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Arbitrary Datum Scheme

coremt_bat
Hi all

I am measuring a large part (> 6' wide x 4' high) with a complex datum scheme specified by the print. The print calls out several datum points that are theoretical surface points based on the holding fixture. This makes defining the datums extremely confusing for me. I am not a GD&T or metrology expert, but my knowledge and recent readings makes me believe this translates to making the datums related to a set of arbitrary planes based off of these surface points. Again, this is a very large part, for the automotive industry, which has a node set to math data's theoretical (0,0,0) point.

I set the datum scheme as follows:
A- a plane to define X plane
B- a plane to define Y plane down center of part (part centered @ Y=0)
C- a plane to define Z plane
All of these planes are based off of the datums in the print. I constructed an offset point off a tooling ball from the fixture to create the node mentioned above. Then all print datums are offest points from the node. The points then are used to construct planes to create the datums above.

The alignment to the fixture is done with tooling balls which aligns the Romer arm to the math data and fixture. The data seems to be skewed in a pattern that doesnt seem to be very consistant. Please let me know if the datum scheme I created is a viable way to set datums in a metrology program or if there is any way to improve upon this.
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  • Due to legal constraints, I am not allowed to post the actual print, so I made some rough sketches of the part's shape and where the datum locations are. The datum callout for most of the part is A,B-C,D.

    This is a side view (interior) of the part. The reverse side is identical and the datum surfaces are labeled as such. The tooling balls are 4 deep from the side view (total of 12 on fixture)
    Hood Side Datum.pdf

    This is a front view of the part which has a secondary datum scheme E,F,G. I included a callout for one of the slots because I do not quite understand how to set this datum/tolerancing into PCDMIS.
    Hood Front Datum.pdf

    How I interperet this is that there is a center plane between B-C datum surfaces, A is a line across an X location of the part creating a plane at that location, and D is a line across a Z location of the part creating a plane at that location.

    To align to the fixture, I input 3 tooling balls and do a best fit. There are 4 seperate programs for this part since it is so large and to eliminate leap frogging.
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  • Due to legal constraints, I am not allowed to post the actual print, so I made some rough sketches of the part's shape and where the datum locations are. The datum callout for most of the part is A,B-C,D.

    This is a side view (interior) of the part. The reverse side is identical and the datum surfaces are labeled as such. The tooling balls are 4 deep from the side view (total of 12 on fixture)
    Hood Side Datum.pdf

    This is a front view of the part which has a secondary datum scheme E,F,G. I included a callout for one of the slots because I do not quite understand how to set this datum/tolerancing into PCDMIS.
    Hood Front Datum.pdf

    How I interperet this is that there is a center plane between B-C datum surfaces, A is a line across an X location of the part creating a plane at that location, and D is a line across a Z location of the part creating a plane at that location.

    To align to the fixture, I input 3 tooling balls and do a best fit. There are 4 seperate programs for this part since it is so large and to eliminate leap frogging.
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