Quick parallelism question

[ET50]

Datum A has 2 surfaces parrallel to it. One surface to the right of Datum A, and 1 to the left of Datum A. Should there be 2 seperate dimensions originating from Datum A with 2 seperate parralelism callouts, or can you dimension from Datum A to one surface, and then a second dimension from the first surface to the second surface with 1 feature control frame? I'm looking at a drawing dimensioned the second way. The first dimension does not have feature control frame, but the second dimension does. The intent of the designer is to have each surface parralel to Datum A within the same tolerance.

 

[KENAT]

If the surfaces are located by some kind of GD&T (such as profile of a surface) such that your chained dimensions are 'basic' then either way is fine.

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[Belanger]

If each surface is called out with parallelism, those two parallelism zones act independently. In other words, one "shelf" on the left could be offset too much (higher) than the other shelf on the right, yet they are both parallel. If you want them to be parallel to datum A, but still in line with each other, you'd have to use profile of a surface (either with or without a basic dimension to datum A, depending on the desired location tolerance).
If you are able to post a sketch, that always helps.

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

http://www.gdtseminars.com

 

[ET50]

It's a little odd (to me) how they have it called out. What I want is for each surface (1 to the right and 1 to the left)to be independantly parallel to Datum A. I would think the best way is to have 2 callouts, each independant and with it's own dimension from A. The way they have it is a dimension from A to one surface (on the right), and a second dimension from the surface on the right to a surface on the left. The second dimension has a prallelism feature control frame callout below the dimension. Does this then control both surfaces to be parallel to Datum A even though it isn't dimensioned from Datum A? Just tring to keep it as simple and clear as possible to the person manufacturing the part.

 

[KENAT]

I'm confused, a surface be parallel to A doesn't have to be directly toleranced to A.

The way you specify the parallelism to datum A is in the FCF(s) not the dimension scheme.

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[dtmbiz]

ET50

IMO either method described is valid.
If you have a dimension from Dat A to one side and an overall dimension;
or two dimensions from Dat A to each surface (more tolerance stack); or basic
dimensions with a profile control; all dimensional schemes are locating those surfaces.
The parallelism (orientation) back to Dat A would prescribe a boundary of 2
parallel planes (for each parallel control) which are aligned / oriented
in relationship to Datum A as a refinement
of orientation to those surface within the size limits.

 

[ET50]

I should clarify my answer above. There are no basic dimensions and no profile callouts, just parallelism

 

[ET50]

Okay and thanks to all. I understand now that where the feature control frame is makes no difference. I will have a tolerance stack up going from one dimension to another however. One dimension is +/- 0.30 and the second is +/- 0.60.

Thanks again!