Datums BC Sometimes A

[gregfox]

On a simple bock with a hole or boss, I’ve seen three datum references ABC and a single positional tolerance. I understand the need for the xy placement tolerances of B and C relative to the two side planes, but not the top surface A.

Obviously a feature has to be on the surface referenced as A, but the positional tolerance relative to datum A does not make sense. it’s not like it is saying it can be some amount from the surface.

Separate parallel and perpendicular control frames make sense. I’ve read where perpendicularity is assumed from the positional, but in that case the actually tolerance value does not seem to unless the value is simply a catch-all tolerance for features.

I’ve also seen just B and C, with A treated separately




Fox Manufacturing Group

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[powerhound]

It means that the hole has to be perpendicular to surface A first, not parallel to B first. There is a difference.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
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[gregfox]

So this assumes that the XY tolerance is same as that required for perpendicularity, just like having a global tolerance in the title block?

Fox Manufacturing Group

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[Belanger]

The hole is positioned in a "datum reference frame" made up of A, B, C collectively, so you can't really chop it up and think of the separate datums individually. Powerhound is correct that it shall be perpendicular to A. And also keep in mind that datum A sets the stage for contact against B and C.

If you lop off the reference to datum A, then you would be saying that datum B is primary, and that would not accurately simulate that functional contact situation in 3-D space.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[gregfox]

O.k., but it you wanted a tighter tolerance for perpendicularity, and placed an additional reference say on the feature's edge or center line with a tighter tolerance, would they conflict or would the second simply supersede the first tolerance?

Fox Manufacturing Group

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[Belanger]

A separate perpendicularity tolerance on the hole (think of its axis) would be a refinement (a tighter tolerance) of the perpendicularity aspect that position was doing. IOW, superseding. You'd still leave the reference to A in the position callout.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

This is not the first person asking about difference between |A|B|C| and |B|C| in positional callout for this scenario.
Perhaps attached pdf will be useful.

http://files.engineering.com/getfil...-992c-41ee-b284-0289f285c16d&file=pos_tol.pdf

Of course this does not mean that there are no real life situations where |B|C| couldn't be used - I just wanted to show a difference between these two tolerancing schemes.

 

[gregfox]

Thanks all. It was one of the more confusing aspect of gd&t

Fox Manufacturing Group

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