Flatness question 3

[SeasonLee]

A simple question arose while I was reading a textbook from Alex, please see the attached for the question, anything I missed? Thanks for the help.

Season

 

[pmarc]

CH,
I had no intentions to insult you. I was just stating the fact - "G" is not applied to median plane.

The quote from 5.4.1.2 does not support your point of view. It says: "This violation is permissible when the feature control frame is associated with the size dimension or attached to an extension of the dimension line."

Here "G" is applied to the extension line, not to "an extension of the dimension line".

 

[pmarc]

Oh, and the example of FCF applied to an extension of the dimension line is in fig. 4-38 - it is the position callout applied to dia. 38.00.

 

[pmarc]

Season,
This does not help me to understand why in the same time "D" is standard-compliant and the maximum flatness deviation allowed on the surface "I" is 0.4. What is CARE test?

 

[CheckerHater]

OK, me no spik English good - I missed the difference between "extension dimension line" and "dimension extension line".

Now, with both questions and answers available, can you tell why "D" is still legal?

The only explanation left to me is that in absence of <CF> modifier, 12+/-0.2 envelope does not apply to the entire length of the part.

 

[CheckerHater]

CARE test

 

[SeasonLee]

CH
For the OP case, I think labeled "G" is applied to a surface, pls see first page.

pmarc
The second page is CARE test.

Season

 

[CheckerHater]

I guess we already agreed on both

 

[Belanger]

Here's another thought -- would you all say that the 12 ± 0.2 is a "feature of size"? How does Rule #1 play into that?

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

 

[greenimi]

[Here's another thought -- would you all say that the 12 ± 0.2 is a "feature of size"? How does Rule #1 play into that?]

I guess for a certain portion/area IT IS a feature of size, so subject to rule#1, but still the flatness of any of these 2 surfaces –which are part of this feature of size--- cannot be more than the size tolerance (0.4), right? Therefore, rule#1 is covered.

 

[Belanger]

But what does that say about the left-hand surface, at the portion which is directly across from the open-spaced slot?

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

 

[greenimi]

I don’t know, probably that surface --- at the portion which is directly across from the open-spaced slot----is NOT subject to rule#1 because does not have the opposing surfaces/points.

 

[Belanger]

That is what I'm thinking about -- thus the flatness is compliant with the rules of GD&T, and not redundant.

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

 

[pmarc]

J-P,
How can it be that in the same time flatness tolerance "D" = 0.5 is standard-compliant AND the maximum flatness deviation allowed on the surface "I" is 0.4?

 

[Belanger]

pmarc, I never claimed that the flatness tolerance was limited to 0.4. I was actually restating what you had written earlier: since there is no CF, the size tolerance (and thus Rule #1) doesn't go all the way across. So the flatness "D" is standard compliant.
If CF were added, then your question would be valid: "D" would not be standard compliant.


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

 

[pmarc]

J-P,
Let me put it this way:
If per the answer-sheet flatness callout "D" is standard-compliant (which I think is correct, exactly because of the reason you described), then how is it possible that maximum flatness deviation allowed for the surface "I" (which is the very same surface the "D" callout is applied to) is 0.4? In other words, how can a surface controlled by flatness tolerance = 0.5 have maximum flatness error = 0.4? What determines that this is 0.4, and not 0.5?

 

[greenimi]

I would say surface “I” ends right at the horizontal black line shown on the side view (and runs below this line) and the surface “D” begins on that line and runs above it.

I say this because these pictures/examples are to be understood at the fundamentals levels and I don’t think the author intentionally made them to be tricky (at least not for beginners).

So, surface “I” it is a regular feature of size (at least part of it: a little bit below the black line and more below the slot) and then subject to rule#1 (maximum flatness error = 0.4 and maximum parallelism error probably the same value = 0.4—between the surfaces)

 

[axym]

Hi All,

I agree with pmarc on all counts. The Flatness tolerance D cannot be standard-compliant if the maximum flatness error allowed on surface I is 0.4. Also, I agree that Flatness callout G applies to surface F, not the derived median plane.

I would say that the 0.5 Flatness tolerance D is most likely a typo. It was probably supposed to be 0.3 or some other value smaller than 0.4. This is a fundamentals textbook, and I would not expect this author to intentionally include tricky nuances of discontinuous features of size.

I would also say that part of the blame here lies in the concept of "perfect form at MMC" and extending this concept further than it should be. Here is what Y14.5-2009 actually states:
-The actual local size of an individual feature at each cross section shall be within the specified tolerance of size.
-The surfaces or surfaces of a regular feature of size shall not extend beyond a boundary (envelope) of perfect form at MMC. This boundary is the the true geometric form represented by the drawing.
-No variation in form is permitted if the regular feature of size is produced at its MMC limit of size unless a straightness or flatness tolerance is associated with the size dimension or the Independency symbol is applied.
-Where the actual local size of a regular feature of size has departed from MMC toward LMC, a local variation in form is allowed equal to the amount of such departure.

So we have two requirements. One is that the local sizes must be within the size tolerance, and the other is that the surfaces must conform to a perfect form boundary at MMC. Many authors extend this to say that Rule #1 imposes an indirect form control on the surfaces, but the standard does not actually say this. It is true that the form error of a feature conforming to both the local size requirement and the boundary requirement could not be more than the size tolerance. But this only applies to features in which the surfaces are fully opposed (i.e. in which the local sizes are defined everywhere). The problem, of course, is what to do in cases like the one in the example where the 12 +/-0.2 width feature has gap in one of the surfaces.

This has also got me wondering about the effect of the CF symbol. Exactly what does the CF symbol on the 20 +/-0.2 feature give us? Does it mean that the size tolerance includes the N surface, which would not be included if the CF symbol were not there? Surely not, or we would need a CF symbol for the 12 +/- 0.2 feature in order to include the E surface.

Yikes, another can of worms. The standard does not tell us what to do with partly-opposed features of size.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Sorry, pmarc -- I hadn't read all of the questions on the exercise, so I didn't see anything about the flatness of I being queried. But yes, something seems to be wrong there between questions 3 and 6.

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