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Position without DRF/Datums

edw.jack23
Hey all,
Wondering how to best measure and report a pattern position without datums. (See Dim labeled 11 on attached print view). The three holes are properly dimensioned to each other with basics and are the primary datum so the GD&T checks out but I'm not sure how to do it properly on PCDMIS. I've used a constructed feature set for that Datum A made from three cylinders, and it seems to be the centerpoint between the three, but I cannot report position with GeoTol as it says datums are required. Do I have to use Legacy? And can MMC be applied in Legacy? Thanks in advance for any advice!
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  • I would say to get an orientation you are going to have to make a line from A to B , then rotate to that, then do a basic rotation of 120°. This will then " square " it back up to the print. Yes you can use MMC in legacy. The 3 holes for A will just be best fit alignment using those 3 holes.
  • in reply to Schlag
    Thank you! Is there a usual best way to do patterns as datums like this? I also thought of doing a constructed circle best fit to get the centerpoint. Not sure what the most accurate way to represent this is.
  • in reply to Schlag
    You would have to do surface hits for them. Idealy I guess they should be cylinders. Circles are 2D and this is a 3d callout. How thick is it ? Im guessing the face should be A and the bolt pattern B but I didnt draw it. Its so rare an engineer or drafter pay attention to this.
  • in reply to Schlag
    Yes I made them cylinders, they are just thick enough for me to get two concentric circle scans in. I think it is done this way because they MUST be the primary datum for the position without datums to be viable.
  • in reply to Schlag
    I don't think that's a valid statement. Pretend its thin stamped sheet metal. How would you make cylinders in that ? The callout is just making the sure the 3 holes are in position to each other. Think of it as the bottom segment of a composite position callout. If it was that thin you would rally have to use surface hits next to each of the 3 holes. Then in reality your leveling to the " face " of the part and not A. Sometimes theory and how you actually have to do it don't quite match up.
  • in reply to Schlag
    I think that orientation does not really matter. All that is called for is that the holes do not violate the maximum material boundary which is a cylinder 11.9mm in diameter located exactly by the basic dimensions, which also indirectly control the orientation of those tolerance zones. They are only relative to one another so the surface hits won't add anything because I'm already measuring them as 3D features.......I think
  • You need to measure the three diameters as cylinders making sure they all have the same nominal diameter and length. Next create a geometric tolerance command - position 0.1 (M) - referencing all three as the considered features and be sure to enter the correct tolerance values for the diameters so that the MMC is calculated correctly. Following that, INSERT>DIMENSION>DATUM DEFINITION, select all three cylinders from the feature list and define them as datum A. PC-Dmis will treat all three cylinders as a pattern and apply MMB based on their previously defined size tolerance (from the position command).
  • in reply to Neil Challinor
    So that's how you do it. That makes a lot of sense. I was using an alignment to one to get the basics to the the others, sort of a fake datum. It worked but this seems easier and more correct. Thank you!
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