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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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  • 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

    Bill,
    I have used a great many arms in hundreds of shop environments, all over the world and that is what I’m basing my assertions. Typically the arm was either delivered in advance or pulled out of the back of a truck; it doesn’t get much more portable than that.

    I’m not saying that you are wrong about the numbers you are seeing, just that if 1) your arm is still in specification, and 2) your arm mounting and 3) part setup are good, you should expect to see numbers consistent with the spec. If you don’t, something is wrong, usually with one of the 3 things I list above.
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  • 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

    Bill,
    I have used a great many arms in hundreds of shop environments, all over the world and that is what I’m basing my assertions. Typically the arm was either delivered in advance or pulled out of the back of a truck; it doesn’t get much more portable than that.

    I’m not saying that you are wrong about the numbers you are seeing, just that if 1) your arm is still in specification, and 2) your arm mounting and 3) part setup are good, you should expect to see numbers consistent with the spec. If you don’t, something is wrong, usually with one of the 3 things I list above.
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