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GEOTOL help

browningdmis
management had me run a test. I taped a 2,4,6 block to the table. took the top plane, x+ and y- and created my alignment. I then called them datum a top b x and c y. I measured 2 holes on each side and did a position in geotol. I used the same measured data in legacy and got a .010 difference. How can this be?

What management is saying is the position formula. 2 x (dx^2 + dy^2)^1/2 when you figure it manually using the measured values matches legacy perfectly but it is not matching in GEOTOL.

I understand this is rough and dirty test but I can not explain why parts we measure good with second methods and legacy are good. GEOTOL (which our customer uses and requires) is failing the part. I have asked Hexagon this question and keep being told that GEOTOL is correct to the standard. Which is great but the standard use the 2 x (dx^2 + dy^2)^1/2 formula and is passing this but failing in GEOTOL.

I am not bashing PCD Hexagon or anything but I can not explain it to management.

What am I missing??​
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  • When you are using legacy, the features (both the circles and planes) will be calculated using whatever algorithm is specified in the feature itself. By default this is Least Square, which is an averaging method.

    The legacy position call out is using these calculated features (centres, vectors and sizes) in its analysis.

    If you're looking at the measured values of the features themselves and seeing correlation with the position calculations this is why, because it's looking at the same values.

    When you are using GeoTol, the previously calculated features don't actually come into play. All the hits of both datum and considered features are recalculated.

    The primary datum will be constructed external to material on the top plane.

    The B datum will be constructed perpendicular to datum A then external to material on the B face.

    Datum C will be constructed perpendicular to both A and B external to material on the C face.

    The hole will be created external to material, much like Max Inscribed (but not exactly).

    If you construct a primary datum plane from your A plane, a secondary datum line from your B plane and tertiary datum point from your C plane, then do a 3-2-1 alignment from those. Then change the calculation for for the circle to Max Inscribed and report using legacy you should get very similar numbers to the GeoTol output.

    That said, it's likely the block isn't that far out, more likely that form of the circle or the datums (maybe a hit on a burr or bit of dirt) swaying the results.

    If you've measured only 3 points on the datums you won't see form error, but if you check perpendicularity you will likely see them showing not square.

    Alternatively, if the GeoTol diameter is showing small, it's likely this causing the issue.



  • in reply to NinjaBadger
    mike my alignments are almost always my datums. Why would I want to probe the same thing twice to get an alignment and a datum from the same feature. Plus up until 2022 I could program with less points normally 4 for a plane and have it give me results I could trust and verify.
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