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Question about straightness of a planar surface

Question about straightness of a planar surface

Question about straightness of a planar surface

(OP)
I am a new one. I have a question for you guys.

Question: Is the straightness of a considered planar surface (usually no modifier indicated) used under the regardless of feature size (RFS) concept?

For example, please see the Fig.5-6 Specifying a straightness of a flat surface in ASME Y14.5 2009.

The straightness for a planar surface rather than derived median line or derived median plane never uses MMC or LMC modifiers. And RFS concept is implied for all geometric tolerances where no modifier is specified. So the straightness of a planar surface mostly like to be under the RFS condition.

Is it correct or not?

RE: Question about straightness of a planar surface

Straightness applies to linear elements, which don't have size, so no modifiers based on size are allowable.

RE: Question about straightness of a planar surface

...unless it's applied to a feature of size. (I know the question was about a planar surface, but I had to throw that in there so the OP doesn't take that as an across-the-board statement for all straightness applications.)

When dealing with surfaces, there really is no such thing as MMC or LMC. So I suppose it's legal to speak about a surface tolerance "RFS" but it doesn't really add any value since surfaces have no size, as Dave mentioned.

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

RE: Question about straightness of a planar surface

JP - I thought about adding that, but then it requires repeating every section of the standard and then adding a complete tutorial with more examples.

RE: Question about straightness of a planar surface

Good point CH. And still could not get 100% agreement due to the "actual local size" improper definition/ interpretation.

RE: Question about straightness of a planar surface

I would consider this a feature of size. I don't know of a definition of "partial feature of size" in the standard. Maybe I just haven't seen it. That being said, I don't know what it has to do with the exercise question. Were you looking for help with that or are you just providing the figure for the graphic only?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
Hi powerhound, I am really looking for help with that.

Personally I believe that the first priority is to solve the problem whether or not using RFS concept in this case.

Talking about partial FOS, there is not authorized standard statement on it. But most cases in Y14.5, you will see no perfect opposite sides of those parts. Such as Fig. 5-7, Fig. 6-2, and so on.

RE: Question about straightness of a planar surface

Quote: “Personally I believe that the first priority is to solve the problem whether or not using RFS concept in this case”

Sorry for jump a little bit into the conversation, but I am not sure how we can help if we don’t know what are the functional requirements.
The standard allows you to use both (MMC or RFS) depending of the functional requirements/ design intent.


RE: Question about straightness of a planar surface

The standard does not allow MMC in the presented case so RFS is the only option. This what 3D Dave has already explained.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
Hi greenimi, you are right, the standard allows you to use both (MMC or RFS). But standard requires you use MMC when the drawing specifies it with MMC modifier.

RE: Question about straightness of a planar surface

Quote:"But standard requires you use MMC when the drawing specifies it with MMC modifier."

What is this? I am a little bit confused now.

RE: Question about straightness of a planar surface

(OP)
Without MMC or LMC modifiers, you have to use RFS concept, to my knowledge.

RE: Question about straightness of a planar surface

That's kind of true but your phrasing is a little strange. Keep in mind that if a tolerance is met at the RFS condition then it will also be met at MMC, thus you don't HAVE to verify at MMC if specification is met at RFS.

So what do you think the answer to the question is?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
I believe the answer is B(0.2).

RE: Question about straightness of a planar surface


MQSCI651

Did you read the thread CH pointed to?
If yes, did you see what is the answer?

RE: Question about straightness of a planar surface

What would you consider "worst case"?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
"The worst case" may be the most important point in my opinion, because no definition about this. Please let me take certain time to think about it. I will try my best to describe it later on.

RE: Question about straightness of a planar surface

(OP)
Hi greenimi, I will take time to read that thread. Thank you for your help,

RE: Question about straightness of a planar surface

It usually means an MMC condition. If that's the case, would your answer still be 0.2?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
Because of RFS, no bonus tolerance is allowed in this case. So I don't think "the worst case" is matter with the size tolerance.

There are two kinds of tolerances, size tolerance and geometric tolerance. Straightness and flatness are geometric tolerances.

RE: Question about straightness of a planar surface

Does anyone remember that "F" in RFS stands foe "feature" and "S" stands for "size", and RFS HAS ABSOLUTELY NOTHING TO DO with features that don't have size, such as planar surfaces?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: Question about straightness of a planar surface

MQSCI651,

No, that is incorrect. In this case, the worst case is dependent on the size. As CH pointed out, RFS means Regardless of Feature Size so it absolutely has to do with the size of the feature.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

(OP)
Hi powerhound, I love your direct explanation.

My current understanding is still that the geometric tolerance is not relative to size tolerance in this case. They are individual each other. RFS = no matter what feature size is, geometric tolerance should be kept in a certain level. There are only two choices, the worst one is 0.2.

Please help me to point out what's wrong with my opinion.

RE: Question about straightness of a planar surface

Size tolerance is a geometric tolerance. It just isn't a geometric characteristic.

"tolerance, geometric: the general term applied to the category
of tolerances used to control size, form, profile,
orientation, location, and runout."

What most do is read between the lines and fill in the word 'symbol' for the word tolerance in that definition. It would be nice if the Committee had managed to actually change the definition, but they either overlooked it, or something.

Anyway, a size tolerance does control size, and often form and sometimes profile. Just not explicitly, but that's another word that isn't in the definition.

RE: Question about straightness of a planar surface

Quote (MQSCI651)

My current understanding is still that the geometric tolerance is not relative to size tolerance...

General rule #1 directly relates size tolerance to form tolerance. If you don't know about general rule #1 then maybe that's the disconnect. If you do know about it then maybe we need to understand it better. Basically general rule #1 says that as the size of a feature departs MMC towards LMC, that much deviation in form is allowed. At MMC, the form must be perfect. So in your case 30.3 would be the MMC size. At this size, the form must be perfect. Straightness is a form tolerance so at 30.3 the straightness error must be 0. If the feature were to actually measure 30.2 then the allowable straightness error would be 0.1 because 30.2 is 0.1 away from the MMC size. There are some exceptions to rule #1 and one of them has to do with straightness but this is not that case so there is no exception here.

My biggest concern with this exercise is with what is meant by "worst case." Do you know who authored the material? Also what version if the standard is the material teaching? 1994 or 2009?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

I was deliberately avoiding answering the question so we could work through it but you seemed to be hung up on size being unrelated to geometric tolerances and I was afraid you would lose interest before we got to the bottom of it. If this exercise is in accordance with the 1994 standard then there are only 2 ways to override rule #1. One is a note stating "PERFECT FORM AT MMC NOT REQUIRED". The other is applying straightness to the feature of size (derived median line). The method shown here is applying straightness to line elements of a surface so rule #1 is not overridden. Thus when it is produced at it's MMC size, it must have perfect form--zero straightness error--even though 0.2 and 0.5 are allowed.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

RE: Question about straightness of a planar surface

Looks like you guys are back to the thread CH pointed/ indicated.
Rule#1 and where is applicable and where is not.
Don't reinvent the wheel. Read the previous thread (the entire thread not just portions of it) and agree or disagree with the statements presented there.

RE: Question about straightness of a planar surface

(OP)
Thanks to 3DDave and powerhound. All your inputs are very good for my understanding on this problem.

Hi powerhound, I read this problem from the other forum. I agree with what you said about this. My direct feeling is different resource has different background, and different language. I don't know who authored this material. I don't know what does the "worst case" mean under what kind of background. In the other words, when we say the "worst case", what will be based on?

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