Accuracy and spec

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  • Accuracy and spec

    When my quality manual states use measuring equipment with a minimum accuracy ratio of 4 to 1, meaning product tolerance to equipment tolerance. How does that works with CMM's? what is my equipment tolerance, I am assuming its the same thing as accuracy?

    What do I look for in the Calibration certificate? is it MPE? or what?

    B&S One
    PC-DMIS CAD v2014

    Romer Infinity

  • #2
    I would just use the tolerance formula of the cmm (x+L/xxx) on the min case.
    I don't mean that the uncertainty is this value (there are very different !), but in a quality world, there's no reason to use another value for a cmm than for a caliper.
    For a caliper, you can use the calibration value, here, it's hard, so using the mpee should be enough.
    Just my opinion...


    • WolfMan
      WolfMan commented
      Editing a comment
      There is a formula for Maximum Uncertainty of Length Measurement and The maximum permissible indication error. I am assuming the MPE is the one I need.
      Last edited by WolfMan; 11-20-2019, 03:55 PM.

  • #3
    Even if it's not really true, the MPEe certify that the measurments error on a length are lower than the corresponding value.
    I don't know exactly about the max uncertainty, but I believe that it's the uncertainty calculated during the calibration, takig into account some parameters like temperature, MPEp, uncertainty of calibration of the artifact, of the standards used (...). It's valid only on 2D distances.
    It's imposible to use this value if you measure a classical part, like a LS circle.
    If your QM doesn't know to much about cmms, using MPEe should be enough.
    If he knows a little more than nothing, then you will have to proove what you say, and in this case, good luck !
    Calculating an "global" uncertainty seems (to me) to be impossible, you should have to calculate it for each feature (dimension, location, form...), taking into account the cmm, the probe, the part and the method !!!!!

    bis, just my opinion...


    • WolfMan
      WolfMan commented
      Editing a comment
      Jefman, I am not trying to calculate uncertainty. I want to know which of the values I need to use to comply with 4:1 rule.

  • #4
    If your dimension is approaching "state of the art", that 4:1 ratio is subjective to your level of risk.
    I would consider any non-linear dimension tolerance of <0.0004" would be considered approaching state of the art.
    Linear tolerances, I would consider state of the art <0.0001"
    Again, these values are subjective. Maybe if I had better equipment, these arbitrary values would differ. Other contributing factors will always compound this confidence interval (environment, bias in fixturing or part setup, etc)
    Last edited by louisd; 11-20-2019, 05:15 PM.


    • #5
      I know some customers require a Gage R&R study on critical dimensions to verify repeatability and reproducibility of the measurement system used. Error must be no greater than 30% (measurement deviations/tolerance), anything under 10% is excellent. Perhaps that's why its in the Quality manual, so if it ever was required, there's a good chance you'd be able to meet that requirement.


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