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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?
  • 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.

    +1 Have seen many, many ppl here at work doing the same because they didn't know different. Educating yourself and others in this area will benefit your position tremendously.
  • 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


    You get what you pay for. But as a special favor to you, I will make it a point to not offer my NSHFO the next time I see a thread by you.


    Innocent
  • Now look and see what you've done, you've gone and gotten Wes POd now Stuck out tongue closed eyes

    Just one more little comment.. You've been programmin for 20+ years and don't understand the concept of datum shift and applying it to your positional tolerancing Astonished Astonished
  • Now look and see what you've done, you've gone and gotten Wes POd now Stuck out tongue closed eyes

    Just one more little comment.. You've been programmin for 20+ years and don't understand the concept of datum shift and applying it to your positional tolerancing Astonished Astonished



    I am not POd, just not very patient threads that go the way of that television series "Lost", where there is never enough information to really know what is going on, and as soon as you get a couple of things figured out another 3 or 4 puzzles arise to take their place. It seems there are some trolls in the world who think that is fun, but I have more amusing ways to occupy my time. Add to that the fact that the only reason I still frequent this board is because I feel I owe it to all of the people who have taken the time to explain and teach things to me, to in turn, teach others when I can.

    We all have our share of bad attitude days and brain malfunction days, so I try to give the benefit of the doubt, but when my attempts to help, albeit littered with my perverse humor, are belittled by the people asking for help. . . well, I will be the first to admit I am not the smartest man in the world, but I know enough to not keep trying to feed a dog that only wants to bite me.
  • Sorry if I offended anyone, I do understand Position and MMC, but the question I was asking was in general terms.
  • In general terms:


    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.



    As to how how it can effect a group of holes to each other well it could be good or it could be bad there is no way to tell from the information you have provided, but the more important question is do you care? If the holes aren't toleranced to each other you can't hold someone to that standard if it's not on the print. All you can do is hope that whoever designed the part took all of that into account when they toleranced the part.

    As wscmm said:

    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.

    But again based on what you have given us we can't answer that. To paraphrase Wes, if you want more you have to give us more. Until then all you will get is GIGO. And in that vein I think that chromatically the parts are noncompliant and should be rejected on that basis.
  • I guess I was pretty vague on the MMC thing. The feature hole and two datum holes (B and C) are at MMC. Thr vendor sent me reports using the PC-DMIS text only format. I had never seen this type of report before so I did not understand what the Datum Shift Rotation arguments were doing, The lines for the Datum shift don't even show on my reports when I do the PC-DMIS text reports, so I guess it threw me. I use another software to create my formal reports and position dimensions use a position/bonus tolerance calculator. So I don't identify datums when reporting features, just their size and location relative to those datums.
  • You've been programmin for 20+ years and don't understand the concept of datum shift and applying it to your positional tolerancing Astonished Astonished

    +1
  • Just curious, are you using legacy dimensioning and your vendor using XactMeasure?
    XactMeasure uses the datum shift dimensiong scheme in its reports.
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