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Diameter of a Cylinder -> Size Feature vs constructed cylinder

Alvaro
Dear PC-DMIS users, colleagues, and experts,

In the last months, in the company where I'm currently working, we've updated our PC-DMIS software to one of the newer versions (2018 R1), and have found a new possibility to report sizes with all the ISO modifiers integrated. For us this is great news, since now we can apply the modifiers directly on the measure rather than through the elements construction.


Doing so, we've found that using the "enveloppe" requirement, that for an external diameter should grant the GN for the USL, and LP for the LSL, gives the same result as demanding each modifier separately (which is good), however, the GN hands a different value than when constructing by Best Fit & Min Circumscribed cylinder (same points, same element, by scanning and then constructed).


and then demanding the diameter through the loc feature

We observed that the result is the same regardless of alignment, we also observed that the Mean Squares and GG modifier result on the same diameter (which is at is should be).
We tried the same idea with the GX diameter, and a Max Inscribed Best Fit Cylinder, and the problem is reproduced. The value for diameter is different in a magnitude close to 0.01 mm.

I'd like to know which of our assumptions is wrong, (that a min circumscribed best fit cylinder should be equal to GN for example, or maybe the diameter calculation through the localization feature), and why is it wrong. Or, if it may just be a problem of our programming that can be solved.

As a bonus question, I'd like to know if the size feature with modifiers that I show in the first screenshot will be available for two parallel planes (or it already is and we didn't find how to use it correctly, which is certainly possible).

Thank you very much for the time taken to read this far.

Best regards,

Álvaro
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  • Dear JEFMAN,

    I don't know how PC-DMIS calculates it, but following the ISO norm, GN & GX should never be a two point size. Two point sizes are just LP (by definition, which is also the "default" modifier should none be present), GN and GX should be calculated for the whole dimensional entity (same thing applies to GG for example). If you have at hand the ISO 14405-1:2016, check 3.4, 3.6, 3.6.1 (for local sizes), and 3.7 (for global sizes) which is the case of all modifiers that start with a "G".


    You're right on this point (so I'm wrong Disappointed ), it's a global size (the 10:33pm of the post can explain some errors Slight smile)
    In ISO, they still use minimax, instead of L2 constraints, which can give some differences (you can test it easily on a curved plane)



    And also, I think that theoretically, a max_inscr should not forcefully be held by 3 points, it may be 2 as a minimum, and an infinite number of them should the form be perfect as a maximum.

    I may be wrong here, so please feel free to correct me, I'm always open to be proven wrong.



    In theory, you're right, 2 are enough (ellipse case), but on real parts, when you dimension the roundness (not the circularity !) of a max_inscr or min_circ, there are often (always ?) three points with a t_val=0.

  • in reply to JEFMAN
    Working late hours makes weird things to everyone (for me it was about 9 in the morning, so fresh like an apple :P). Thanks for your answer anyway, the will to help is always to be praised.
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