the subset. The plane’s point data must be dense enough to contain at least four points in each data subset. After the software calculates flatness for all unit areas, it uses the worst-case measured value. The plane passes the flatness tolerance per unit area if the worst-case measured value is within the per unit tolerance value.

Considered Features: Measured Plane, Auto Plane, and a Constructed Plane.

1. From the Insert | Dimension menu, or from the Dimension toolbar, choose Flatness to display the XactMeasure GD&T dialog box.

2. From the Features area, select a plane feature that contains at least four data points.

3. From the Feature Control Frame editor area, define the tolerance value for the Feature Control Frame (FCF).

4. If you want to refine your tolerance on a per unit basis, select the Per Unit check box.

5. If you marked Per Unit, In the newly-added line in the Feature Control Frame editor area, define the tolerance and size of the unit.

6. Click Create to calculate the flatness dimension.

7. Click Close to close the XactMeasure GD&T dialog box.

8. View the results by opening the Status window (View | Other Windows | Status Window) or the Report window (View | Report Window). If the plane feature fails the flatness tolerance, PC-DMIS colors the text red.

Flatness of a Plane

(A) The measured value is the minimum distance between two parallel planes that contain all of the measured surface points of the plane. The plane passes the flatness tolerance if the measured value is within the tolerance value.

Flatness of a Plane Per Unit Area

(A) The measured value is the minimum distance between two parallel planes that contain all of the measured surface points of the plane. The plane passes the flatness tolerance if the measured value is within the tolerance value.

(B) The measured value for the lower segment is the minimum distance between two parallel planes of the per unit area. This is the worst deviation amongst all per unit area segments.