×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Form error in the stackup calculation
3

Form error in the stackup calculation

Form error in the stackup calculation

(OP)
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,

RE: Form error in the stackup calculation

gabimot,

Minimum wall thickness is 10.8, and form error is involved for both features. It determines how far the actual surfaces can deviate from the unrelated actual mating envelopes. Such deviation decreases wall thickness.

Maximum wall thickness is 17.4, and form error is not involved for either feature.

pylfrm

RE: Form error in the stackup calculation

Max is 17.4
Min is 12.6

Form tolerance is directly from the size tolerance in your example, so I'm not sure why it would be of concern.

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

RE: Form error in the stackup calculation

I got “almost” the same values.

X max: 17.4 (same value as everyone)
X min: 12.1. I am not sure why I am missing another 1.3.
X min:
-20.5/2 = -10.25
+48.70/2 = +24.35
Ø3 / 2 = -1.5

Total: 24.35-10.25-1.5 = 12.6
12.6- form error on datum feature A (0.5) = 12.1 (I am working on the radius, so everything is divided by 2)

Pylfrm and everyone,

Just for my own education, how the requested values (X max and x min), would change, if datum feature A would be called at MMC/MMB or LMC/LMB?
Adjusted case 1 OD: 50 ±1.3, position Ø3 (RFS) to A (MMC/MMB)
Adjusted case 2 OD: 50 ±1.3, position Ø3 (RFS) to A (LMC/LMB)

I really appreciate your contribution here on the forum and the amount of education you provided. “Datum shift” thread was another excellent example. At least for me was an outstanding piece of “training material”

RE: Form error in the stackup calculation

For my adjusted scenarios I got:
Adjusted case 1 OD: 50 ±1.3, position Ø3 (RFS) to A (MMC/MMB)
X max: 17.4
x min: 12.1

Adjusted case 2 OD: 50 ±1.3, position Ø3 (RFS) to A (LMC/LMB)
X max: 17.9
x min: 12.6

Am I correct?

RE: Form error in the stackup calculation

Nescius,
I reconcile the x min value. Should be 10.8.

Also, do you agree with the values for the "adjusted cases":

Adjusted case 1 OD: 50 ±1.3, position Ø3 (RFS) to A (MMC/MMB)
X max: 17.4
x min: 12.1

Adjusted case 2 OD: 50 ±1.3, position Ø3 (RFS) to A (LMC/LMB)
X max: 17.9
x min: 12.6

RE: Form error in the stackup calculation

I calculate:

10.8 min/17.4 max when A is modified at MMB
11.3 min/17.9 max when A is modified at LMB

RE: Form error in the stackup calculation

Quote (Nescius)

10.8 min/17.4 max when A is modified at MMB
11.3 min/17.9 max when A is modified at LMB

I agree.

pylfrm

RE: Form error in the stackup calculation

Okay, so looks like I have a problem (of understanding) how to calculate the minimum wall when the considered feature is RFS. (for the maximum wall I guess I am fine, even the considered feature is RFS).
My brain just does not get it. (along with the issue raised on VC/ RC thread earlier this week)

Adjusted case 1 OD: 50 ±1.3, position Ø3 (RFS) to A (MMC/MMB)
X max: 17.4
x min: 12.1 (should be 10.8)

Adjusted case 2 OD: 50 ±1.3, position Ø3 (RFS) to A (LMC/LMB)
X max: 17.9
x min: 12.6 (should be 11.3)

Now, the question is why?
Why, I have to subtract another 1.3 from the min wall ?
First of all, everyone agrees with these numbers?

RE: Form error in the stackup calculation

greenimi,

The drawing posted by Nescius shows the minimum wall condition for both the original RMB case, and for your modified MMB case.

For your modified LMB case, the minimum wall condition could look the same except with a perfectly cylindrical datum feature having diameter 20.5.

Once you have drawn or imagined the geometry that results in the desired extreme value, the correct numbers should fall right out. Trying find and remember a calculation method that skips this step doesn't seem very useful to me.

pylfrm

RE: Form error in the stackup calculation

Thank you pylfrm,

As I said it before: you can explain it to me, but you cannot understand it for me.

Also, a big thank you to Nescius for the sketch provided earlier.

I will get it..hopefully. I am not at your, Nescius or pmarc's level yet, but I am working on it..... wink

RE: Form error in the stackup calculation

Ha! I'm not sure you'd want to be at my level. bigsmile I am nowhere near as experienced as the famous names in this sub forum. Having Solidworks at my disposal helps me tremendously.

What pylfrm said above is key:

"...how far the actual surfaces can deviate from the unrelated actual mating envelopes."

Spoken more broadly, the way GD&T "touches" a part is not necessarily the same way other components in an assembly may "touch" the part. In the case of a planar datum feature, a simulator touches the highest points, where a mating part might not touch those points at all due to form errors.

RE: Form error in the stackup calculation

Quote (Nescius)

11.3 min/17.9 max when A is modified at LMB

These numbers are true for LMB size of datum feature A = 20.5.

My qustion would be: Is the LMB size really 20.5?

RE: Form error in the stackup calculation

Quote (pmarc)

Quote (Nescius)

11.3 min/17.9 max when A is modified at LMB

These numbers are true for LMB size of datum feature A = 20.5.

My qustion would be: Is the LMB size really 20.5?

Good point. I guess the LMB size is actually 21.5, which would make the answer 10.8 min / 18.4 max. Right?

pylfrm

RE: Form error in the stackup calculation

Dang. Position tolerance at LMC requires perfect form at LMC. I don't know why my brain assumed perfect form when a datum feature is referenced at LMB.

RE: Form error in the stackup calculation

Quote (pylfrm)

Good point. I guess the LMB size is actually 21.5, which would make the answer 10.8 min / 18.4 max. Right?

Right, Sir.

RE: Form error in the stackup calculation

Where 18.4 max is coming from?
I guess I got 10.8 min for A(LMB)

Are 10.8 and 17.4 for A(MMB) the correct answers?

RE: Form error in the stackup calculation

I know I am “abusing and bullying” pmarc and the rest of the quiet members of this forum, but I like to confirm the values below, and especially the ones where the datum features is called at LMB.
Does anyone have any books where these "KIND" of calculations are explained in details? Kurlikovski tolerance stackup does not have the form error included.

ID: 20 ± 0.5—datum feature A

What is the minimum wall thickness:

Adjusted case 1 OD: 50 ±1.3, position Ø3 (RFS) to A (MMC/MMB)
X max: 17.4
x min: 12.1 (should be 10.8)

Adjusted case 2 OD: 50 ±1.3, position Ø3 (RFS) to A (LMC/LMB)
X max: 17.9, (should be 18.4)
x min: 12.6 (should be 11.3) (should be 10.8)

RE: Form error in the stackup calculation

Here is my calculation, please let me know if I am wrong.

Season

RE: Form error in the stackup calculation

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

RE: Form error in the stackup calculation

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

RE: Form error in the stackup calculation

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

RE: Form error in the stackup calculation

New calculation.

Season

RE: Form error in the stackup calculation

Well, I think I am still wrong again. Tired

Season

RE: Form error in the stackup calculation

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

RE: Form error in the stackup calculation

2
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.

http://files.engineering.com/getfile.aspx?folder=7...

RE: Form error in the stackup calculation

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 ?


RE: Form error in the stackup calculation

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

Neither of the features must have perfect form at LMC.

RE: Form error in the stackup calculation

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?




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.

RE: Form error in the stackup calculation

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)

RE: Form error in the stackup calculation

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.

RE: Form error in the stackup calculation

pyromech

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

Season

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources