What's the difference?
The clearance distance is how far the probe will clear a feature before and after measuring.
It is applied in the normal direction of the feature.
For example, in plane feature below, the probe will pause slightly at .120” from the surface before touching the first point. It will also move .120” away from the surface after the last point.
Note: The dialog box above shows the default clearance/retract distances that were set in this program, but you can override them by un-checking the checkbox and entering your own numbers.
Clearance distance is especially useful when you need to measure a hole with a probe that approaches from a different direction than the hole axis.
The light blue line shows where the probe will travel.
Notice how the probe will travel down from the +Z clearance plane, then clear the hole by 10mm before entering the angled hole.
In this case, the clearance distance represents the distance between the center of the probe and the first scan path or touch point.
Another use of clearance distance is shown in the example below. Sometimes, you might want the probe to travel between holes using a Sub-Clearance Plane (while the SCP has no “Retract to inner plane” distance).
If you would like the probe to drive slowly for a short distance before measuring the first point, use a clearance distance and adjust the feature speed in the Measurement Plan Editor.
Retract distance is more straightforward.
The probe will move to the retract distance to allow space before and after each probing point or each scan path. It is always perpendicular to the nominal feature surface.
In the image below, the blue line shows the probe travel. Before and after touching each of the six points, the probe will clear the surface (in this case, by .040”).
Set the retract distance according to the part variation and feature size. A 1mm retract is good for many types of parts, but you will need to reduce it for tight spots or increase it for more expected part variation.
If the probe needs to jump over multiple obstacles while measuring points on a plane, set the retract distance higher than the obstacles.
One final note to keep in mind:
If the clearance distance is set to zero, and you are measuring a plane or point, the probe will clear the surface by the retract distance on the first and last points. That means the clearance distance can be set to zero, but the probe will still “clear” the surface by the retract distance.