Datum Translation Modifier on Tertiary Datum Feature (FOS) at MMB

[powerhound]

Does this make any practical sense? I just saw a figure in Y14.43-2011 (Fig. B-11) with this and I'm trying to wrap my head around whether it's actually valid and what could be the use of it. I didnt' find any explicit prohibition of it but that shouldn't automatically mean that it can be done. It still has to make sense, right? I'm completely on board with its use at RMB but I'm having a little trouble with this one. How would the gage work and what could the mating part do to warrant a callout like this?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

So is everyone stumped, like me?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[greenimi]

http://www.eng-tips.com/viewthread.cfm?qid=363878

See Pawel's attachment.

 

[greenimi]

Also the whole discussion is interesting including the links to some additional threads, specially because Jim Meadows, the chair of Y14.43 committee is involved providing some insights. Hope this helps more than confuse.

 

[3DDave]

I'll re-up my previous comment:

"What I implied was that the committee should not have added it unless it was a frequent application, and had seen at least one useful example, preferably many.

So far no support for this approach has included even one application. All examples have been artifically constructed for the purposes of explaining how the concept works, but not a single one explaining why anyone would use it or showing how it is used in a real assembly."

Meadows apparently wrote "What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement?"

The answer to this question is to define the location of the tertiary feature with a separate 'width' tolerance at right angles to the mutual plane and use that width as the tertiary datum so that its location closer or farther from the secondary does not affect it's limitation on angular rotation - a facility which was available since 1982. There was never a need for this modifier as the facility was already available to handle that case.

 

[powerhound]

Wow! Pawel's attachment explains it. I'm with Dave though because there's already a way to solve the exact problem that Jim is introducing the concept for. How did this even make it's way into the standard? I think it only serves to muddy an issue that was pretty clear.

Thanks greenimi. This helps and gives me a better feeling that I wasn't as lost as I was thinking.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II