Y14.5, Question on Parallelism Application

[cbrf23]

Hello all, I have an interesting question on using GD&T to control parallelism between 2 theoretical planes.

In the example attached, I'd like to use a parallelism control frame to say "DATUM E MUST BE PARALLEL TO DATUM D WITHIN .01 INCHES."
As of now, I'll probably add this as a note like I wrote above, but I'm very curious what the proper way to do this would be following ANSI Y14.5-2009.

I'd like to keep the drawing true to how the CMM program runs, which in this case is to create two planes from the identified features and check parallelism between them.
My initial thought was to use datum targets on each feature to establish the two planes, but I'm not sure how I could apply a tolerance to one of the planes (preferably plane E).
I dont know where I would attach my feature control frame, as the "feature" I want to control is actually a datum plane, and this is not really allowed by Y14.5 to my knowledge.
I know I could use a compound true-position callout, but this doesnt match our actual inspection method at the CMM, and I want it to be obvious to the operator what is being checked at each stage of the CMM program.


Thanks!
~Steve

 

[powerhound]

You need to think about it differently. Theoretically datum D and datum E will be perfectly parallel to one another, it's the datum features that will not. You CMM is not checking the parallelism of the datums, it's checking the parallelism of the datum features. I don't know how this part functions but first of all it would be better to have 3 datum targets if possible. This will stabilize the part much better then two. I can't tell why you need that datum reference frame like that. It doesn't look right. Can you elaborate some more on what you're trying to do? I have a sneaking suspicion that there might be more to this than you think.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[drawoh]

cbrf23,

You cannot control datum[ ]E like that. ASME AY14.5 requires you to control the actual features. In this case, you are willing to tolerate a significant positional shift of the E[ ]datum holes with respect to the D[ ]datum holes.

Is there some parallel feature that datum[ ]D and datum[ ]E can line up with? If there is, you can apply a composite positional tolerance.

Parallelism has no meaning here.

--
JHG

 

[SeasonLee]

You can’t to tolerance the datum targets, the link below will give you a clear interpretation.

http://www.tec-ease.com/gdt-tips-view.php?q=133

I will assume that your datum target plane D and E at different level, in this case you may use basic dimension and profile control to define the offset surface, please see Y14.5M-1994 Fig.4-33 page 75 (or Y14.5-2009 Fig.4-22 page 67 ) for more detailed information.

Season

 

[cbrf23]

Hi guys,

Thanks for all the replies.
I do understand that you cant tolerance the datum planes - thats why in the original post I said to my knowledge this was not allowed by Y14.5.
Normally, I would control this with a compound true position - however since this drawing is not intended to be used as a blueprint for machining, but simply as a work instruction for the CMM inspection process, we want to represent exactly how the CMM program is written and the actual checks being performed.
The decision was made to call this out as parallelism between the planes, because that is how the CMM program is written - I just was curious if there was any way to do this following Y14.5.
It would add no value to change the CMM program to check as true-pos rather than parallelism, so for now we are going to leave the drawing the way it is, even though it does not fully conform to Y14.5.

To answer questions on what I am trying to do, we are performing a quick check to make sure that the offsets on the machine were correct, as this is pretty much the only thing that can go wrong at this stage.
This is the first thing inspected at the CMM, and if this is off we dont have to run the rest of the program, which is pretty lengthy.
All other features are measured from a different datum structure.



Thanks again for all the responses.

 

[powerhound]

I do understand that you cant tolerance the datum planes

The note you want to add says "DATUM E MUST BE PARALLEL TO DATUM D WITHIN .01 INCHES." This in itself implies that you don't understand that you can't tolerance datum planes. That's why everyone is going in the direction they're going with this.

I think the best way to call this out may be to dimension a section view where datum feature E is actually shown parallel to datum D. Then do as SeasonLee suggested and use Profile of a surface to control the locationa nd orientation of datum feature E to datum D.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II