Legacy TP with Datums: Does Not Work Like XactMeasure

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  • Legacy TP with Datums: Does Not Work Like XactMeasure

    When using a TP Legacy, when you select a datums with in TP, do they work as they should like xact measure does? or they will give you the results based on your Alignment ?

    i am not refering to modifiers.
    Last edited by WolfMan; 09-07-2017, 09:53 AM.
    B&S One
    PC-DMIS CAD v2014

    Romer Infinity

  • #2
    With legacy TP, you have to have alignment setup to the required datum structure.

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    • AndersI
      AndersI commented
      Editing a comment
      Doesn't match the info in the Help file - source?

    • bfire85
      bfire85 commented
      Editing a comment
      AndersI I haven't used legacy TP for a long time, but that is what I was told when I was trained. I took the trainer's word for it.
      Last edited by bfire85; 09-07-2017, 01:58 PM.

    • vpt.se
      vpt.se commented
      Editing a comment
      Not needed as you can select "Use datums" in legacy. Without datums however, it is evaluated to the current alignment.

  • #3
    They don't work right. Long ago, I was taught to just add bonus tolerance from datums to the position tolerance. That's what PC-DMIS does in legacy. That is very wrong since at least Y14.5-2009, if not earlier (with the revision of 14.5.1 in 2004 maybe).
    "This is my word... and as such is beyond contestation."

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    • AndersI
      AndersI commented
      Editing a comment
      I.e. they don't work according to current standards when you have modifiers on the datums.

    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      I'm not entirely sure that they ever work, but that is a fair summary of my post.

  • #4
    I'm pretty sure (just tested it) that the datums do work properly in a sense that if you callout a feature under an alignment and just select that 1 feature, it gives you a value, I then went other some other alighment and selected the feature then picked the datums and it did in fact return the same value. However, like Vinnie said it does not calculate bonus correctly.

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    • #5
      I would hope that in the newer version they would work ...silly me

      i wasn't referring to the modifiers, I wanted to know if using the datums, will it actually calculate the correct TP.

      The best way to test it, is. just to measure features without any alignment, then plug them inside the TP and then compare to an alignment results
      Last edited by WolfMan; 09-07-2017, 09:48 AM.
      B&S One
      PC-DMIS CAD v2014

      Romer Infinity

      Comment


      • AndersI
        AndersI commented
        Editing a comment
        Yes, legacy TP should calculate the correct TP, with datums, as long as the dimensions are single features, and there are no modifiers on the datums.

      • WolfMan
        WolfMan commented
        Editing a comment
        Really, ok thank you

    • #6
      Im actually trying to test a dimension that I have in legacy (which is all I ever use) to the same dim done in xact.... but I don't know how to use xact.... very frusterating lol

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      • #7
        XactMeasure and legacy does not do the calculation "the same way". Read up on Josh Carpenter replies regarding this - the suggestion is to move from legacy to XactMeasure.
        PC-DMIS CAD++ 2o17 R2

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        • WolfMan
          WolfMan commented
          Editing a comment
          Can you give us a short version??

      • #8
        I believe you will also see a change in results for Legacy TP and Xact TP when there is not a tertiary datum involved as well. Recall running into this issue.

        Comment


        • #9
          Originally posted by vpt.se View Post
          XactMeasure and legacy does not do the calculation "the same way". Read up on Josh Carpenter replies regarding this - the suggestion is to move from legacy to XactMeasure.
          From Josh's post somewhere in the forum:

          When using Legacy Position,

          1) Do not check any of the datum boxes because it's useless - there is no internal creation of a new alignment to those datums, like there is in XactMeasure.


          2) Do not put MMC on any datum - those don't work correctly. This is due to a long-running GD&T misunderstanding. For decades, many people (including some big names in GD&T who wrote and sold lots of books) thought that MMC on a datum would simply give more bonus tolerance - and this was written into PC-DMIS in it's early development.

          Unfortunately this is incorrect. MMC on a datum feature-of-size really allows for 'datum shift' where the position of the feature being measured can be reported from the center of a virtual MMC datum shifted within the actual over-minimum-size datum.

          Think of a part with two holes: one hole is the datum, the other needs to be held position to that datum hole.
          So we build a hard gage to check the part.
          The gage has two pins:
          Pin A is the datum pin, it's longer and made to the minimum allowed diameter of the datum hole.
          Pin B is the checking pin, it's shorter and made to a special size: the minimum allowed diameter of the hole minus the allowed position deviation.
          For sake of arguement, let's say the print allows position deviation of .001"

          If we make a part with both holes at their minimum size, the datum pin fits tightly into the datum hole with no wiggle room, therefore the hole-to-be-checked had better be very close to perfect position for it's checking pin to get into it. There's only .001" allowed deviation for position.

          If we make a part with the datum hole at (or over) nominal, then we have some wiggle room to shift the minimum-diameter gage-datum pin over in the part's datum hole. Let's say the datum hole is .005" over minimum - now we can shift the datum pin over .0025" left or .0025" right within the larger-than-minimum datum hole to get the checking pin into the hole to be checked.

          This is Datum Shift, and XactMeasure does it perfectly while Legacy incorrectly gives more bonus tolerance.

          HTH
          PC-DMIS CAD++ 2o17 R2

          Comment


          • VinniUSMC
            VinniUSMC commented
            Editing a comment
            Point number 1 is what I was referring to when I wrote: "I'm not entirely sure that they ever work". Thanks

        • #10
          I use legacy T.P., select datums, and apply bonus tolerance, have been doing so for years, have never had an issue, and can verify my dimensions on the plate within +/- 0.0002" of the CMM. You all are starting to scare me now... lol

          Comment


          • VinniUSMC
            VinniUSMC commented
            Editing a comment
            Does it give you different numbers than if you were to not select datums? I have a feeling you are just seeing numbers to your alignment, which happens to be to your datum structure.

          • DAN_M
            DAN_M commented
            Editing a comment
            What I have always done is: do a rough alignment to find my part. Then zoom around, pick up my datums, and build alignments to them. Then I recall the alignment I want & go to town. Once I'm ready to dimension stuff, I recall that alignment & workplane and make my dimensions. I have never had an issue.

          • VinniUSMC
            VinniUSMC commented
            Editing a comment
            That's not what we're referring to (I think). I believe we are talking about in the legacy position dimension, you have the option to select your datum features. This selection of datum features within the position dimension dialog is what does nothing.

            Aligning to the datums and then dimensioning with legacy is perfectly normal. Thinking legacy position dimension datum selection works, is the problem.

        • #11
          Originally posted by DAN_M View Post
          I use legacy T.P., select datums, and apply bonus tolerance, have been doing so for years, have never had an issue, and can verify my dimensions on the plate within +/- 0.0002" of the CMM. You all are starting to scare me now... lol
          When you say apply bonus tolerance, are you referring to the feature or to the datum? Bonus of the feature works fine, but bonus on the datum is applied incorrectly.

          I usually create the alignment called out in the FCF, as you mentioned, and use legacy TP. The exception is when I want to utilize the datum bonus, then I'll use XactMeasure.
          PC-DMIS 2016.0

          Jeff

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          • #12
            Originally posted by vpt.se View Post

            From Josh's post somewhere in the forum:

            I think it's called Maximum Material Boundary (Datum Shift)...Co-workers mistakenly think of it as MMC as in more bonus = more betterer! These are the same people who don't understand basic dimensions either and measure from feature to feature, breaking away from their DRF, when politely confronted they flare up and say the person who taught me has been doing it for 30 years! LOLZ I love my (previous) co-workers.
            PcDmis 2015.1 SP10 CAD++
            Global 7-10-7 DC800S

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            • #13
              Originally posted by Schrocknroll View Post

              When you say apply bonus tolerance, are you referring to the feature or to the datum? Bonus of the feature works fine, but bonus on the datum is applied incorrectly.

              I usually create the alignment called out in the FCF, as you mentioned, and use legacy TP. The exception is when I want to utilize the datum bonus, then I'll use XactMeasure.
              I apply bonus to the feature and the datum and it works just fine for me (I thought)... =/

              Comment


              • VinniUSMC
                VinniUSMC commented
                Editing a comment
                Datum features allow datum shift, not bonus tolerance. Adding "bonus tolerance" from datum features is wrong. Not that you are alone in thinking this. It was "common knowledge" when I started programming. It was further clarified by the 2009 revision, but I think the math definitions standard cleared it up first.

            • #14
              Well I just want to know for sure if datums can be used in legacy or not, NO Modifiers of ANY kind.

              Example, say I have a program that does not do any kind of rotation (it really needs to ), but if I were to use the correct datums insider the Legacy TP, am I ok?
              B&S One
              PC-DMIS CAD v2014

              Romer Infinity

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              • #15
                Wow, now I feel vindicated for arguing this point 20+ years ago. I must have been ahead of my time.
                Systems Integrator
                Hexagon Manufacturing Intelligence

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