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?

 

[powerhound]

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

 

[MQSCI651]

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.

 

[CheckerHater]

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

 

[powerhound]

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

 

[MQSCI651]

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.

 

[3DDave]

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.

 

[powerhound]

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

 

[powerhound]

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

 

[greenimi]

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.

 

[MQSCI651]

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?