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Implement support of a compound datum made of two non coaxial circles.

natanderson 4 weeks ago in Metrology Software / PC-DMIS updated by nrowland 3 days ago 3

Example A is a plane, B and C are two circles laying in plane A. I need to evaluate GD&T relative to A|B-C. B-C should be treated a as pattern. ref case #00757938

Note: B and C are measured on a thin glass lens with a Vision probe on an Optiv Reference machine: they are 2D and cannot be measured as cylinders.

This functionality is supported in other metrology software. This is very frustrating and embarrassing for my customers.

A work around:  you can create a line from circle -B- to circle -C- and name that datum B-C.  The FCF should be A|B|B-C.

I hope this helps.

hi Chris, I think if B is used a secondary, it will set the origin on B and orient to the line B-C while being still centered on B. This is not the intent as B and C should have equal weight on positioning the origin. I need the origin to be half way between B and C. But you gave me an idea, if I construct a midpoint M between B and C, then I could use DRF A|M|B-C where B-C is the line from B to C, and I believe this should work. I will give this a try.

Thanks for the quick response.

Note: I think if PC-DMIS does not support using B-C as a pattern of circles as a secondary datum, it should pop a warning instead of computing a solution that does not raise concerns unless noticed (in my case, the suspicion came from the fact that the graphic display of the DRF was originated on circle B, where my customer and I expected the origin to be half way between B and C.

After reviewing this case in detail, I was able to determine that this case of a primary datum plane, secondary datum made up of two non-coaxial circles (as a common datum) is currently supported with the Geometric Tolerance commands in PC-DMIS 2023.1 versions and newer.    Additional support for many combinations of common datums were added in that version and I encourage all customers to upgrade to 2023.1 or a newer version of PC-DMIS.   

Unfortunately, there is a note in the current PC-DMIS help menu that can cause some confusion regarding the use of circles as a common datum.   If you have a common datum that has at least one circle and it is a primary datum, then the circles (features) must be co-axial.   The coaxial requirement does not apply when the common datum is a secondary or tertiary datum.   We are currently updating the help menu documentation to clarify this.   

Regarding the graphic display of the DRF, the Geometric Tolerance commands have rules on where the "trihedron" is placed in the graphics display window when the display coordinates are set to "DRF".   In cases where the datum is multiple features (patterns and common datums), that location can vary depending on how many features are used and the order of their selection (in the case of common datum).   In this particular case, the rules place the trihedron on the first feature of the common datum (B) if the callout was A|B-C .    However, if the callout was changed to A| C-B , the trihedron will move to the location of C.   This gives the user the ability to have some control over the placement of the trihedron.   If the user desires a different location than the two options mentioned, the Geometric Tolerance setting for display coordinates can be changed to "current alignment".   Then the user has the ability to create an alignment command with the trihedron in the desired location so that it can be used for the XYZ reporting in the Geometric Tolerance command.   We have provided the "current alignment" setting as a way to alter the XYZ reporting in cases where the automatic "DRF" solution is not what the user desires.   

Please see the help menu documentation regarding the "display coordinates" setting in Geometric Tolerance Commands if you have additional questions regarding how to you use this functionality.


Nathan Rowland

Product Owner HxGN GDT Library