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When to use Primary Datum Plane construction

JFournier
The newer version of PC-DMIS has a constructed plane option of Primary Datum Plane, and I'm not sure when I should be using that.

Should it be used in my DCC alignment for the primary (leveling) datum?
Or should I continue to use the auto-feature plane that defaults to LSQ for alignment, and use the constructed Primary version as a Datum Definition that is used for evaluating GD&T only?

Is there any concern that if I don't use enough points to construct this Primary Datum plane, it will skew alignments/results if there is too much part variability and the points aren't exact from part to part? (example: if there's an errant hit on a burr or something, the plane is far more skewed compared to an averaged plane)







  • Read the help files by searching for "datum plane" as there is some fairly decent info for that
  • I dont use Geotol but my understanding is that your primary datum plane sets your A datum for that program so you can use it for alignments and dimensioning. Everything checked to "A" is referenced back to that feature.
    • L1 - This is equal to the sum of the distances.
    • L2 - This is equal to the square-root of the sum of the squares of the distances. Minimizing the L2 norm is the same as a least-squares fit.
    • L∞ - This is equal to the maximum distances between the ideal surface and the non-ideal surface. Minimizing the L∞ norm is the same as minimizing the maximum deviation, so we use the term "minmax" for this norm.
    Please correct me if I am wrong.
    ​
  • I dont use Geotol but my understanding is that your primary datum plane sets your A datum for that program so you can use it for alignments and dimensioning. Everything checked to "A" is referenced back to that feature.
    • L1 - This is equal to the sum of the distances.
    • L2 - This is equal to the square-root of the sum of the squares of the distances. Minimizing the L2 norm is the same as a least-squares fit.
    • L∞ - This is equal to the maximum distances between the ideal surface and the non-ideal surface. Minimizing the L∞ norm is the same as minimizing the maximum deviation, so we use the term "minmax" for this norm.
    Please correct me if I am wrong.
    ​

    We've got 2019R2 so XactMeasure has that capability also
  • I'd say that the constructed Primary Datum plane would be more relevant if using Legacy dimensions, as the GeoTol commands will do the proper (external to material) datum constructions when they're evaluated.

    I primarily use them when explaining why we get differences between Legacy and Geo Tol.

    If you use the Primary Plane /Secondary Line / Tertiary Point and then either MaxInsc / MinCirc (depending on ID or OD) we can usually get a fairly close match between Legacy and GeoTol.

    Typically they won't be exactly the same as the MaxInsc and MinCirc aren't exactly the same as the default math type used by GeoTol.​
  • in reply to NinjaBadger
    I should add, this would be for GD&T without MMB on datums. Legacy can't do that!
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