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Datum Selection

xbigfatgoat
I have multiple parts where a cylinder is called out as a datum and another cylinder has either a runout or concentricity callout back to the datum.
What I'm finding is that making the cylinder the datum I get bad results for the runout or concentricity. If I construct an axis line from the cylinder and make that line the datum
then my runouts match very closely to the real world and my concentricity improves. I figured assigning the cylinder as the datum in PCDMIS would automatically use its axis
but the results are outright wrong. My question is why?

Im using PCDMIS 2022.2..
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  • The cylinder feature itself will likely be measured Least Squares (a sort of Average), so when you construct a line from it, the line will replicate the the axis exactly.

    When you defined the cylinder as a datum, GeoTol is making a different cylinder from all of its hits (using either Constrained Min/Max or Constrained Leas Sq depending on which standard you are working to)

    I'm assuming that the form of this cylinder is kicking it over somehow, you might get some clue as to why by reporting the cylindricity of the datum and performing a graphical analysis.



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  • The cylinder feature itself will likely be measured Least Squares (a sort of Average), so when you construct a line from it, the line will replicate the the axis exactly.

    When you defined the cylinder as a datum, GeoTol is making a different cylinder from all of its hits (using either Constrained Min/Max or Constrained Leas Sq depending on which standard you are working to)

    I'm assuming that the form of this cylinder is kicking it over somehow, you might get some clue as to why by reporting the cylindricity of the datum and performing a graphical analysis.



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