Assuming drawing standard Y14.5:
Use gages at virtual condition when the tolerance is specified at MMC to account for a feature produced at MMC size and positioned as out of true position as the tolerance allows. The use of virtual condition gages is the same whether the datum is specified at RMB or at MMB (that allows datum shift, i.e. some amount of movement in the fixture to help the feature fit in the VC gage).
Simply because such a gage imitates the "usable space" of the feature in question. For instance, if a hole is given a position tolerance with the MMC modifier, that recognizes that a bigger-sized hole will be able to walk around more. However, the "winking" effect of that hole creates the exact same "usable space" as a smaller hole that can't walk around as much.
Thus, the virtual condition is the one thing that is constant, and that's what the gage can instantly verify.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
Of note: One should understand the difference between related and unrelated actual mating envelopes (RAME and UAME) to appreciate the application of gaging concepts for datums at MMC.
A cylinder by itself is unrelated. There is no datum reference.
A cylinder specified as having a perpendicularity to a datum reference plane has an actual mating envelope that is related to that plane.
In the first case, it's just the cylinder and the actual mating envelope is allowed to align as required to fit closely to the cylinder.
In the second case the actual mating envelope is restricted to being oriented perpendicular to the plane, so if the cylinder is tilted the related actual mating envelope will be larger than the unrelated actual mating envelope. In practice this is similar to the diameter of the smallest clearance hole in a mating part that fits up against the datum feature plane that the cylinder is specified relative too.
Babu96,
What standard for dimensioning and tolerancing do you use? If, by any chance, Y14.5-2009, figure 1-1 is a good place to go to understand the difference between related and unrelated actual mating envelope (RAME vs. UAME).
I would personally say that although knowing the difference between RAME and UAME is important, even more important is to know rules for proper calculation of Maximum Material Boundaries of datum features since gages are supposed to simulate these boundaries. For that para. 4.11.6.1 and associated fig. 4-16 in Y14.5-2009 may be helpful to some extent.
3DDave and pmarc drilled in to the details of my post - thanks guys. J-P Belanger really defined the underlying concept - "usable space" - that one must grasp before the calculations make any sense. I use the thinking that the gauge is worst condition of the metal that a mating part would "see", i.e. the GD&T error would be at the maximum allowed acceptable.
