Composite Tolerance

So, our shelf has 10 mm holes in it and we've improved our drawing to include MMC for the features. We want hard gages for this because, while business is good, we can't yet afford a CMM

For the top segment, the gage would be 3 mutually perpendicular planes. In datum A there would be 2 pins with a diameter of 5 mm. Pins would be located as close as the toolmaker can place them to the holes basic dimensions.

Middle segment gage would have 2 perpendicular planes at -A- and -B-. Datum A would still have 2 pins at their Basic location, but now the diameter would be 8 mm.

Bottom segment gage is a single plane, again with two pins, but now the Pin diameter is 9.5 mm.

This setup makes sense to me.

However, I'm having trouble trying to imagine a hard gage that fulfills both segments of the callout in the original post. Gage would have to make contact with both -A- and -B- with a chuck to restrain -A- while holding the part against -B-. Assuming that the position should be at MMC, the Top segment pin size would be .001 smaller than the smallest allowable hole size. Bottom segment pin should be .0005 smaller than the smallest allowable hole size.

I don't see how the lower segment is refining the relationship of the holes.

Not trying to be difficult here, just trying to understand the logic.​

So, our shelf has 10 mm holes in it and we've improved our drawing to include MMC for the features. We want hard gages for this because, while business is good, we can't yet afford a CMM

For the top segment, the gage would be 3 mutually perpendicular planes. In datum A there would be 2 pins with a diameter of 5 mm. Pins would be located as close as the toolmaker can place them to the holes basic dimensions.

Middle segment gage would have 2 perpendicular planes at -A- and -B-. Datum A would still have 2 pins at their Basic location, but now the diameter would be 8 mm.

Bottom segment gage is a single plane, again with two pins, but now the Pin diameter is 9.5 mm.

This setup makes sense to me.

However, I'm having trouble trying to imagine a hard gage that fulfills both segments of the callout in the original post. Gage would have to make contact with both -A- and -B- with a chuck to restrain -A- while holding the part against -B-. Assuming that the position should be at MMC, the Top segment pin size would be .001 smaller than the smallest allowable hole size. Bottom segment pin should be .0005 smaller than the smallest allowable hole size.

I don't see how the lower segment is refining the relationship of the holes.

Not trying to be difficult here, just trying to understand the logic.​

imagine the part in the OP is going into a "D" shaped slot. if the bolt pattern moves in X or Y no big deal as long as the slot has enough offset from the part, but once the bolt pattern starts to rotate the flat on your part will not be parallel to the flat on the slot and cause interference.

The datums don't have a Maximum Material Boundry so there is no datum shift. D shaped slot has to collapse around the part eliminating all movement except along the Z axis.