Concentricity reporting

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RodT
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Concentricity reporting

Post by RodT » Tue Aug 26, 2014 12:38 pm

Hello,

Why is it that when reporting the concentricity of 2 circles (neither of which are an origin) I get a lot of error, but when reporting each others Y and Z values those check good. This is a simple round part (attached) with a thru hole. The drawing requires each end of the hole to be concentric to each other within .005". When I report it I get .008". However, each one of their Y,Z vaules is within .0007". How can this be? I even tried making one of them XYZ zero and then reported the other. I got the same result.

Thanks
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Ryan Christopher
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Re: Concentricity reporting

Post by Ryan Christopher » Tue Aug 26, 2014 1:02 pm

No matter how the holes locate to your alignment, that doesn't really matter. The relationship between the feature and the datum determines error here.

I like to imagine situations like you are showing by thinking of them as something I could inspect with a vee block. Then the relationships between d1, d2, and the datum -A- axis angle relative to the feature become simpler to visualize. With a simple picture, now I'm able to simulate errors for the d1 length or the Datum angle and trig out how much error is needed to get the amount that I am seeing. Then I know if my error is reasonable or not.

Most importantly, read through ASME Y14.5 for no B.S. explanation!
vee block pickture.png
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Ryan Tackes
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Sideoiler427
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Re: Concentricity reporting

Post by Sideoiler427 » Tue Apr 17, 2018 12:54 pm

Hi I am new to this forum and just recently upgraded our manual machine to the CMM Manager software. I am having issues when inspecting concentricity and was hoping that someone could provide additional information being my issue is 4 years after the last post. The requirement for my feature is diameter .627/.625 concentric to [A] within .002. Datum [A] is a cylinder 2.877 / 2.875 x 2.875 deep , and when the two are checked using the concentricity icon on our manual machine the results are .003 - .0037. If I just use the location results between the two diameters, I get .001 - .0014. To confirm my results I inspected the two features in three other methods, conventionally, with our DCC CMM, and with our Micro-Vu. This lead me to believe that there was something off with the results on my manual machine, running the CMM Manager S/W as all the other forms of inspection reported acceptable results (.001 - 0014).
Can anyone explain why when I select the concentricity option to report the results it more than doubles the reported results from the feature locations or the results from other inspection? Keep in mind I tried inspecting as circle / circle and as circle / cylinder features, with the end results being the same. I also was using 6 - 8 points on the circles and 12 - 14 on the cylinder. No matter how the features were inspected on this machine, the end results showed out of tolerance.

Any input will be greatly appreciated!
Thank you,
Chris

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Re: Concentricity reporting

Post by medupriest » Tue Apr 24, 2018 1:24 pm

When doing concentricity with two circles, the calculation will be using your reference frame for the axis for the concentricity. When you use a cylinder for the datum, it will use the axis of the cylinder at the axis for comparison which will provide a more accurate result.

Reporting the basic dimensions doesn't provide a clear comparison as concentricity is looking at a median axis from the points and on a 3D feature, there isn't a clear way to do that.

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