"I'm not sure what will happen with the definition of actual local size - to me, this is one of the most difficult problems in GD&T. There is not even agreement on the meaning of the current definition in Y14.5 applied to cylinders, as "any individual distance at any cross section" is interpreted differently by different people. Some interpret it as a distance extracted from 2 opposed points (as one would measure with a mic), and some interpret it as a diameter extracted from a cross-sectional circular element. Unfortunately, the standard contains text supporting both interpretations and no figures that clarify the meaning (only side views are shown, obscuring what is really happening within a given cross section). So we don't even know for sure whether a cross section of a cylindrical feature has one actual local size or many. The committee is in a difficult position, because it is now impossible to choose one or the other without contradicting past practices in some way. The Y14.5.1M-1994 mathematical definitions standard created a novel definition based on an LMC sphere, but this has been largely ignored in industry (partly because it conflicts with the idea of 2-point opposed diameters, and partly because the mathematical definitions standard was itself largely ignored).
The fact that there are camps favoring different interpretations makes it likely that different types of local size are needed for different applications. The ISO GPS standards define several different types of size, but so far Y14.5 has not embraced this approach."
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca"
