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Stack-up concept

Stack-up concept

(OP)
I would like to get a little help in understanding the stackup concept on the cases below. I have a hard time grasping the concept on how the X maximum and X minimum are to be CORRECTLY calculated (maximum and minimum distance between OD and ID). The numbers (X max. and x min.) are just all over the spectrum....
I have studied some of the examples found on eng.-tips, but looks like none of the existing shown cases are helping. Just some small tips and tricks on how to judge or think them would be really needed.
Thank you
Ani

RE: Stack-up concept

What calculations did you do and what sketches did you make to figure this out?

RE: Stack-up concept

(OP)
Here are my values and how I came up to them:

Case a.) position Ø3 MMC to A LMB
X min: (49 (LMC) -1 (form error) - 30 (LMC) -4 ( pos. 3 and bonus 1) )/2 = 7
X max: (50 (MMC) – 29 (MMC) +3 (pos.) ) /2 = 12

Case b.) position Ø3 to A LMB
X min: (49 -1 – 30 -3 (only pos. but no bonus ) ) /2 = 7.5
X max: (50 (MMC) – 29 (MMC) +3 (pos.) ) /2 = 12


Case c.) position Ø3 LMC to A MMB
X min: (49-1 -30 – 3 (only pos. no bonus at LMC available) )/2 = 7.5
X max: (50 (MMC) - 29 (MMC) +3 (pos.) + 1(bonus) ) / 2 = 12.5

What am I missing? Which calculation is incorrect?

RE: Stack-up concept

You're missing what's known as "datum shift." You have accounted for one modifier (the one that follows the geometric tolerance number -- which yields "bonus" tolerance), but you didn't account for the second modifier (the one that follows the datum letter).

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

RE: Stack-up concept

(OP)
Could you, please, advise how datum shift will be taken into account?
Not really sure how to use it.
I appreciate your help.
Thank you
Ani

RE: Stack-up concept

See attached Excel file for the 3 calculations. Notice the tabs at the bottom of the screen -- each scenario is on a different tab.
You may not be familiar with that format of doing stacks (it already halves each diametric number as entered, rather than the final answers). But the thing you were missing is shown on the row called "shift." It has to do with the datum feature deviating from the prescribed boundary (MMB or LMB).

I must emphasize that these calculations are solving for a consistent wall thickness. If we wanted to know the max/min wall thickness at any possible cross-section then the answers would be different (because perfect form is not required at LMB, but for now let's skip it).

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

RE: Stack-up concept

I think I made a goof on my calculations! My parenthetical comment at the end of my last post is indeed relevant to the first two calculations, even for constant wall thickness. This is because the LMB itself consists of a 49 mm cross-section that can still bend as much as 1 mm. (Bending any further than that would violate the actual mating envelope.). So the LMB is 48 mm -- which means that the max "shift" could be 2 mm.

I should have remembered this from the similar question that began on this forum on Sept. 8!
I've changed the spreadsheets and reattached them here. Somebody let me know if it still doesn't look right.

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

RE: Stack-up concept

(OP)
John-Paul Belanger,

Thank you for the corrected Excel file!

My follow up questions:

On case A, why the form error on datum feature A is not conisdered, otherwise speaking why the answer is not driven from 48, but from 49 (LMC)?

On case B, again, the driving term, per your spreadsheet is 49. Why not 48 since the form error is to be considered as the perfect form at MMC is enforced (not the perfect form at LMC). Am I correct?

Case C, is the only one in agreement. Interesting fact, I considered form error on datum feature A on all three cases, but you didn't and even that we got on case C same result. Go figure. !

RE: Stack-up concept

I calculate the following wall thickness extremes:

(a): max = 13; min = 7
(b): max = 13; min = 7
(c): max = 12.5; min = 7.5

pylfrm

RE: Stack-up concept

Pylfrm,
May I ask, your calculations are in the consistent wall thickness scenario (described by J-P) or the absolute min/max in any possible cross section case?
Thank you for your input

RE: Stack-up concept

And I forgot:
Merry Christmas Ho! Ho! Ho!

RE: Stack-up concept

greenimi,

I basically assumed a tube of infinite length, in which case I don't believe there's a difference.

I will also note that form error is only required for the minimum wall thickness of case (b). The other five values can be achieved without form error.

pylfrm

RE: Stack-up concept

pylfrm -- you got the same answers for case A and case B. But notice the different modifiers. Certainly the MMC modifier after the tolerance number in case B would impact the calculation somehow.

Also, I'm not sure what you mean when you write that "form error is only required for the minimum wall thickness of case B."

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

RE: Stack-up concept

Belanger,

The calculations may change, but not the results. The equivalence is discussed in thread1103-430008: Stackup RFS-MMC combo, simple part, same results?.

With a perfectly cylindrical hole, every point on the surface will be within 15.0 of the feature axis. The smallest possible wall thickness in case (b) would be 7.5. Introducing form error to the hole allows the surface to be 15.5 away from the feature axis in one direction, reducing the wall thickness to 7.0.

The LMB being 48 instead of 49 affects the minimum wall thicknesses as well as the maximums, but I don't believe you accounted for that in your revised spreadsheets. That should explain the rest of the difference between your answers and mine.

pylfrm

RE: Stack-up concept

Earlier you said that the min answer for case B is 7, but you've revised that to 7.5? (I'm not sure if you are familiar with the spreadsheet method I used, but I'm just trying to account for the correct inputs to see if we can agree on the answer.)

In the meantime I'll read that other thread; I never really went through it when it popped up back in September.

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

RE: Stack-up concept

Belanger,

No, I stand by my original answers. I was just trying to explain my earlier comment about form error. If OP's question were modified to require perfect cylindricity on the ID and OD, then the answers would be the following:

(a): max = 13; min = 7
(b): max = 13; min = 7.5
(c): max = 12.5; min = 7.5

The only one that changes is the minimum for case (b).

As for the spreadsheet inputs, I'd say you need to add 0.5 of shift to the case (a) and (b) minimum calculations to account for the difference between the datum feature size you used (49) and the LMB (48). You also need to change the "axis of ID / edge of ID" value for the case (b) minimum calculation from 15 to 15.5 to include the effect of form error.

Perhaps thread1103-426288: Form error in the stackup calculation would actually be more useful. Nescius posted a good illustration.

pylfrm

RE: Stack-up concept

Yes -- last night I found Nescius's picture. These questions are fun but they get tricky when there are so many caveats.

edit (adding a comment): It's also good to note that this stack question would seem to yield a different answer if the position symbol were changed to concentricity (ASME style). Just about every stack textbook says that position and concentricity are treated the same way in a stack, but Nescius's picture shows the difference (position is based on the AME and concentricity would be based on the median points).

RE: Stack-up concept

(OP)
pylfrm,
Could you, please attache/ post a sketch (similar to Nescius's picture) on how YOU came up with the values shown above?

"(a): max = 13; min = 7
(b): max = 13; min = 7
(c): max = 12.5; min = 7.5"


You know a picture worth a thousand of words. I appreciate your help.

Thank you
Ani

RE: Stack-up concept

Quote (aniiben)

Could you, please attache/ post a sketch (similar to Nescius's picture) on how YOU came up with the values shown above?

I agree. It would be helpful for me too.

Thank you pylfrm.

RE: Stack-up concept

aniiben, greenimi,

I'm not feeling terribly inspired to draw circles at the moment. Perhaps if you post your own sketches, I (or someone else) can provide some further insight.

pylfrm

RE: Stack-up concept

Thank you very much pmarc.
Happy New Year !

RE: Stack-up concept

(OP)
I am really impressed and for the moment I would just say THANK YOU pmarc.

RE: Stack-up concept

Happy New Year! And glad I could help, of course.

RE: Stack-up concept

The drawings posted by pmarc show pretty much the same geometry I was imagining. They look better that what I would have come up with, so it's probably for the best that I wasn't feeling inspired.

The only thing I can add is to note that the minimum and maximum can both exist simultaneously on a single part for each case.

pylfrm

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