GD&T Interpretation Needed 2

[Kenneth E]

The attached file is a screenshot of one of our engineering drawings.

The feature is an internal pocket. Question is regarding the .025 callout behind the Unilateral symbol.

What does it mean in this application?

Thanks

 

[greenimi]

If the drawing is according to ASME Y14.5-2009 means that the internal pocket is only allowed to get smaller by the size tolerance 0.025 on each side.
Lets assume the basic dimension between the opposite sides is 10, then the maximum dimension is 10 and the minimum dimension is 10 - 0.025 - 0.025 (for both sides)

 

[Kenneth E]

Thanks greenimi.

The attached file is of the mating part, a lid which fits just inside the pocket. It appears to me that the GD&T on the pocket was applied incorrectly.

 

[mkcski]

The number after the U is always that portion of the total profile tolerance that adds "weight" to the part feature. Because the tolerance and number are the same in your case, all of the tolerance (material) is "inside" the basic shape. That's a smaller interior pocket. And the opposite if your drawing was for a raised "boss".

Certified Sr. GD&T Professional

 

[greenimi]

If the lid goes inside the pocket I would say you care about its outer boundary, therefore zero tolerance zone shown on your drawing is probably correct.
As shown on your lid drawing, the shape (outside shape) is allowed only to get smaller.

Not sure if referencing B and C in the profile is correct.

 

[Kenneth E]

Thanks for your help greenimi.

 

[AndrewTT]

I would be interested to see the full drawing of these parts. It is possible that the GD&T is applied incorrectly.

For the "lid", does the profile feature control frame have a circle at the knee of the leader? Does it have a between "x" and "y" designation for where this control starts and stops? The surfaces designated as datum features B and C lead me to believe that there may still be problems with the GD&T application.

 

[chez311]

Kenneth,

I am going to echo Andrew a little - is that profile tolerance meant to be all-around or just applied to the flat, planar face on which the leader line is directly attached? If the latter I have no issue, if the former I don't *technically* have an issue, but you should definitely be aware of the implications.

 

[Kenneth E]

Andrew,

I believe you are correct in that the GD&T is applied incorrectly for the part the lid is fitted to.

It's obvious to everyone involved in making the parts that the lid has to fit down inside the pocket, and they will machine accordingly.

Thanks for everyone's help.

 

[chez311]

Okay so since you didn't really quite answer the question, I'm just going to go ahead and forge onward with the assumption that the profile tolerance is applied "all-around" (which would require a different leader line type as Andrew noted to be unambiguous - Y14.5 does little to define which types of transitions link features together without the explicit use of the "between" or "all-around" notation).

If this is unwarranted/unwanted information I apologize - I also do not desire to once again open the can about self-referencing since it has been discussed here many, many times but I feel that people should know about it since its very common when people apply datum references and then unwittingly wrap them in an all-round profile tolerance which references those same datums. The short version is that from what I gather this practice is simultaneously not illegal but also not well supported (or at least well defined) by Y14.5-2009 (I have yet to fully read through 2018 but I doubt this has changed much if at all) - it exists in a gray area where there exists some level of consensus among the community on how it should be treated, but also brings up some of the finer aspects of GDnT - which can really throw a lot of people through a loop when either interpreting drawings or inspecting parts if they are not familiar with it.

I'll get off my soapbox here - as I said I don't want to drag out a long discussion on a well covered topic, I only desired to provide information. Heres some previous threads I've gathered which have some more in depth discussions:

https://www.eng-tips.com/viewthread.cfm?qid=405622

https://www.eng-tips.com/viewthread.cfm?qid=404621

https://www.eng-tips.com/viewthread.cfm?qid=438924

A few documents from said threads which explain how this is treated:

https://files.engineering.com/getfi...=212727627.13.1552575076512&__hsfp=2021768341

https://files.engineering.com/getfi...=212727627.13.1552575076512&__hsfp=2021768341