PC-DMIS allows you to type a reference distance in the Distance box in the Parallelism dialog box (Insert | Dimension | Parallelism). PC-DMIS uses this value with cylinder, cone, and line features to calculate the parallelism dimension.
For these features, the points used for the parallelism calculation are the end point of the axis and a point projected from this end point along the feature axis.
The distance between these two points is the referenced distance. For other features, this distance has no effect on the dimension. This option is useful when the dimension needs to be calculated a certain distance along the feature.
Understanding Parallelism's Distance Projection
Usually a parallelism dimension takes the datum line's vector, and checks the endpoints of the dimensioned feature to see if they are within tolerance as shown here:
A - Tolerance width (or cylinder).
B - Datum vector positioned at measured feature.
C - Datum line.
Sometimes, however, you can’t always measure right to the edge of the line, so you will need to extend the line out by a certain distance.
Since the extreme deviations are usually at the ends of the line, if you extend out the line to a distance longer than the actual length of the line you will usually get larger deviations. You can also extend the line's opposite direction by using a negative reference length.
An extended line may look like this:
A - Reference Length
B - Extended point, from reference length greater
than line length
C - End point of line, or reference length same
as line length.
D - Start point of line, or 0.0 reference length.
You can also use the Distance box to specify a reference length to check the axis of a cylinder.