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Form error in the stackup calculation 3

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gabimo

Mechanical
May 2, 2013
124
Calculate the minimum wall thickness


When both features (considered features and datum feature) are at RFS, do we need to include both form errors in the stackup calculation?
Cylindrical part with a thru hole

Dimensions in mm

ID: 20 ± 0.5—datum feature A
OD: 50 ±1.3, position Ø3 (RFS) to A (RFS)
What is the minimum wall thickness:
Form error on A: 2x 0.5 = 1mm
Form error on OD: 2x 1.3 = 2.6mm

X max, x minimum,
 
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SeasonLee,

Looks like the form error is not included in your calculations. I made exactly the same "mistake" .....at least according to pmarc/ plyfrm/ Nescius we are wrong.
Still not understanding how and why.
Usually pmarc is correct and able to explain in the layman terms on what I was doing wrong....

Thank you for your input and help

 
pmarc,
A couple of months ago you had a thread named "I hate MBD, Why" or something like that.
Maybe I will initiate a new thread called "I hate form error", but will be no need to ask "why?" [banghead]
 
greenimi

I think the mistake is not on the form error, since we didn't consider the datum shift, here is my new calculation on the 2nd case.

Season
 
New calculation.

Season
2017-06-20_021622_h1zwu2.jpg
 
SeasonLee,

Looks like your case 3 also needs "some adjustements" for "datum shift" and might still not be enough to get the correct values [surprise]
 
greenimi,

You said you knew how to get to 10.8 for min wall thickness (with A referenced at LMB), so in the attachment I am just showing how to get to 18.4 for max wall thickness. Again, the key thing is to realize that the size of datum feature A's LMB is not 20.5, but 21.5. The rest is nothing but a trivial arithmetic.

 
pmarc,

I am trying to get the concept of LMC.
Yes, I got 10.8 as a min wall thickness (with A referenced at LMB), but probably just because of stroke of luck not because I fully understand the concept. (my monthly moment of honesty:):) )
How I got x min (minimumm wal thickness when A is LMB) is the following:

+48.7 (OD at LMC)
-20.5 (ID at LMC)
- 3 (poisitional tolerance)
- 0 (no bonus for the considered feature, in this case OD at RFS)
- 1 (datum shift , datum feature is at LMC, 20.5 shown before, but the callout in position is at MMB, therefore shift is available)
-2.6 (form error on the considered feature, since the OD is at LMC, shown above 48.7)

Total: 21.6
21.6/2 = 10.8

I am trying now, to make the same sketch concept for x min similar with the one you showed for X max. Not sure I am able to, but at least I am trying....

Just a follow up question for you:
Is the perfect form at MMC applicable for the considered feature (OD in our case)? I would say : yes.
Is also the perfect form at MMC (NOT at LMC) applicable for the datum feature (ID in our case): I also say: YES----even the datum feature is called at LMB.

Am I correct ?


 
Perfect form at MMC applies to both, datum feature (ID) and toleranced feature (OD).

Neither of the features must have perfect form at LMC.
 
SeasonLee

I think you have the word terminology mixed:
ID MMB=20.5 ID LMB=19.5; The hole at is MMB is the smallest hole or 19.5". It is not a shaft

greenimi

Alex covers form controls on Chapter 14 of his Tolerance Stack workbook though no examples similar to this thread using position tolerance, I found something similar but instead of form tolerances he uses datum shift tolerances. Would it be consider the same?

washer_levazz.png

stack_jifmgo.png


Threads like this spike more my interest. I always thought that datum shifts or form controls can only apply when the M/L modifier is called after the feature control tolerance (per washer drawing; Modifier after 0.2 Tol), just like bonus tolerance. But i see not necessarily, even if is called after the datum features such as Case 2-3. I find that this is the opposite of finding the gaps limits where usually datum shifts or form error are not involved.
 
pyromech,
I am the least qualified person to answer or address the issue above.....I am stuggling myslef....
I think the form errors enters into "the equation" if the considered feature or the datum feature are RFS/RMB. If they are MMC/MMB/ LMC/LMB then the bonus/datum shift will "play" and no form error is to be considered/used.


I found this statement from Bryan Fischer, statement I consider relevant to your question:

For a part dimensioned and toleranced in accordance with ASME Y14.5, the resultant condition is either
• the sum or difference of the size, size tolerance, the geometric tolerance at the specified material condition, and the additional (bonus) tolerance for the opposite material condition, or
• the sum or difference of the size, size tolerance, the geometric tolerance at the specified material condition, and the allowable form error at the other material condition, or
• some combination of these.
To reinforce the last paragraph, the full additional (bonus) tolerance is only allowed (and in spec) if the toleranced feature has perfect form at the other size (the non-specified size). E.g., if a hole has a positional tolerance applied with an MMC modifier, the only way that the full additional (bonus) tolerance is allowed and in-spec is if the hole has perfect form at LMC, which is not required. If the hole has any form error, the amount of additional (bonus) tolerance is decreased by that form error. The resultant condition worst-case boundary remains the same (it has the same value), but the variation that leads to that boundary differs.


There are some experts on this form that can explain it better than me. (why bonus/ datum shift and no form error)

 
As a challenge, I would ask anyone to calculate the X max. and x min. for Exercise 12-2 (your embedded pictures) when the position and the datum feature are RFS/RMB

Maybe the form error will be considered or maybe not since datum feature B is secondary and the related actual mating envelope (not the unrelated one) will have to be used.

Position Ø0.2 (RFS) to A primary and B (RFS) secondary.
Perpendicularity stays the same.
 
pyromech

I did mixed the terminology, thanks for your correction. Most important is I do learned lot here.

Season
 
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