faulty diameter measurement = to faulty TP measurement output?

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  • faulty diameter measurement = to faulty TP measurement output?

    I have a part with diameter and true position call out. If you have an scenario where the CMM is not measuring the holes correctly, and I decide to manually inspect them with pin gages and not report diameter on the CMM, is it true that if I do this then I cannot rely on the CMM true position measurement since this is “diameter” dependent. I thought that the size of the hole doesn’t effect TP bur rather just the position?

  • #2
    diameter CAN have no effect on Position (or true position) if you are using RFS (regardless of feature size). Hole can be 20mm over-size or 10mm undersize and the position value will remain the same.
    Of course, if you are getting bogus numbers for diameter, then the calculated center point (where all results come from) will be bogus as well. If you are talking a few microns 'big or small' for the size, then position is fine, but if it says a 10mm hole measures 12mm, but checks to a pin of 10mm, you have issues that MUST be addressed before you do anything else.
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    Originally posted by AndersI
    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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    • gdntnovice
      gdntnovice commented
      Editing a comment
      The issue is due to the holes being drilled in a conical shape. Btw, in this case is there any standard proper way of measuring? Should I measure the min or the max? Is pin gage a valid option on this scenario?

  • #3
    Based on the calculation of the hole size, the center of the hole will change. The "center" of the hole is the location. I believe that Least SQ and Max inscribe can/will result in different locations. Now, what type of tolerances are we talking here ? This will determine if this is an issue or not.



    What do you mean hole size will not effect TP and just the position ? TRUE POSITION is just an ancient term that doesnt seem to die, POSITION is the same thing....

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    • Douglas
      Douglas commented
      Editing a comment
      this reminds me of a guy who went out of his way to correct me... he was talking about micro-meters and did not accept my reply in microns. I think I actually made him look up what a micron is

  • #4
    What’s the reason for conical shaped hole?

    Locating on the cone? or locating on the through hole?

    Anyways, the entire feature length must be to size and position.

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    • #5
      You can measure a circle/cylinder and then report the T.P. without reporting the diameter size AS LONG AS your T.P. zone is RFS (No "M" or "L" symbol).

      That being said, WHY is your diameter NOT a diameter? What shape is it supposed to be per print?

      We'd love to help you figure out the best way to report your geometry but we'll need more information from you otherwise we'll just have to keep guessing.
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      • gdntnovice
        gdntnovice commented
        Editing a comment
        Can you please refer me to which ANSI or ISO standard allow me to do this: "You can measure a circle/cylinder and then report the T.P. without reporting the diameter size AS LONG AS your T.P. zone is RFS (No "M" or "L" symbol)."

    • #6
      Using Maximum Material Boundary (MMB) and Least Material Boundary (LMB)


      The ASME Y14.5-2009 standard specifies how both the size and position or orientation tolerances of a datum feature are to be used in calculating the datum MMB or LMB. Refer to "ASME Y14.5-2009 Dimensioning and Tolerancing" section 4.11.6, "Determining Size of Datum Feature Simulators at MMB", and succeeding sections on LMB. Figure 4-16, "Example Calculations of Maximum Material Boundary", in the ASME standard provides examples of how the MMB boundary is calculated for a datum feature, taking into account both the size of the datum feature and position or orientation tolerances on the datum feature.

      Following the Y14.5-2009 standard, PC-DMIS calculates datum material boundaries as follows:
      • MMB - When you specify circle-M https://www.pcdmisforum.com/i_mmc.gifon a datum feature of size, PC-DMIS calculates the datum MMB by including the effects of size and any preceding position or orientation tolerances on the datum feature, while respecting the datum order of precedence as specified in the ASME Y14.5-2009 standard. Precedence in a FCF moves from left to right.
      • LMB - When you specify circle-L https://www.pcdmisforum.com/i_lmc.gifon a datum feature of size, PC-DMIS calculates the datum LMB by including the effects of size and any preceding position or orientation tolerances on the datum feature, while respecting the datum order of precedence as specified in the ASME Y14.5-2009 standard. Precedence in a FCF moves from left to right.

      Stating a Specific Boundary

      If you want to define a specific boundary, perhaps because the implied boundary is not clear, you can use the Feature Control Frame Editor to explicitly state the boundary by enclosing it within brackets immediately following the applicable datum feature reference and any modifier following that datum feature reference. This value represents the size of the material boundary. PC-DMIS uses it to calculate the bonus on the datum feature, disregarding any preceding position or orientation tolerances on the datum feature:

      https://www.pcdmisforum.com/example_fcf_mmb.gif

      Feature Control Frame Editor showing the MMB symbol and the square brackets that will hold the boundary value
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      • BIGWIG7
        BIGWIG7 commented
        Editing a comment
        In other words, if you dont have a modifier, THE SIZE OF YOUR FEATURE WILL NOT IMPACT HOW POSITION IS MEASURED.

    • #7
      I think the solution has been stated in a couple of different posts. Hopefully this will summarize everything.

      The deviation in size of your feature will not be used to modify the *positional tolerance* if the feature is being measured RFS (no modifier).
      If the feature is being analyzed at an alternative material condition (MMC, MMB, LMC, LMB), then the deviation in size will be used to increase the available tolerance (MMC, LMC) or even for "datum shift" (MMB, LMB) to optimize your results.

      It is crucial that you are not allowing more tolerance than you actually have. For example, if you are measuring a datum feature that is referenced at MMB and it's size measures .002 from MMC on an alternate gage, but the CMM is reading it at .005 from MMC, PC-DMIS will allow more datum displacement then would actually be allowed, thus not reporting accurately.

      As has been stated, if the deviation between CMM size measurement and an alternate gage size measurement is insignificant, then I'm sure you are OK.
      If it is significant, you will have larger problems on your hands. The CMM is not reading the part correctly (weak alignment, weak partholding, etc...). You will reject good parts or pass nonconforming parts depending upon the direction of your error.

      If I were you, I would attempt to validate your program by checking to an alternate gage. So long as the two size measurements are within 10-20% of the allowable size tolerance, I would say the CMM is close enough.

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      • #8
        If you are measuring circles in the cone, make sure the hits are taken perpendicular to the cone centerline otherwise the measured circle center could end up deviating from the cone centerline (measuring a tilted/skewed circle in the cone). To get rid of this, measure the cone as a cone and construct circles using intersections with planes and evaluate these instead.
        PC-DMIS CAD++ 2o19 R1 SP11

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