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Datum Shift

cmm-measurer
ConfusedI received a report from a vendor where they are showing everything on a part 100% in tolerance. I have checked the part myself and show quite a few things out of tolerance. On their report I noticed a term "DATUM SHIFT" and some variables "X,Y,Z", etc. just before each dimension is reported. My question is this, What exactly is "DATUM SHIFT" and what is it doing? Is there some kind of data manipulation going on here?
  • ConfusedI received a report from a vendor where they are showing everything on a part 100% in tolerance. I have checked the part myself and show quite a few things out of tolerance. On their report I noticed a term "DATUM SHIFT" and some variables "X,Y,Z", etc. just before each dimension is reported. My question is this, What exactly is "DATUM SHIFT" and what is it doing? Is there some kind of data manipulation going on here?


    Datum Shift is a sort of "best fit" that is either trying to mimick a hard gage or account for a material condition modifer on a datum. At least those are the ways it should be used. You are correct in suspecting that it can also be used to made bad parts look good.

    Whether that is the case here or not can not be determined without examining CMM code, (or at least the report) and drawing requirements.
  • Do you have Maximum Material Condition callouts on your datums? MMC "bonus" tolerance allows for datum shift. If you have the ASME Y14.5-1994 standard see page 85.
  • just remember ALL the holes on your part have to be moved the same amount and will be part of your TP calculation. You can't move 1 hole one way and then another hole the opposite directions to make it in tolerance.
  • I understand the concept of MMC. But I don't understand what is happening with the "rotations" within that shift. Now this shift is not actually moving the datums around, but moreso it is applying this "allowable bonus" tolerance to the feature in some form or another. Is that a correct statement? I'm just curious as to how this might stack up when reporting on a group of several holes. Theyll all look good relative to the Datums, but to each other could potentially have some problems, with no exact numbers to relate to a tool correction. Am I trying to play hockey on a basketball court with that approach?
  • I understand the concept of MMC. But I don't understand what is happening with the "rotations" within that shift. Now this shift is not actually moving the datums around, but moreso it is applying this "allowable bonus" tolerance to the feature in some form or another. Is that a correct statement? I'm just curious as to how this might stack up when reporting on a group of several holes. Theyll all look good relative to the Datums, but to each other could potentially have some problems, with no exact numbers to relate to a tool correction. Am I trying to play hockey on a basketball court with that approach?





    Maybe, or you could be trying to take a mountain bike up a luge run backwards. Without actual drawing callouts, code, and reported numbers there is no way for us to tell you anymore than we really have. But several of us excel at making stuff up, so if you would like, we can start throwing things at the wall to see what makes the most arousing racket.     . .
  • Why not call the vendor and ask what they are talking about? It may be that they don't know what they are talking about or mis-interpreted the print. Ya never know. Slight smile
  • I guess one thing I could do would be to spend all day just making silly responses to posts and that way I can get to the 1000 post plateau and become a Demi-Guru. (Like that means anything) LOL But I thank you for your input anyway. It does break up the monotony
  • I guess one thing I could do would be to spend all day just making silly responses to posts and that way I can get to the 1000 post plateau and become a Demi-Guru. (Like that means anything) LOL But I thank you for your input anyway. It does break up the monotony

    Your the one that posted the question without enough info to answer it correctly, First thing is does the feature control frame (thats the little box with the A, B, & C's or other letters) have modifers on them? If so the vendor might just be doing it correctly.
  • I have seen folks use "datum shift" as bonus tolerance and just increase the TP tolerance.
    That is not correct. It could be that your vendor is doing that. It is not uncommon.
    You need to educate yourself about this completely and then re-evaluate those reports.
  • Datum shift link below from Tec-ease


    www.tec-ease.com/gdt-tips-view.php%3F
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