Effective Training Inc. Message Board

I've repeatedly run into a problem when dimensioning long thin-walled tubes under ANSI Y14.5. Because they are stock sizes and subject to free state variation, the envelope rule clearly does not apply. That's good for straightness, but there are situations where I would like to require perfect circularity at MMC, or at least some kind of variable allowance depending on size. Unfortunately, Y14.5 forbids MMC on circularity or cylindricity. If the envelope rule can do it, why can't I?

Section 5.5 says that where circularity is applied in a free state, the diameter should be qualified with AVG. That suggests to me that if I don't apply a form tolerance and don't qualify the diameter with AVG, then the envelope rule still applies to circularity even if the part is flexible. Is that right? The standard certainly isn't clear on this point if that's the case.