Min Wall Thickness - GD&T v/s Note 2

[Madhu454]

Hi,
Please see the attached drawing. The left side of the drawing where the min wall thickness is controlled using a note. Where as the right side shows the same part is made with GPS to control the Min wall thickness. Is the drawing with GPS adding any additional value compare the one shown in the left side? In this case won't the note eliminating all the uncertainties in the drawing?

 

[pmarc]

Madhu,

Two things:
1. Both tolerancing schemes are not equivalent. On the left side min wall thickness can never be less than 1.75. On the right side it can go down to 1.65.
2. I would not say that the scheme on the right introcudes more uncertainty than the scheme on the left. In fact, I do not see anything uncertain/unclear/not standardized on the right, while on the left interpretation of the wall thickness dimension has not been given in any ISO GPS standard, as far as I can tell. These are subtleties, but the fact that for the wall thickness dimension the direction of actual measurement is unclear, makes the drawing on the left more ambiguous, in my opinion.

 

[Madhu454]

Hi pmarc,
Thanks for the reply.
I was calculating the minimum thickness as below.
The boundary for inner dia = 16.5 + 0.1 + 0.2 (pos tol) = 16.8
The boundary for outer dia = 20.3 (lmc)
Minimum wall thickness = (20.3 _ 16.8 )/2 = 1.75
But, you have mentioned that the min wall thickness is 1.65mm from the drawing on right side. Am I missing anything in the calculation? Please let me know. Also could you please explain how the direction of measurement for GPS drawing, is it normal to the datum axis A.

 

[pmarc]

Madhu,

See attached graphic for min wall thickness calculations.

http://files.engineering.com/getfil...f3d-8c9b-814e790cb71d&file=Min_distance_2.PNG

For the drawing on the right, according to ISO 14660-2:1999 all actual local size measurement shall be normal to the axis of LSQ cylinders of both diameters (see below).

http://files.engineering.com/getfil...c5c-abf1-5b030dbd1423&file=ALS_ISO14460-2.PNG

So for the datum feature A, if the axis of its LSQ cylinder is parallel to the axis of minimum circumscribed cylinder, the actual local size measurements will be normal to the datum axis A. Now, the position tolerance zone is centered about that datum axis A, meaning that position tolerance requirement shall be also verified in direction normal to the datum axis A.

 

[Madhu454]

HI pmarc,
Thanks for the explanation.
It is clear now,

 

[Madhu454]

Hi,
In continuation to the above discussion, now I got some more doubts.
1) Is adding the LMB for datum feature A, change the Min wall thickness?
2) Does it also means, when datum feature A produced at LMC, it should have perfect form?

 

[monkeydog]

Take your side by side drawing to the machinist, and ask him how he would make and check both parts.

 

[greenimi]

Re:"Hi,
In continuation to the above discussion, now I got some more doubts.
1) Is adding the LMB for datum feature A, change the Min wall thickness?
2) Does it also means, when datum feature A produced at LMC, it should have perfect form? "

I am sure pmarc will correct me if I am wrong, but I am trying to answer:
Q1: No.
Q2: Yes.

2.7.1 “In cases where a geometric tolerance is specified
to apply at LMC, perfect form at LMC is required. See
para. 7.3.5.” (if you are using y14.5-2009).

ISO with E symbol, probably is equivalent with the default in y14.5
7.3.5 LMC as Related to Positional Tolerancing
Where positional tolerancing at LMC is specified, the
stated positional tolerance applies at the feature size limit
that results in the least material in the part. Specification
of LMC requires perfect form at LMC. Perfect form at
MMC is not required. Where the feature departs from
its LMC limit of size, an increase in positional tolerance
is allowed, equal to the amount of such departure. See
Fig. 7-12. LMC may be specified in positional tolerancing
applications where the functional consideration is to
ensure a minimum distance is maintained while allowing
an increase in tolerance as the feature of size departs
from LMC. See Figs. 7-13 through 7-17. LMC is used to
maintain a desired relationship between the surface of
a feature and its true position at tolerance extremes. As
with MMC, the surface interpretation shall take precedence
over the axis interpretation. See para. 7.3.3.1 and
Fig. 7-6.

 

[greenimi]

http://www.eng-tips.com/viewthread.cfm?qid=351570

http://www.eng-tips.com/viewthread.cfm?qid=348558

two more threads for min wall conditions.

 

[pmarc]

In continuation to the above discussion, now I got some more doubts.
1) Is adding the LMB for datum feature A, change the Min wall thickness?
2) Does it also means, when datum feature A produced at LMC, it should have perfect form?

1. Yes, it will change the min wall thickness to 1.75.
2. Yes, when specified at LMR in position tolerance frame, datum feature A produced exactly at LMC (that is, with all actual local two-point sizes equal 20.3) must have perfect form.

 

[greenimi]

So, the natural question is why the datum shift will have an influence on this case (and did not for the cases discussed in the additional threads)?
Thank you pmarc

Probably, because A is primary here versus B was secondary on 9-13 (page 264 , Alex K book)—B @RFS or B @ MMC get the same X distance.

 

[pmarc]

greenimi,
My natural counter question is why do you think that datum shift has influence on min wall thickness?

 

[greenimi]

Actual minimum material envelope does take in consideration all the form errors of the ID which is not the case when the datum feature is specified RFS or MMC. For the min wall calculations you would need to consider the form errors. Therefore, the datum driven from datum feature specified RFS or MMC might not have the form errors imbedded so they need to be considered separately.

For RMB and /or MMB: If the feature it is a perfect cylinder for 270° degrees, but for a small area has an irregularity / form error, then the datum is the axis driven from the unrelated actual mating envelope which does not include the form error just mentioned.

For LMB: If the feature it is a perfect cylinder for 270° degrees, but for a small area has an irregularity / form error, then the datum is the axis driven from the unrelated actual minimum material envelope which DOES include the form error just mentioned. It is the axis of a perfect cylinder, within the material, unrelated with any datums, which contacts the datum feature in the highest points (again, within the material).

Hmm, I think I lost myself……

 

[pmarc]

For LMB case (using ASME terminology), datum axis is not the axis of the unrelated actual minimum material envelope, but the axis of the least material boundary cylinder fixed at the diameter that equals LMC size of the datum feature. So in Madhu's modified scenario datum axis A will be derived from a perfect virtual cylinder of dia. 20.3, which in ISO is called Least Material Virtual Condition (LMVC) of datum feature. The datum feature itself (external cylinder) must always have a material inside of that LMVC.

For the internal feature, the LMVC boundary is 16.8 (16.5+0.1+0.2). That boundary is coaxial with the datum axis A.

So the min wall thickness is (20.3-16.8)/2 = 1.75.

 

[greenimi]

pmarc,

Okay. Now understood.

And the datum shift is the difference between the unrelated actual minimum material envelope "cylinder size" and the size of the cylinder fixed at the diameter that equals LMC size of the datum feature.

My follow up question is my previous assessment correct in ASME?

"minimum material envelope does take in consideration all the form errors of the ID which is not the case when the datum feature is specified RFS or MMC. For the min wall calculations you would need to consider the form errors. Therefore, the datum driven from datum feature specified RFS or MMC might not have the form errors embedded so they need to be considered separately."

For RMB and /or MMB: If the feature it is a perfect cylinder for 270° degrees, but for a small area has an irregularity / form error, then the datum is the axis driven from the unrelated actual mating envelope which does not include the form error just mentioned.

For LMB: If the feature it is a perfect cylinder for 270° degrees, but for a small area has an irregularity / form error, then the datum is the axis driven from the unrelated actual minimum material envelope which DOES include the form error just mentioned. It is the axis of a perfect cylinder, within the material, unrelated with any datums, which contacts the datum feature in the highest points (again, within the material)."

 

[pmarc]

greenimi,

Assuming I understand you question correctly, my answer would be following:
For min wall thickness calculations, if datum feature A is referenced RMB or at MMB in the positional callout, its maximum possible form error (prescribed by total size tolerance and Rule #1) should be taken into account, because there is no requirement for the datum feature to have perfect form when produced at LMC. If, however, datum feature A is referenced at LMB, its possible form error is not taken into account, because when produced at LMC it must have perfect form.

Does this answer your question?

 

[greenimi]

Yes. Thank you very much!
That is the exact answer I expected from you and you fulfill my expectations, as usual. Your answer is clear and concise.

Thank you again pmarc