Question about straightness of a planar surface 1

[MQSCI651]

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?

 

[3DDave]

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

 

[Belanger]

...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

 

[3DDave]

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.

 

[MQSCI651]

Thank to 3DDave and Belanger for their explanations.

Talking about FOS, how should we consider this part which is displayed as attached file as below?

Should we treat the part as one that has FOS or partial FOS?

 

[CheckerHater]

We already have spent two months trying to figure that out:

http://www.eng-tips.com/viewthread.cfm?qid=391408

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

 

[greenimi]

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

 

[powerhound]

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

 

[MQSCI651]

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.

 

[greenimi]

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.

 

[powerhound]

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

 

[MQSCI651]

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.

 

[greenimi]

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.

 

[MQSCI651]

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

 

[powerhound]

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

 

[MQSCI651]

I believe the answer is B(0.2).

 

[greenimi]

MQSCI651

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

 

[powerhound]

What would you consider "worst case"?

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

 

[MQSCI651]

"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.

 

[MQSCI651]

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