Runout Question

As JacobCheverie said, the correct way to measure the datums is to measure them as cylinders, using enough hits and levels to ensure you capture a true representation of the surface. xbigfatgoat, you say you are using 2022.1 which means you have access to the geometric tolerance command. That makes it really easy - measure datum A as a cylinder and then define as datum A, measure datum B as a cylinder and define as datum B then, from the datum definition dialog, click the "common datum" checkbox and select datum A and B from the list to define common datum A-B. Finally, create your runout.

Whenever you are using geometric tolerance commands, you should always use the simplest approach possible, sticking to measured-features, auto-features or scans wherever possible. This is because the geometric tolerance command takes a completely different approach to previous dimensioning methods, acting on the hits of the features and datums rather than taking their pre-resolved solutions. BF/BFRE constructions can be used where necessary, because they store the hit information. However, most other constructions just make life more difficult and lead to problems because they do not contain any hits.

Most of the other approaches discussed on this forum - constructing points from centroids, measuring as circles, building lines - are attempts to work around the limitations of old CMM software and dimensioning tools. With PC-DMIS legacy dimensioning for example, the user is required to interpret the print and then build an alignment representing the datum reference frame - hence the common approach of constructing a 3D line through the centroids of circles.

For an alignment, the line you described in your first post would be ok - as long as it is a 3D line.

I measured three separate circles for each journal, constructed cast points at each circle and then used those points to construct a line. I levelled to this line.


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Just remember that the geometric tolerance command works independent of any alignment so, if you create a geometric tolerance runout following the approach I described earlier and try to compare those numbers to a legacy runout using the alignment, the numbers won't match.

For an alignment, the line you described in your first post would be ok - as long as it is a 3D line.

I measured three separate circles for each journal, constructed cast points at each circle and then used those points to construct a line. I levelled to this line.


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Just remember that the geometric tolerance command works independent of any alignment so, if you create a geometric tolerance runout following the approach I described earlier and try to compare those numbers to a legacy runout using the alignment, the numbers won't match.