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I/D Cylinder Probing Variance

Yamadog
Hello All,
Im quite new to CMMs and PCDMIS. I've only been actively working with the Romer Arm for maybe a month. Today I was probing a feature on a very large connecting rod for a frac pump which is a knuckle type feature consisting of an I/D cylinder and O/D Cylinder. I was using the 6mm Ruby and getting consistent oversized results on the I/D. I/D is to be 2.999 +/- 0.001. My readings were all ranging 3.002/3.004. When checked with tri-mic I was in tolerance at 3.000. I finally decided to switch to a 3mm Ruby and what do you know....now I'm in at 3.0002. So I'm not sure if my 6mm is damaged or if it's something I'm doing incorrectly. Any advice? Also my results checking the O/D with the 3mm vs 6mm only yielded a variance of 0.0012 substantially less than the I/D variance I saw. I appreciate the help. Thank you kimdly.
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  • In my experience, holding +/-.001 with that large of a diameter with the arm is difficult. With the arm, the fewer points you take, the less amount of error you will have, for a cylinder I only go 8 hits max. The best way is to have a reference gauge with a known diameter, measure and compensate.
    that

    Another trick is to calibrate to a sphere. Calibrate to a sphere by taking your points with the arm in the same orientation/angle that you are taking the cylinder points at. This brings the error down significantly, but will add more error if using that calibration at different arm angles. You are basically not compensating to for the full range of the arm and only focuses on the range in which you are trying to measure. If you have a repeatable setup, this is a good option, just use one probe with that calibration for just that feature.
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  • In my experience, holding +/-.001 with that large of a diameter with the arm is difficult. With the arm, the fewer points you take, the less amount of error you will have, for a cylinder I only go 8 hits max. The best way is to have a reference gauge with a known diameter, measure and compensate.
    that

    Another trick is to calibrate to a sphere. Calibrate to a sphere by taking your points with the arm in the same orientation/angle that you are taking the cylinder points at. This brings the error down significantly, but will add more error if using that calibration at different arm angles. You are basically not compensating to for the full range of the arm and only focuses on the range in which you are trying to measure. If you have a repeatable setup, this is a good option, just use one probe with that calibration for just that feature.
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