GRS&S said:
I have a feature control frame of
True position | 2.0 | A | B | C M (where C is Ø 8.0 ± 0.1)
It is my contention that the 2.0 applies when the diameter of the hole is at it's minimum size of Ø 7.9. Bonus tolerance would be added as the Hole Ø increases. Therefore, if the Ø measured at Ø 8.1, I would be able to apply the most "bonus" to the exisiting position of 2.0.
Any feedback?
I'm don't consider myself a GD&T expert, so take my comments with a grain of salt, but this is my understanding of it:
When the MMC modifier is applied to a datum, it allows what's termed "datum shift" which is similar but not exactly the same as bonus tolerance gained from MMC on the feature itself. In this case, the MMC condition is noted on the tertiary datum. Now, not knowing what the actual datum features are, I can only give example of how it should be used.
Assume for example that the A datum is a plane, B datum is a wall perpendicular to A, and C datum is your 8.1 diameter. The part would be staged on the A datum feature which would restrict pitch, roll, and translation in X. It would then be aligned to datum B which would further restrict yaw and translation in X (or Y if aligned to Y). Finally the part would be restricted in Y by the C datum, removing the last degree of freedom.
Noting that the MMC condition is allowed for the C datum only, the part could be shifted to the extents of the C datum feature size, but must remain restricted in translation or rotation by the A and B datums.
It gets more complicated if there are multiple features that are to be evaluated using the same datum structure, as by GD&T rules, features controlled by the same datum structure with the same modifiers are to be evaluated simultaneously. This means that the datum shift must apply to all these features in the same direction and by the same amount, i.e. a position error of -.1 in the X direction and another of +.1 in the X could not both be accomodated by shifting the part in one direction or the other. Two features with positional error in the same direction could be acceptable assuming the datum structure allows shift in that direction.