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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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  • Hey Alvaro

    I noticed the same issue a few months ago. However the differences where much smaller. If I remember correctly the max difference between the 2 approaches were around 0.001mm.
    I asked our support in Germany about it and they also didn't know the answer but asked the programmers at Wilcox.

    This is the answer they gave us:
    Re: different values between location dimension and size dimension for max_insc and min_circsc
    It is a different calculation by design. We are using constrained L2 math for Maximum Inscribed and Minimum Circumscribed in the Size dimension but are still using the original Max_Inscribed and Min_Circumscribed math everywhere else in PC-DMIS. That answers your question, but the next question is probably why? Daniel and I are members of ASME and ISO committee's and have been involved in studies that resulted in changes in the standards. Since the new ASME and ISO standards are not published yet, I am apprehensive about making any further changes in PC-DMIS because it would impact program migration and prompt questions from customers. The Size dimension command is new so does not affect program migration but does causes confusion in the case you describe in this report (comparing with math from other commands). The results from constrained L2 are more precise and more repeatable, so we are planning on changing the math in other areas of PC-DMIS beginning in 2019 R2. I also expect ASME Y14.5, Y14.5.1 and ISO 5459 to be Published by then.

    Regarding your second question, you first have to create a width feature. Then you'll be able to select it.

    Best regards

    Aaron
  • Hello Aaron,

    Thank you very much for the quick, concise, and perfectly understandable answer(s).

    I'll keep all this in mind! (probably the 0.01 mm difference is due to the diameter being with a huge & irregular shape defect.

    Best regards,

    Álvaro
  • In addition, I would say that size (GX or GN) are calculated between two points on each side of the diameter (approximately), but max_inscr and min circ are calculated on 3 points.
    The rsult can't be the same...
  • 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". 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.

    Regards,

    Álvaro
  • It is not clear from your description if you constructed the circle with BF or BFRE - this might make a difference, especially if the centre point of the GN is different from the GG. Maybe that plus what's described by @ Aaron Baldauf in post #2 ( https://www.pcdmisforum.com/forum/pc-dmis-enterprise-metrology-software/pc-dmis-for-cmms/442930-diameter-of-a-cylinder-size-feature-vs-constructed-cylinder?p=442940#post442940) can explain the total diff?

  • 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.

  • PC-DMIS... The metrology software where the "same" calculation on the same diameter yields two different results...
  • Now that's a bit unfair - there are quite a lot of definitions of 'size', and not always a definition of the algorithm in the standards. The blue answer in post #2 - https://www.pcdmisforum.com/forum/pc-dmis-enterprise-metrology-software/pc-dmis-for-cmms/442930-diameter-of-a-cylinder-size-feature-vs-constructed-cylinder?p=442940#post442940 - explains it quite well, IMO.

  • 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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