Should this dimension be boxed? 2

[illini8181]

If you have a part with two parallel surfaces which are controlled with GD&T (flatness and parallelism), should the dimension defining the distance between them be boxed? Or, is this optional? I am looking at two drawings put out by my company. In this scenario, one of them boxes this dimension, and one does not.

I could see this being optional. In other words, if it is not boxed, that means you go to the standard drawing tolerance block for the tolerance on the dimension, and the GD&T only controls the flatness of the first surface, and the orientation of the second surface relative to the first surface. But, if it is boxed, the GD&T would additionally provide the tolerance on the dimension defining the distance between the surfaces.

For the example part in the attached document, the tolerance on the dimension circled, if boxed, would be +/- .010. If not boxed, it would be +/- .040 (per our group's tolerance block).

This is probably a pretty simple question, but I'm really not sure!

 

[illini8181]

Oops, forgot the attachment.

 

[Belanger]

In your attachment, that dim of 8.00 must NOT be boxed (basic dim). This is because parallelism's job in life is to control the orientation (and form, indirectly) of the surface, never the distance. If you used the symbol for profile of a surface rather than parallelism, then you would have a choice of boxed or not boxed, depending on whether you wish to control orientation or location.

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

 

[pmarc]

No, this dimension should not be boxed.
In fact boxed (basic) dimension would be meaningless in this case. It defines perfect distance between left face of the block and datum plane A established from right face of the block, but since parallelism FCF does not have power to control any locational aspect of the relationship between two faces, left face could be at any distance from datum plane A.

 

[pmarc]

This is what I call perfect timing, J-P

 

[illini8181]

Thanks for the clear and helpful responses.