ISO vs ASME on XactMeasure

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  • ISO vs ASME on XactMeasure

    My question is about difference between this 2 standards.
    I have part where datum "A" is a plane. All planes parallel to "A" need to be measure and reported position tolerance (0.05mm in relation to "A")
    When I using ISO standard results is about 0.05-0.07 out of tolerance, but when I switch to ASME then results are green (0.02-0.03).
    Just for test, I check locations of this planes and distance from "A" and it's look like ASME showing me right results.
    It is possible that this 2 standards is so much different?

  • #2
    Hmmm, that's odd.

    Position in ASME applies to feature's of size only (Diameters & Widths) and can't be applied to a face as such.

    When you're checking using location / distance, what's the deviation of the plane? Can you post a section of the report?
    Automettech - Automated Metrology Technology


    • #3
      Probably need to do Profile. Look at your Max and Min measurements. They should be the same on both.

      ASME: Adds the max and min (if on opposite sides of the tolerance bands). Otherwise, if the measurements are on the same side of the tolerance band, it uses the worst one. You need both MAX and MIN measurements to determine whether the profile is good. Do not use the MEAS number.

      ISO: Uses the MAX and MIN to determine the worst deviation. It takes that worst deviation and multiples it by 2. For ISO, use the MEAS number.


      • #4
        I'm quite certain that ISO spec utilizes datum simulators as datums (it should calculate based upon virtual condition IE three highest points of your plane), whereas ASME will let you use best-fit planes to be datums.

        Per the Help file: Information on FCF Dimension Calculations

        PC-DMIS and ASME Y14.5M-1994
        • PC-DMIS GD&T follows ASME (ANSI) Y14.5M-1994. The mathematics for this standard are described in "ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles". This standard is similar to ISO 1101. The main difference is that Y14.5 requires Position datum fitting to find the candidate datum reference frame that minimizes the deviation of the considered feature. PC-DMIS allows the user to turn this off with the Fit to Datums check box.

        Profile Calculations
        • In v4.2 and higher, the registry setting UseISOCalculations, found in the Options section of the PC-DMIS Settings Editor, can be set to 1 to report profile as two times the maximum deviation. This setting only affects Profile (not Flatness). Also, when the GD&T Standard on the Advanced tab of the GD&T dialog is set to ISO 1101, the Profile reports two times the max deviation and formonly Profile ignores size.
        • In PC-DMIS 2009 and higher, FCF Profile with formonly takes into account size when UseISOCalculations is set to 0 (ASME) or the GD&T Standard selected on the Advanced tab of the GD&T dialog box is ASME Y14.5. This is also true for legacy Profile formonly. A new registry entry called UseSizeForProfileDimensions in the Option section of the Settings Editor, lets you revert to the V42 and earlier behavior for legacy profile dimensions only. This registry entry defaults to 1 (TRUE), so you must use the PC-DMIS Settings Editor to change it to 0 (False). If set to 0, legacy Profile formonly will ignore size. However, the FCF profile will always take size into consideration even when this entry is set to 0 if the GD&T Standard is set to ASME Y14.5.

        Some Calculation Differences
        • Legacy dimensions for roundness such as a Location dimension's RN line or a legacy Circularity dimension, are computed using the Least Square solution. On the other hand, FCF dimensions for roundness (Circularity and Cylindricity) in version 4.2 and higher are computed using the Tchebychev algorithm (min/max) as required by the Y14.5 standard. Because of the change in calculation, Circularity and Cylindricity FCF dimensions will generally compute to a slightly smaller value than their legacy counterparts.

        Form Calculations
        • FCF dimensions in PC-DMIS 4.2 and later support the Y14.5 definitions of Circularity and Cylindricity.
        • FCF dimensions in PC-DMIS 4.3 and later support the Y14.5 definitions of Flatness. FCF flatness uses a Tchebychev (min/max) algorithm. Legacy flatness still uses the LS method. FCF flatness generally gives a somewhat smaller flatness value than the LS flatness. This is independent of the UseISOCalculations registry setting.
        • FCF dimensions in PC-DMIS 2009 and later support the Y14.5 definition of Straightness when using FCF dimensions.
        • Legacy form dimensions still calculate the least squares form.

        A Note on Parallelism Evaluation
        • The evaluation of parallelism is 3-dimensional regardless of the workplane or the feature being dimensioned.


        • #5
          There is eksempel of what is different between iso and asme. PLN10 in Y axis is parallel to Datum E (also plane) with distance between 0.125mm EXEMPEL.JPG


          • #6
            Neither of the agree with the location distance. Note they both agree with the Nom / Meas / Dev in the FCF Summary.

            Note this is just the centroid of the plane. You need to look at the flatness / angle / individual hits to see why you're getting what you're getting.
            Automettech - Automated Metrology Technology


            • #7
              Enable TEXT=ON in the FCF.

              I know some versions have had some wonky results dealing with planar TP tolerance zones as well. Make sure that the correct axis for evaluation is selected in the planar tolerance setting, or enable the other two axises in the report to see if anyone of their deviations matches the TP result...
              PC-DMIS CAD++ 2o18 R2 SP5


              • #8
                I agree, turn on text analysis. For kicks and giggles, turn off Fit To Datums and see what happens.


                • #9
                  darius Position should not show any difference at all between ASME and ISO. And doesn't do it when I do a quick test in 2019 R1. Verify that the THEOs are identical in the two dimensions. What's the value of UseISOCalculations in Settings Editor - I believe this should be zero nowadays as we have the ASME/ISO choice in the dimension itself.

                  louisd I have never seen PC-DMIS re-compute a datum plane (only circles and cylinders) and a quick test with a LSQ plane and the tangent plane created from the same points show that PC-DMIS is *not* re-calculating the datum plane according to the ISO standard, you have to do it yourself (and that's why "Tangent plane" has been renamed "Primary datum plane" in 2019 R1).
                  SW support - Hexagon Metrology Nordic AB


                  • #10
                    I found this while doing the google. some slides don't apply as they are draft revs of 2019 proposals

                    page 28 is particularly applicable
                    Last edited by louisd; Today, 10:46 AM.


                    • louisd
                      louisd commented
                      Editing a comment
                      Slide 18 says planar construction with ISO 5459 (2011) rev is a constrained Chebyshev, whereas Slide 5 shows ASMEy14.5 2009 is technically undefined aside from it needing to be a datum plane simulator (giving three possible options). As far as what rev of PCDMIS applies which interpretation... that's anyone's guess considering they are making such dramatic changes to 2018R2 as proposed in this pdf

                  • #11
                    Originally posted by AndersI View Post
                    darius Position should not show any difference at all between ASME and ISO. And doesn't do it when I do a quick test in 2019 R1. Verify that the THEOs are identical in the two dimensions. What's the value of UseISOCalculations in Settings Editor - I believe this should be zero nowadays as we have the ASME/ISO choice in the dimension itself.
                    Good catch, if I recall correctly, 2015 was the last version that used that registry setting or it was the first version that didn't use it. Can't remember which.
                    PC-DMIS CAD++ 2o18 R2 SP5


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