Flatness/Form Error Reporting

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Clouseau
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Flatness/Form Error Reporting

Post by Clouseau » Fri Oct 18, 2019 2:23 pm

I’m very suspicious of the data that CMM-Manager is outputting to my report. How can I have:
a max deviation of .031”
a min deviation of -.002”
but my RMS deviation is .004”?

Max Dev is defined in the Help section as “The maximum deviation between a point and its corresponding best fit point.”
RMS Dev is defined as "The average (root mean squared) deviation."

I’m not sure where CMM-Manager is going with this; I want a measurement based on the ASME Y14.5M 1994 definition of Flatness:

6.4.2 Flatness. Flatness is the condition of a surface having all elements in one plane.
6.4.2.1 Flatness Tolerance. A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface must lie.

I want a single number that reports the minimum distance between two parallel planes that encompasses all points on my surface, and I'm not sure that CMM-Manager is giving me that.

Kind Regards,
Doc
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Clouseau
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Re: Flatness/Form Error Reporting

Post by Clouseau » Wed Oct 23, 2019 1:51 pm

I believe I’ve figured out what you get when you click Form Deviation.

Firstly, what you don’t get is Form Error, but a break-down of form error. After the software calculates the location of the plane (in the case of flatness), Form Deviation reports the highest point and the lowest point relative to what it calls “best fit,” plus the “RMS Deviation,” which is a misleading term. I interpreted “RMS Deviation” to be the flatness of a plane whose location was calculated via the root-mean-square algorithm. I was wrong and gave incorrect information to my engineering team because of this misunderstanding. “RMS Deviation” should actually be called Standard Deviation as it’s known in statistics, because it is the average deviation of all the points used to define the location of the plane.

If I add the absolute values of the Min and Max deviations I get the form error measurement I’m looking for (see attached). I’ve noticed that in one of the reporting areas called “Feature Report” there are selectable values for each feature you can choose to have reported, including “F” which presumably means “Form” or “Form Error.”

I haven’t figured out how to access this yet : (

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CrashN8
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Re: Flatness/Form Error Reporting

Post by CrashN8 » Fri Oct 25, 2019 1:41 pm

CMM-Manager is NOT reporting Standard Deviation that I know of. Form Error is in fact the delta between highest and lowest points as compared to best-fit (RMS) feature.

I think you need to review tie results again.

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