Position tolerance using envelope requirement @ RFS

[Evom]

Cannot seem to find any example from my references to enforce my theory on this so looking for clarification here.

Have a hole say 5mm, tolerance of +/- 0.2, position tolerance of 0.05 applied controlling it with respect to 3 datums A, B & C using basic dimensions.

From what I understand, although technically they state you need a maximum material requirement (MMR) to invoke a maximum material virtual condition (MMVC) scenario, an "extreme boundary" is still in effect with the envelope boundary @ RFS as envelope requirements do not control the orientation or location of a feature of size FOS

So theoretically, the extreme boundary would still be @ maximum material state (MMS) 4.8 - 0.05 = 4.75. The only difference between RFS & MMR in this scenario is the position tolerance will change for MMR based on the size due to the collective requirement where as with RFS it will not, but they both have boundaries that go beyond the size tolerance of the hole due to addition of the position tolerance.

Does this sound correct?

 

[Belanger]

Yes, I think that sounds OK. Anything that moves around creates a boundary. The difference is that the MMR modifier creates a constant boundary, whereas the boundary for an RFS tolerance is constantly changing -- but it's still a boundary of sorts.

That's why the RFS state is usually described in terms of the axis, not the boundary. I know the ASME standard is pretty explicit about this, but I'm not sure of the exact wording in ISO (since it sounds like you're using ISO).

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

 

[Evom]

Correct in regards to ISO being the standards I'm using.

RFS isn't a boundary as you said, but rather is the default condition of a tolerance unless specified otherwise (M or L modifiers) in the tolerance frame.

The envelope boundary in ISO is akin to rule #1 in ASME. The way I visualize it is the envelope boundary shifts due to orientation and location tolerances while keeping the MMS of the feature of size (FOS) contained. They won't affiliate the term "virtual condition" of any sort with in an RFS scenario to prevent confusion but the term "extreme boundary" is mentioned on other tolerance controls in RFS scenarios.

So I presume for position it's the same as it controls the form, orientation and location requiring an "extreme boundary" pin size equal to 4.75 for a pass/fail inspection

 

[Belanger]

I agree that the term virtual condition wouldn't be used in an RFS scenario. Virtual condition is a boundary but a special kind of boundary (a constant boundary) only when MMR (or LMR) is invoked.

However, I wouldn't say that the boundary stuff we're talking about with position is akin to Rule #1 in ASME. That's a quite different thing, where the size tolerance of a feature inherently controls the form (nothing to do with position). In ISO terminology this gets into the circled E and other goodies detailed in standard ISO 8015.

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