Min wall thickness (question #6)

[gabimo]

I am able to calculate the minimum wall thickness specified on question #6, but I am not able to get the “correct” answer from the book.
Anyone, would be interested to get their GD&T and math skills sharpened?

I can post my answer: 6.1655.
Book’s answer is 5.6083, but I do not understand why.

Thanks
P.S. I agree with 1-5 answers from the book, but not with 6.

 

[3DDave]

What contributors did you include in the calculation?

 

[gabimo]

LMB for B: 59.7 and Resultant condition boundary: 10.1 + 0.1 + 0.1 = 10.3

 

[3DDave]

I believe they lied.

Use 13.5 X 13.5 as the coordinates, making the TP radius 19.09188309

59.7-10.3= 49.4 => /2 = 24.7 => -19.091...= 5.60811691

 

[greenimi]

Hmm, Are you saying that the book has a mistake?

3DDave,
Could you, please, post a little more details of your calculations?

 

[3DDave]

Here's a GeoGebra diagram.

 

[HGMorgan]

I went with making a drawing (attached) and I am getting different numbers.
The view on L/H shows the 13.5 x 13.5 hole pattern, min thickness = 5.8081
The view on R/H shows the 13.5 x 12.7 hole pattern (per OP sketch), min thickness = 6.3652

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

 

[3DDave]

HGMorgan,

You did not include the effect of the geometric tolerances on the features which allow the boundaries to move relative to one another. The outer surface can tilt per the perpendicularity amount as the outside diameter gets smaller; the holes can translate due to the position tolerance as the hole gets larger.

 

[Belanger]

The perennial problem with this type of question is what they mean by "wall thickness." Do they mean the thickness at two points, anywhere along the depth of the OD? Or do they want to know the minimum consistent thickness throughout?
(Either way, yes the GD&T has to be factored in.)

 

[greenimi]

The perennial problem......

I do not think this case is about "the perennial problem". It is about a mistake in a (probably published) book.

 

[HGMorgan]

3DDave,
You are correct, the Maximum Material call-out with the Diametric Zone Tolerance (even though the Diametric Zone Tolerance is Zero) does affect the boundary of the feature.
The feature diameter tolerance would apply. In this case .1 + .1 = .2
The view on L/H shows the 13.5 x 13.5 hole pattern, min thickness = 5.8081 - .2 (MMC of both features) = 5.6081 (Same as what you got and with-in .0002 of book)
The view on R/H shows the 13.5 x 12.7 hole pattern (per OP sketch), min thickness = 6.3652 -.2 (MMC of both features) = 6.1652 (With-in .0003 of what OP got)

So, there is still an issue with the sketch and the 12.7 dimension.

Harold G. Morgan
CATIA, QA, CNC & CMM Programmer

 

[Belanger]

I do not think this case is about "the perennial problem". It is about a mistake in a (probably published) book.

True -- but it's another angle to the story that should be clarified, don't you think?
I can't find it right now, but there was a thread from 2 or 3 years ago where we did a wall thickness calculation on a very deep hole, and there was confusion about calculating a consistent thickness versus a single pinch-point.

 

[greenimi]

Belanger,

https://www.eng-tips.com/viewthread.cfm?qid=419571

Correct. Here is one of the discussions and I am sure there are more where this subject of what means “minimum wall thickness” has been debated.

However, as far as I understand (from 3DDave and from OP) and able to calculate (with my not so good memory and skills) here the issues are the basic dimensions and how to get the correct basic dimensions for the holes: using 13.5 and 13.5 OR using 13.5 and 12.7.
Looks like there is an inexcusable (for this level) mistake in the book.
Is my understanding correct?

 

[Belanger]

greenimi -- no, it was actually a different thread:
[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=335464[/url]

OK, it's way more than the 2 or 3 years ago that I estimated!
But if anyone really wants to dive into that aspect of wall thickness, it was a pretty good discussion back then (too back the graphic is no longed linked in the OP -- if anyone wants it I can post it again).