### Per Unit Check Box

The Per Unit check box appears for flatness and straightness dimensions. It lets you specify a flatness per unit area or a straightness per unit length:

Per Unit check box

When you select the Per Unit check box, PC-DMIS changes the Feature Control Frame Editor area, adding an additional line where you can enter a tolerance value as well as the unit area or unit length values:

Feature Control Frame Editor (per unit flatness)

Feature Control Frame Editor (per unit straightness)

PC-DMIS now automatically uses a step increment that ensures that the "per unit window" will include maximum overlap as it "slides" over all the point data.

Once you enter in the desired information and create the Feature Control Frame, PC-DMIS also applies the flatness per unit area or straightness per unit length in the Feature Control Frame:

For example, a flatness FCF with an overall flatness tolerance of .05 and a per unit tolerance of .01 per unit area of 10 x 10 would look like this:

Example of a Finished Per Unit Flatness FCF

For example, a straightness FCF with an overall straightness tolerance of .0254 with a per unit tolerance of .012 / per unit length of 10 would look like this:

Example of a Finished Per Unit Straightness FCF

PC-DMIS also supports the "square area" symbol that was added in the Y14.5 2009 standard. This symbol is used to indicate when the area is exactly square, meaning the width and the length values of the per unit area are identical. To use this symbol, select the <UA> field and choose the Square Area symbol from the drop-down list:

Example of Selecting the Square Area Symbol

The FCF would then appear as follows, the square area symbol indicating that the per unit area value following it has an identical width and length (in this case the 10 meaning a 10 X 10 area):

Example of a Finished Per Unit Flatness FCF with Square Area Symbol

The square area symbol will also appear in your report output.