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| Does your cmm software report concentricity correctly ？ |
| Written by Ray Xing |
|
Recently I provided a one day customized GD&T training to one of my customers, who’s been in pump manufacturing business for long time. Their drawings are very old school because you can tell from all those GD&T symbols, e.g. datum letter is before tolerance in FCF, etc.. But the most interesting thing is that concentricity is the major callout on their drawings, appears on almost every single drawing because co-axiality control is their major concern on their products. That’s why I gave them a customized training so they’ll understand how to achieve co-axiality control by calling out (total) run-out, profile, position or concentricity. I know talking about concentricity as per ASME Y14.5 standard itself can open a can of worm but right now I’d like to be focused on how cmm report concentricity. If you search around on the web you’ll find that people on many forums conclude that most QC guys report true position instead of concentricity as a “shortcut” when using cmm and most cmm software doesn’t calculate concentricity correctly. Therefore I kind of designed the following simple test to see if cmm will correctly report concentricity or not. Even though there’re so many different cmm software out there I choose to use pure dmis code so many of you should be able to run or import these codes to see if you can duplicate the result on your system. Everything is done off-line.
Concentricity is exactly same as true position! Is it right though? Here’s the definition from ASME Y14.5: “…Median points of all diametrically opposed elements are congruent with a datum axis (or center point) …”. So let’s do this on cmm:
Now report these points: So according to ASME definition, the real concentricity reading should be the worst of these 4 median points, which is PT004 showing 0.323 deviation radially then double it to report 0.646 as final result. See graphic below: Now if you construct a circle from these 8 points and then report concentricity and true position you’ll get same result as previous. This is just to verify that the software construct the circle correctly. PC-Dmis users can also import these dmis codes but need to play with THEO/ and ACTL/ to get same results (I tried). I couldn’t do dmis in to Calypso but I tested same way in Calypso and get similar result. If you’re interested to show me your test results or share your opinion, feel free to contact me at: xray@rxmetrology.ca Ray Xing |
