GDT Question - is this even correct?

Ron Rompen

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#1
I am attaching a file with part of a drawing on it. My question relates to the callout on the LEFT side of the drawing.

This calls the position of the 71.5 diameter (tolerance 0.25 diametrical) when restrained to A and B (B is on the right side of the drawing)

My interpretation of this is that the diameter (which is also identified as datum A) can NEVER have a position error greater than zero (when called out the way it is). Since it is a datum feature, and is used to create the alignment, it can only be at the 'perfect' position.

Am I interpreting this correctly?

Thanks for any feedback.


gdt.jpg
 
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John Predmore

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#2
Ron, you should have this conversation with the design-responsible engineer. FWIW, my interpretation is the 71.5 diameter is located with a tolerance of position related to the centerline connecting the A feature and the B feature, which is not the same as A datum. In the feature control frame, both A and B and the 0.25 tolerance use the MMC modifier, which is done to enable fixed gage tooling. I don't know how the tube was made, but the centerline A on the physical part might not be perfectly aligned with the centerline of B, so the position of 71.5 is measured with respect to the combination of A-B, plus bonus tolerance from the MMC modifier. BTW, the datum of a cylindrical feature is the centerline, not the diameter.
 

Sebastian

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#3
Positional Tolerance - an overview | ScienceDirect Topics
"Common datums"
1. Position of cylinder with diameter 71,5, height 22 shall not exceed 0,25 MMC against common datum consisting of cylinder with diameter 71,5, height 22 MMC and cylinder with diameter 70, height 27,5 MMC
2. Position of cylinder with diameter 70, height 27,5 shall not exceed 0,25 MMCagainst common datum consisting of cylinder with diameter 71,5, height 22 MMC and cylinder with diameter 70, height 27,5 MMC
Correct me, if I am wrong.
 
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