Position tolerance zone question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Position tolerance zone question

    hi all friends and family

    i have a question about position tolerance (and i guess datum & basic/ted dimensions too)
    Ill upload a picture of a drawing i made for this example (PLS NOTE ITS NOT A REAL PART, PLS DONT BAN ME)
    The "flip view" in the drawing is EU btw, and I will asume that datum A = ZPLUS, B= XMIN, C = YPLUS.

    Are both drawing 1. and drawing 2. in the picture valid GD&T callouts?

    I've always thought that a datum locks the degrees it can control, assuming the degree has not been locked by previous datum. (like drawing two where the orgins are one the datum faces). But I always see these types of position callouts on drawings.

    So in drawing 1. Zplus should lock Z-orgin, B, X-orgin and C Y-orgin, but the dimensions seems to go from the diameters too (which are not datums). Would not the correct way to do this is set one hole as a datum that locks just one degree of freedom? (X or Y-orgin)

    OR have I been misunderstanding the way position tolerance callout works and the position of the tolerance zone doesnt have to be from the datums, but from specific TED/Basic dimensions like drawing 1.?
    So in picture 1 the circles are projected on datum A; between a tolerance zone positioned, 10mm from hole ⌀12 and ⌀10 and oriented parrallel to datum B/C (i.e. the basic dimensions steer the position, datums orientation?)

    Reason why im asking is because im curious: but also because when you use position in PC-DMIS it will always use the dimensions like picture 2 (i.e. nominal values from datums) DWADOAWD.png
    Sorry for rough explaining. im noob.
    Last edited by pcdmisstudent; 06-17-2020, 11:10 PM.

  • #2
    I would just say drawing one is wrong. If the FCF says ABC, that is what you use. Ultimately, it's up to the customer though.

    Comment


    • UKCMM
      UKCMM commented
      Editing a comment
      It is not wrong it is fully compliant with GDT requirements you need to read a copy of the standards

    • RanDawgg
      RanDawgg commented
      Editing a comment
      .... I already conceded that it is legal. Just giving my opinion.

    • NinjaBadger
      NinjaBadger commented
      Editing a comment
      I agree 100% with UKCMM.

      Both are equally valid and will give the same results.

  • #3
    Regardless where it shows the basics coming from ( 20 mm from the edge or 10 mm from hole to hole ) Your positional callout will use A-B-C as shown in your feature control frame your zero locations. BASIC is just that and doesnt control the actual location. The feature control frame does that. You need to have a copy of Y14.5 and understand it in this business.

    Thanks Matt. Thats a better explanation.
    Last edited by Schlag; 06-18-2020, 10:06 AM.

    Comment


    • #4
      BASIC dimensions show you "how to make a perfect part". Stack-up isn't an issue or a problem, and the basic dimensions are just 'the perfect part'.
      To dimension a hole, you have to know where it is supposed to be, that is what the basics tell you, be it 20mm from the edge or 10mm from the previous hole that is 10mm from the edge. Those are your THEO values for the feature. That is all they do. If you don't have a cad model, then you must use the values from the print.
      sigpic
      Originally posted by AndersI
      I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

      Comment


      • pcdmisstudent
        pcdmisstudent commented
        Editing a comment
        Hmmm.. So lets say the print is like drawing two on the picture i inserted. (no basic dimensions from datums)
        Will you still have to be within nominal position tolerance from the datums, even if the drawing doesnt specify it? (i.e. 10mm and 20mm)
        Last edited by pcdmisstudent; 06-24-2020, 07:46 PM.

    • #5
      ...the TPs should have cylindrical tolerance zones too since the (crude) drawing doesn't state the axis of evaluation.
      PC-DMIS CAD++ 2o19 R1 SP11

      Comment


      • #6
        Maybe this attachment will help
        Attached Files

        Comment


        • #7
          Schlag
          Matthew D. Hoedeman

          So since the FCF controls the orgins, as long as holes are within tolerance from datums (10mm and 20mm, like pic 2), the other basic dimensions from non-datums (like pic 1) can be "whatever it want", since it doesnt have a tolerance?

          Comment


          • Schlag
            Schlag commented
            Editing a comment
            Yes. The hole is not 10 mm from the other hole its 20 mm from the datum. The 10 MM wouldnt be documented or evaluated.

        • #8
          NinjaBadger
          UKCMM
          How will the two different basic dimensions give the same result?
          Lets say its a planar position zone (0.5mm A/B/C) for simplyifing it:

          (drawing 1)
          Hole one (10mm from datum B) can vary from 9.5 - 10.5.
          Hole two (10mm from hole one) can vary from 9.5 - 10.5.
          This means that hole one can be at 10.5 from datum B and hole two 10.5 from hole one.

          (drawing 2)
          Hole one (10mm from datum B), can vary from 9.5 - 10.5 so lets say its 10.5
          Hole two (20mm from datum B), can vary from 19.5 - 20.5, so lets say its at 20.5 mm.

          SO in example one hole one is 10.5 from datum B, and hole two is 21mm from datum B. (i,e. NOT within tolerance if you use cad nominals from A/B/C in FCF Position.
          IN example two hole one is 10.5mm from datum B & hole two is 20.5mm (i.e. both are WITHIN tolerance according to cad nominals from A/B/C in FCF Position.

          Please correct me if im wrong, but if you guys are correct, then PC-DMIS is deviating from ISO / ASME standard?
          Also ty for the source UKCMM

          Comment


          • #9
            Originally posted by pcdmisstudent View Post
            NinjaBadger
            UKCMM
            How will the two different basic dimensions give the same result?
            Lets say its a planar position zone (0.5mm A/B/C) for simplyifing it:

            (drawing 1)
            Hole one (10mm from datum B) can vary from 9.5 - 10.5. NO! The position tolerance 0.5 is total tolerance - so it can be 9.75-10.25 from Dat B
            Hole two (10mm from hole one) can vary from 9.5 - 10.5. NO! Even if your positional tol was 1mm (so ±0.5 as you've written) it's not being applied to the TED/Boxed value. It's applied to the theoretical position of the hole

            This means that hole one can be at 10.5 from datum B and hole two 10.5 from hole one. No! (See above - the tolerance is applied to the theoretical position of the hole, NOT the TED/Boxed dimension)

            (drawing 2)
            Hole one (10mm from datum B), can vary from 9.5 - 10.5 so lets say its 10.5 NO! The position tolerance 0.5 is total tolerance - so it can be 9.75-10.25 from Dat B
            Hole two (20mm from datum B), can vary from 19.5 - 20.5, so lets say its at 20.5 mm. Although you've got the amount of tolerance wrong (Pos 0.5 ~ ±0.25) you've got the principle right here, BUT it's exactly the same for both examples. You DO NOT get tolerance stack on boxed dimensions.

            SO in example one hole one is 10.5 from datum B, and hole two is 21mm from datum B. (i,e. NOT within tolerance if you use cad nominals from A/B/C in FCF Position.
            IN example two hole one is 10.5mm from datum B & hole two is 20.5mm (i.e. both are WITHIN tolerance according to cad nominals from A/B/C in FCF Position.

            Please correct me if im wrong, but if you guys are correct, then PC-DMIS is deviating from ISO / ASME standard?
            Also ty for the source UKCMM

            See comments in bold above.



            Automettech - Automated Metrology Technology

            Comment


            • #10
              NinjaBadger is correct here is a sketch to visualise what he said.


              If you must report the positions with the nominals as per your sketch 1 then you can make a new Alignment and use the Offset distance box to enter the nominals making sure you check the correct axis box click Origin, DO NOT select the measured feature to Reset the nominal.

              Now in the Advanced tab in Xactmeasure change the True position Alignment from Datum Reference Frame to Current Alignment
              Attached Files
              Last edited by UKCMM; 06-26-2020, 06:06 AM.

              Comment


              • #11
                You need to get a copy of the Y14.5 standard and study. You will not less GD&T but more...

                Comment


                • #12
                  UKCMM
                  Thanks, I think I will try this method, since customers always want the basic dimensions reported.
                  NinjaBadger
                  Thanks a lot for helping me. GD&T really isnt my strong side. the 0.5 position zone was my bad, was very tierd. But I think I got the hang of basic dimensions now!
                  One of the reasons i made this thread is because my superiors argue with me that if the drawing is like picture one I should report them linear dimensions 10 +- 0.25, 10, -+0.25 instead of GD&T because it makes it easier to understand..

                  I think this thread is becoming more of a GD&T thread now rather than a pc-dmis CMM thread, so I will PM you a message if you dont mind.

                  Comment

                  Related Topics

                  Collapse

                  Working...
                  X