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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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  • Good luck with that. I wouldn't use a portable arm on anything with tolerances less than +/- a millimeter. The one I've been using isn't anywhere near as accurate as stated. In a perfect enviornment with a perfect operator measuring perfect parts you might have a slim chance of holding toleranes less than +/- .5 millimeters . Being as careful as I can, measuring the same part multiple times, I have a very difficult time repeating within .25mm, and just so you know, .2 (point two) microns is not .008", it's .0000078". if you actually meant two microns then its .000078". Eight thousandths is .203 millimeters (203 microns) which is about as good as it gets with a portable arm, but this is seriously pushing the limits when measuring real parts. Based on my experience I wouldn't even think about making the statement that you've made above to anyone.

    A Webber bar is NOT a real part. Go measure a few of your real parts and check for your own repeatability. Then tell your supervisors what it measures in the REAL world.

    This is based on my own experience, and is my best opinion.

    I just reread your post. Substitute the word milimeters everywhere you said microns and your numbers are correct.

    Bill
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  • Good luck with that. I wouldn't use a portable arm on anything with tolerances less than +/- a millimeter. The one I've been using isn't anywhere near as accurate as stated. In a perfect enviornment with a perfect operator measuring perfect parts you might have a slim chance of holding toleranes less than +/- .5 millimeters . Being as careful as I can, measuring the same part multiple times, I have a very difficult time repeating within .25mm, and just so you know, .2 (point two) microns is not .008", it's .0000078". if you actually meant two microns then its .000078". Eight thousandths is .203 millimeters (203 microns) which is about as good as it gets with a portable arm, but this is seriously pushing the limits when measuring real parts. Based on my experience I wouldn't even think about making the statement that you've made above to anyone.

    A Webber bar is NOT a real part. Go measure a few of your real parts and check for your own repeatability. Then tell your supervisors what it measures in the REAL world.

    This is based on my own experience, and is my best opinion.

    I just reread your post. Substitute the word milimeters everywhere you said microns and your numbers are correct.

    Bill
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