Application of Rule #1 (Taylor Principle)

[shaun8567]

Given the following:

What is the tolerance on the Flatness of Datum Feature A?
a)The flatness is not toleranced.
b)0.3
c)0.5
d)1

Originally, I thought it was (a) because a +/- dimension is equivalent to taking a measurement with calipers/mic (meaning the only control is the width at any given point), so it would be possible for the part bow out and have a flatness greater than 1 while still having a thickness measurement of 24.5-25.5.

However, after re-reading over Rule #1 of the standard it seems that the correct answer is (d). If I understand it correctly, the application of the Taylor Principle to this problem means that the feature must fit within a perfect boundary of the part at MMC. In this case, the flatness on the feature could be as high as 1. Is this correct?

 

[Belanger]

Yes, your latter thinking is correct. A "purist" might also add that the 1 mm flatness tolerance is only available to datum feature A if the opposite side is perfectly flat. But the gist of the question and your answer is all correct.

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

http://www.gdtseminars.com

 

[pmarc]

A "purist" might also add that the 1 mm flatness tolerance is only available to datum feature A if the opposite side is perfectly flat.

Are you sure about that, J-P?
Have a look to attached graphic. The way I see it - even with the opposite side not perfectly flat the flatness error of considered surface may still be 1.0.

http://files.engineering.com/getfil...c-97ba-43fc-931d-f3012d4e2313&file=rule_1.JPG

 

[Belanger]

Good point! I guess I was mixing the idea of feature-of-size flatness (where my extra statement would be true) with surface flatness, which was my thinking as I initially answered the question. Thanks for the graphic.
Actually, reading the original post, we don't know what datum feature A is -- it could be tagged with the size dim. But I suspect Sean meant that it was one of the faces, and either way the answer from the multiple-choice list is still d.
Sorry to muddle things with my extraneous comment

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

http://www.gdtseminars.com

 

[pmarc]

I guess I was mixing the idea of feature-of-size flatness (where my extra statement would be true) with surface flatness

Well, not exactly, J-P. Have a look to another graphic. (I assume that by saying "feature-of-size flatness" you meant "flatness of derived median plane of feature-of-size".)

http://files.engineering.com/getfil...b25-4f74-80e5-4023bf71d742&file=rule_1(2).JPG

 

[Belanger]

Doh -- I'm really not with it on this one, am I?! I obviously know the principles, but I meant to write that the full 1 mm of flatness tolerance (on a FOS) is only available if the actual local size is at LMC.

How's that?

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

http://www.gdtseminars.com

 

[pmarc]

You nailed it.

 

[dgallup]

I see no datum feature A.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[KENAT]

Assuming -A- is the left or right side then I believe (d) is the answer.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

shaun8567,

In ASME Y14.5-2009, see figure 2-6. They do not show the numbers, but it is pretty obviously answer d.

In ASME Y14.5M-1994, the figure still is 2-6.

--
JHG

 

[SeasonLee]

For any form control, the datum reference is not allowed, so the question itself is questionable, am I right?

Season

 

[drawoh]

SeasonLee,

The datum is irrelevant to the tolerance specification. Calling up a datum was a good way to show us what surface he was referring to. It would have been clearer if the datum had actually been applied.

--
JHG

 

[fsincox]

Nice graphic, pmarc, I too did not see that!
Frank