I have been struggling with this for awhile and never really resolved it.

I have parts that get scanned via CMM with hundreds of points. The PCDMIS CMM output is the min and the max along with a value called "measured" which is the absolute difference between the two if different signs and the greater magnitude of the two if the same sign.

example; a) min = 0.05 mm & max 0.10 mm. "measured" = 0.10 mm

b) min = -0.1 mm & max = 0.05 mm. "measured" is 0.15 mm

I am trying to calculate capability on a feature with a bilateral profile tolerance of 0.25 mm. I have thought about a couple different methods, but both seem to have pros and cons. Of course, I want to determine the "worst case" condition and apply it against the tolerance.

1) Use the "measured" value. The downside is that this "measured" value gives very different values depending upon if the signs of the min and max are opposite or the same which may end up overstating the amount of variation from nominal. In exp. b above, my largest variation from nominal is still only -0.1 mm, but the value I have is 0.15 mm.

2) Select either the min or max depending on which one is larger. This option was the one that made the most sense, but has one big drawback. The min and the max can be very close in magnitude, but with opposite signs. When I select the larger of the two, I often end up with values the bounce back & forth between + & -, again now overstating the variation.

exp; part #1 is min.= -.032 & max = .034 use .034 since it is larger

part #2 is min= -.033 & max = .031 use - .033 since it is larger

Now it looks like I have a range of .067 when in actuality it is only .003

Anybody have any suggestions?

I could do min and max capability seperately, but that seems like a lot more work and I'm not sure there is value there either.

I have parts that get scanned via CMM with hundreds of points. The PCDMIS CMM output is the min and the max along with a value called "measured" which is the absolute difference between the two if different signs and the greater magnitude of the two if the same sign.

example; a) min = 0.05 mm & max 0.10 mm. "measured" = 0.10 mm

b) min = -0.1 mm & max = 0.05 mm. "measured" is 0.15 mm

I am trying to calculate capability on a feature with a bilateral profile tolerance of 0.25 mm. I have thought about a couple different methods, but both seem to have pros and cons. Of course, I want to determine the "worst case" condition and apply it against the tolerance.

1) Use the "measured" value. The downside is that this "measured" value gives very different values depending upon if the signs of the min and max are opposite or the same which may end up overstating the amount of variation from nominal. In exp. b above, my largest variation from nominal is still only -0.1 mm, but the value I have is 0.15 mm.

2) Select either the min or max depending on which one is larger. This option was the one that made the most sense, but has one big drawback. The min and the max can be very close in magnitude, but with opposite signs. When I select the larger of the two, I often end up with values the bounce back & forth between + & -, again now overstating the variation.

exp; part #1 is min.= -.032 & max = .034 use .034 since it is larger

part #2 is min= -.033 & max = .031 use - .033 since it is larger

Now it looks like I have a range of .067 when in actuality it is only .003

Anybody have any suggestions?

I could do min and max capability seperately, but that seems like a lot more work and I'm not sure there is value there either.

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