Confused by using UAME to determine bonus tolerance

[dsmemr]

Hi everyone. I've been playing around with representing GD&T concepts graphically in Solidworks, and I'm getting a little confused.

Consider the following hole:
-Size tolerance of 1.4/1.0
-Perpendicularity tolerance of .1 @MMC referenced to datum "A", which is a surface perpendicular to the nominal hole axis.

My assumption is that I can inspect this callout in one of two ways:

1. Calculate the virtual condition of the hole (MMC hole diameter-MMC tolerance zone diameter=.9). Check that a .9 diameter cylindrical boundary, held perpendicular to Datum A, can pass through the hole while datum feature A on the part is also constrained to Datum A.

2. Find the Unrelated Actual Mating Envelope of the hole. Calculate the bonus tolerance ([UAME diameter]-[MMC diameter]). Verify that the axis of the UAME is contained within the tolerance zone.

See the attachment for the example I drew up, showing a section view of an as-produced hole. Method 1 checks out; the hole can (barely) clear a .9 cylinder held perpendicular to datum A. However, with Method 2 the part doesn't pass; I've measured the UAME diameter (1.1494) and used that to calculate the tolerance zone diameter (.2494), but the UAME axis doesn't fit in the tolerance zone. In order for everything to work out just right, I would have to have measured the "size" of the hole parallel to datum A (1.20), instead of using the UAME.

Was I wrong in using [UAME diameter]-[MMC diameter] to calculate bonus tolerance? How can I explain the discrepancy between the two methods?

Thank you!

edit: fixed file upload
edit2: Never mind, all of the files I post have their name appended to their first 2 characters and show nothing when opened. I apologize if the image in the post messes up formatting for anyone.

 

[3DDave]

It's the "&". The upload software can see it; the download software, not so much. It can be fixed by replacing "&" with "%26" when you paste the link. In the browser it will still be automatically converted to show as "&" but the download will work.

The part that is missing is the base location/perpendicularity tolerance. So, in your example, assume it is zero. Because the feature is larger than the MMC UAME size the available tolerance is that UAME size minus the MMC size.

What your are seeing, though, is that there is a defect in the axis evaluation when the size difference is so large - much like how many engineering formulas assume that sin(theta) = theta, but once that slope starts getting large, the initial assumption breaks down.

This is why the surface interpretation typically carries priority over the axis interpretation. They are close, but not the same.

 

[pylfrm]

dsmemr,

The two explanations of the tolerance are not equivalent. ASME Y14.5-2009 says the surface interpretation (virtual condition boundary) takes precedence. The simple solution is to always use the surface interpretation and forget about the axis interpretation.

The two explanations would agree for a perfectly cylindrical hole through an infinitely thick plate. In reality there will always be some level of disagreement.


pylfrm

 

[greenimi]

dsmemr,
The attached file cannot be opened.

 

[3DDave]

You can open it by copying the link, pasting it into the browser bar and replacing the "&" with "%26"

https://files.engineering.com/getfi...4205-849f-558910a6d950&file=GD&T_question.png

You cannot see the "source" that I typed; your browser will automatically replace the "%26" I typed with "&" but it's there.

 

[greenimi]

Got it 3DDave,
Thank you very much

 

[pmarc]

dsmemr,

Why does your hole have UAME size 1.1494? This is outside of the specified size limits.

EDIT: Nevermind, my mistake. I read the numbers too fast.

 

[greenimi]

Why does your hole have UAME size 1.1494? This is outside of the specified size limits.

Why? (is outside of the specified size limits)

Per OP's initial post
Consider the following hole:
-Size tolerance of 1.4/1.0
-Perpendicularity tolerance of .1 @MMC referenced to datum "A", which is a surface perpendicular to the nominal hole axis.

 

[greenimi]

pmarc,
Ok. Good.

 

[3DDave]

The maximum is Dia. 1.4.

No idea where the intermediate size comes from.

Edit: Others seems to have caught up while I was reading.

 

[dsmemr]

The intermediary size was just an arbitrary choice on my part.

Thank you everyone! I had always assumed that the axis/surface methods were just two ways of describing the same overall geometry; it's good to know that this isn't necessarily the case. Of course, the tolerances I chose for my example were very inflated for visual clarity, so I would assume the difference is relatively inconsequential in most real life cases. Still, something I'll keep in mind.

Also thank you for the help with the file attachments, I'll make sure to use a better file name next time.

 

[pylfrm]

I had always assumed that the axis/surface methods were just two ways of describing the same overall geometry; it's good to know that this isn't necessarily the case.

It's never the case. The two definitions describe completely different requirements. They just produce similar answers in some circumstances.

Of course, the tolerances I chose for my example were very inflated for visual clarity, so I would assume the difference is relatively inconsequential in most real life cases. Still, something I'll keep in mind.

Even without the large angular deviation used in your example, the disagreement between the two interpretations can be approximately double the size tolerance. I wouldn't generally call that inconsequential.

Such large disagreements may be very unlikely for some manufacturing processes. Is that what you mean?


pylfrm