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Accuracy

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Accuracy
Does the 4 to 1 rule apply to Portable CMM measuring accuracy, how do I best explain this to my superiors? The arm we use states a length accuracy of .016microns or .00062 inches so do I multiply this x 4 = .0025 and tell them that the expected accuracy of the arm would be this? Hence trying to verify position tolerances of .05 micron might be a stretch of the arms capability, even though I can repeat good measurements on a 20,’’ Weber gage within .0005 Every time no matter where I place it in the reach of the arm, this would include the Z axis.

I said that I could say with out a doubt we could verify position tolerances within .2 microns or .008’’, and I would not want them to lose work over my statement if I am not correct
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  • Point repeatability to 16 microns, sounds like you have a 1.8 meter Infinite, true?

    If so, this is maximum range over 2 of any of the 3 axis so if you turn probe comp off in PC-DMIS and articulate in a seat while taking points, no value of any axis should deviate by more than 32 microns. Of course this is in decent conditions and a good setup. If you don’t have a decent environment or are unable to set it up appropriately, then please understand that is why your numbers are so bad.

    If you cannot get your arm measure in spec under good conditions, please send it in for calibration rather than tell the whole world that the arms are incapable of measuring better than 250 microns.

    The arm itself is in excellent condition, the environment is shop, and the arm is moved from job to job. Its exactly what the Romer is said to be designed for when sold. The product is fixtured with datums on the fixtures to align too. The fact is that in this environment and these conditions, the Romer does not live up to its specifications. This being the case, I personally wouldn't use it for anything that has a tight tolerence. You may have yours in a temp controlled environment, locked down to a table, never move it, and have parts that are conducive to using a portable arm on them. MAYBE it will do better in this case. None of that was stated in the original question therefore I assumed that he was going to be using it as a portable arm in shop environment conditions and if this is the case then holding the tolerences hes talking about is, in my opinion, out of the question.

    Bill
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  • Point repeatability to 16 microns, sounds like you have a 1.8 meter Infinite, true?

    If so, this is maximum range over 2 of any of the 3 axis so if you turn probe comp off in PC-DMIS and articulate in a seat while taking points, no value of any axis should deviate by more than 32 microns. Of course this is in decent conditions and a good setup. If you don’t have a decent environment or are unable to set it up appropriately, then please understand that is why your numbers are so bad.

    If you cannot get your arm measure in spec under good conditions, please send it in for calibration rather than tell the whole world that the arms are incapable of measuring better than 250 microns.

    The arm itself is in excellent condition, the environment is shop, and the arm is moved from job to job. Its exactly what the Romer is said to be designed for when sold. The product is fixtured with datums on the fixtures to align too. The fact is that in this environment and these conditions, the Romer does not live up to its specifications. This being the case, I personally wouldn't use it for anything that has a tight tolerence. You may have yours in a temp controlled environment, locked down to a table, never move it, and have parts that are conducive to using a portable arm on them. MAYBE it will do better in this case. None of that was stated in the original question therefore I assumed that he was going to be using it as a portable arm in shop environment conditions and if this is the case then holding the tolerences hes talking about is, in my opinion, out of the question.

    Bill
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