Tolerance stackup calculation (ref pmarc example Alex Kurlikovski Fundamentals GD and T book) 1

[greenimi]

I am learning tolerance stackups and I am using Alex Kurlikovski book (Fundamentals of Geometric dimensioning and Tolerancing, 2nd edition). I have a question about the stackup tolerance calculation --fig 9-14 page 265—attached—
Minimum distance X min calculation shown is 4.5mm. Someone around here, who has way more experience than me in those kind of calculations, is claiming that the “real” X min calculation should be 4.1 (not 4.5 shown) because the form error was not included.
In the book: X min is :
69.6 (min length) – 50 (basic) - 10(basic) – 8.6/2 (max size for the hole) - (0.6+1)/2 (hole at the LMC, hole position is at MMC) = 4.5mm
Our expert is claiming the calculation should have started from 69.6 - 0.4 = 69.2 and not from 69.6. Therefore, the result would be 4.1mm and not 4.5mm.
Justification: the size of the feature (length) still has to be ±0.4mm (70.4/69.6 = 70±0.4), if the opposing points meet the size specification and the envelope meet rule#1, then the length meet the requirements. The form error was not taking in consideration for the calculation in the book. (a gage can use all 0.4mm in form error and still be making contact with the datum feature simulator)

I know pmarc had some issues with X min calculation in fig 9-12 page 263 (x min should be 2.7 and not 2.9) and here is that specific thread.

http://www.eng-tips.com/viewthread.cfm?qid=335464

And it’s exactly as pmarc stated: “It is weird to disagree with such authority”
Now, going back to our issue (page 265): Ii is our “expert” points us in a right direction or “the unclaimed form error” is not applicable here?

Thank you

 

[powerhound]

Why is he subtracting 0.4 from the 69.6? You can't do that because 69.6 is the shortest that the part can and still be good. Any form deviation can only make the part longer. Maybe I'm oversimplifying this but I'm pretty sure your expert is not correct this time. That being said, I definitely have some problems with some of AK's material and especially with the new 2009 textbook but in this case I think it's pretty straightforward. I'm looking forward to others replies.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
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[pmarc]

Calculations shown in fig. 9-14 are correct, your expert is wrong. Form error has nothing to do with the stackup result in this case. Why is the expert even using 0.4 and not 0.8? After all 0.8 is the maximum form error of 69.6-70.4 width.

 

[greenimi]

"Why is the expert even using 0.4 and not 0.8? After all 0.8 is the maximum form error of 69.6-70.4 width."

I guess because 69.6 is already at its min size, so you subtract only 0.4 (69.6-0.4)
Would be 0.8 if subtracted from nominal 70 (70.0-0.8)


His claim is:
“ Imagine the part has a perfect left edge (datum feature B, in the picture) meaning the surface B is perfectly perpendicular to datum feature A, then the part is at its minimum length 69.6 (let’s say on the top of the part) and the remaining (not included in 69.6) form error 0.4 would be on the bottom of the part. Therefore, the X min dimension is influenced by this form error (no orientation, perpendicularity, angularity requirement between the right edges (2 times) and datum feature A). He said it’s a very common error that the 0.8 tolerance zone for the direct toleranced dimension can move only 0.4 towards the holes related to the DRF. There are actually two tolerance zones when relating to planar features together with direct tolerance zone (± linear dimension) and both tolerance zones have a width of 0.8.”
Hmmm!! I am lost………I might need some help here

 

[powerhound]

As pmarc said, your expert is wrong. The dimension can never be less than 69.6. The form error allowable is not 0.4, it's 0.8 so he's wrong about that too. When the part is at 69.6 the form error can only go towards MMC. The part can not be smaller than 69.6 in any place, at any time.

You are not lost, your expert is.

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

 

[pmarc]

I guess your expert needs much more help than you do
Regarding form error, if left face is perfectly flat and perpendicular to A and the length of the part is 69.6 at the top, the length of the part at the bottom can be 70.4, resulting in 0.8 of both right faces form error and not 0.4.

The only problem with fig. 9-14 I have is that there are no geometric controls between datum features, so theoretically the part can be a parallelogram far different from what pictures with part installed on gage are showing.

 

[CheckerHater]

It looks to me that both threads are falling into the same trap: nobody knows for sure what dimension in question really is.
In older example AK was looking for “thickness”. Is thickness dimension? Is it feature of size? Is it subject to “caliper rule”?
In new problem X called “distance”. Even better. But the same questions still apply.
If X is the smallest point-to-point distance, then we have to take form error into consideration.
If X is “caliper measurement”, we can ignore form error but get different numbers on our stack-up.
Until we agree on our definitions, the whole thing looks like argument about value of 2 + 2 without knowing what “+” means.

 

[pmarc]

CH,
Why do you think that form error has to be taken into consideration in what you call point-to-point distance case? And how would it look like in stack-up in your opinion?

 

[powerhound]

AK's book is a fundamentals book. Fundamentals. With or without CHs graphic, the resident expert is incorrect in his assertion that you can legitimately subtract 0.4 from the LMC value of the part and still somehow have that dimension be compliant. I don't agree with the point you are trying to make with that graphic CH but let's try to get to the bottom of what greenimi is asking about first.

Do we all at least agree that AKs exercise is correct on a fundamental level without adding minutiae that will only serve to further confuse greenimi?

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

 

[Belanger]

Agreed, John. Alex did it right.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

I say that calculations for MIN distance are correct.

However I do not agree with calculations for MAX distance. There is no geometric control between datum features A and B defined on the print, thus the angle of surface B wrt A is not controlled at all. And since 69.6-70.4 width is, by definition, not measured in relation to the datum reference frame A|B|C, the maximum considered distance X stays uncontrolled too.

6.9 MAX is the answer only when the datum feature B is perfectly perpendicular to datum plane A.

 

[greenimi]

Before we go to pmarcs’s issues about how to calculate MAX distance, I would like to ask you guys if you have some examples from Advanced or Tolerance stackup books from AK (Ref quote from powerhound).

[Do we all at least agree that AKs exercise is correct on a fundamental level without adding minutiae......]

I only have Fundamental GD and T (AK) and if this example is treated as fundamental, I would like to know how the Advanced book (or the Tolerance Stackup Book) treat the same example (or maybe a similar one). Can somebody post a picture from other books which consider the stackups at more advanced level?

 

[greenimi]

I guess is the same scenario as we encountered for the composite for single surfaces callout (on the fundamental level is a No-No, but on the advanced level can be done, because the standard does not specifically forbid it).

On my stackup case, on the fundamental level, the X min calculation is okay, but on the advanced level can be different?
Maybe pmarcs’s issue with X Max Distance could be the same (on the fundamental level, the calculation is right/correct, but if we go to more advanced level X Max Distance is different and is based on other requirements (perpendicularity, angularity, form error), etc.
Again, who has some advanced stackup calculations that can be shared with others? (Alex Kurlikovski Advanced or Stackup books or any other)



Mark Foster's answer on the thread below (ref to my post --at the beginning--)
“While I know and respect Alex K's GD&T knowledge, the quote that you are using is from his Fundamentals-level text as well as his online resources. When I teach GD&T to people new to the subject, I also make a similar statement (i.e. that composite feature control frames are to be used on multiple features at a time, not just for one feature) because that IS the intent of that tool, and it is probably the best use of that tool. However, when we get into more advanced topics, we find ways to combine various tools (e.g. composite and simultaneous requirements) in ways that we may not have thought of when we were at a Fundamental-level of knowledge. The Y14.5 standard is intended to be a book full of definitions, rules, guidelines, language tools in general, that we are able to use in order to communicate effectively. So just as there are some people who just barely speak a language and there are others who have a supreme command of that language, we have to learn as we go to move from the former to the latter.”

http://www.eng-tips.com/viewthread.cfm?qid=341776

 

[powerhound]

The example would be treated in the exact same way in an advanced concepts book.

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

 

[pmarc]

greenimi,
There is no such thing like different stack-up results depending on different level of its complexity. Either you include all factors in the calculations or you produce worthless sheets of paper filled up with numbers.

 

[greenimi]

Pmarc,
So the root cause is maybe our interpretation of the direct tolerance dimension (±) which is different than the one in the book.

 

[powerhound]

It seems that this whole thing has been made way more complicated than it has to be. I'm not getting why there's a problem with this exercise.

While the right edge of the part is not related to a DRF, the hole pattern is. It is related to the left edge (datum feature B, secondary). Since datum feature B is secondary and has to have two point contact with the simulator, the right edge, effectively, is controlled parallel to datum B within 0.8. I know this isn't a parallelism callout but my point is that the right edge is indirectly related to the DRF through its relationship to datum feature B. When this part is fully constrained within its DRF, the MIN and MAX calculations are correct. The DRF should match the function of the part so calculating MIN and MAX distances like this is really not a problem.

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

 

[SeasonLee]

greenimi

I can recall pmarc gave us an excellent step by step explanation on the calculation of tolerance stack, pls see the link below.

http://www.eng-tips.com/viewthread.cfm?qid=287922

Season

 

[pmarc]

powerhound,
Allow me to respectfully disagree with you. Please have a look to attached file.

http://files.engineering.com/getfil...-f5bd-471e-96fe-22e09ed6234b&file=stackup.pdf

All 3 cases show...
- width of the part at its MMC = 70.4;
- hole at its MMC = dia. 8.0;
- position tolerance at hole's MMC = dia. 1.0;
- hole shifted towards datum plane B as much as possible within defined positional tolerance...
as the prerequisites to find the maximum distance X.

I hope it also shows that the right edge does not necessarily have to be related to datum plane B within 0.8. It is all because of lack of predefined geometric relationship between datum feature A and B.

 

[CheckerHater]

Pmarc,

May I ask why you measure distance X the way you do and call it “maximum”?

Genuinely curious