Positional Tolerance

[[Da...]]

Hi all, I have a question about the below example of a positional tolerance and would like to know how people interpret it. Mainly the position of Ø0.005 to A.

p.s. sorry for the poor drawing it is just a quick example

[[An...]]

The measured centerline of the hole must be within a 0.005mm diameter cylinder.
The axis must be at the theoretically exact location of the "A" to be measured!

This is how I interpret it!

[[Aa...]]

A obviously doesn't constrain all degrees of freedom in this case. If the z axis is perpendicular to -A-, then it controls [u,v,z]. The rest are left unconstrained. So then, the correct coordinate system for the DRF A in this case would be the one that conforms to A and gives the optimal results for the position 0.005 control on both holes. Normal position characteristics in Calypso will not optimize the unconstrained degrees of freedom--Calypso's best tool for a situation like this is a Best Fit of Bore Pattern.

(If there were only one hole with this FCF, it would essentially be a perpendicularity control.)

[[Da...]]

So, this is really interesting, where there are multiple holes and only a 'perpendicular' datum reference then the 32 TED centres are also included in the positional, but if only one hole is defined then it is perpendicular. I assume in this case also there is an implicit TED of 0 that wouldn’t come into effect so allows for rotational fitting.

[[Br...]]

So the only constraint you have is Datum Feature A. All -A- can do here is control perpendicularity of the two holes, BUT, the pattern must be held at the .005 tolerance simultaneously. What you have is two tolerance zones located exactly 32" apart and are perpendicular to Datum A. They can rotate and move around the part but are always 32" apart and always perp to A. Basically, they just bounce around inside the 0.025 tolerance zone to ABC. Its almost the same thing as a composite tolerance.

[[Aa...]]

Actually, a composite in this case would have the same effect.

But comparing it to a composite is perhaps a good way to explain its meaning.

[[Da...]]

Thanks, I have been struggling to find a reference to this in ISO standards so needed reassurance 🙂

[[To...]]

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Silly American....LOL

[[Br...]]

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Lol minor detail

[[Br...]]

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The only difference would be that in this case, the "A" DRF is subject to a simultaneous requirement, whereas any lower segment of a composite tolerance is not.

[[Aa...]]

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Haha, good point!