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Question I have always used MMC for cloearence hole for screw, but on the tapped hole leave it as RFS. Today a Engineer told me we should be using MMC for tapped hole in the feature control frame.
The right way would be to use the MMC of the fastener and a projected tolerance zone, with a projection height having some relation to the mating part.
To make the measurement of the tapped hole, measure the actual location of the tapped hole as in reference to its actual minor diameter, the center of that diameter being used to define the axis of the tapped hole. The you take the Axis, and sweep the MMC diameter the extent of the projected zone and compare that to the tolerance.
The MMC location would only apply outside the surface from the tapped hole to the limit of the projected zone, not into the tapped hole.
While you can specify all this, can you measure it?
Unless there is a compelling reason to use the projected tolerance zone, I would not use as it will increase the QC inspection time. By adding MMC to tapped hole GDT call-outs you are forcing the inspector to measure with gages either the major, minor or pitch diameter of each tapped hole to calculate the bonus tolerance that can be added based on the delta from MMC. Unless there are problems with parts that specifically require this type of control, I would stick with RFS.
MMC in theory gives you a bonus tolerance on the tapped hole location but in practice it is usually waaaay more trouble than it is worth. There is a really in depth discussion of this on eng-tips.com.
Just do what the place I worked at in Syracuse did. Make the circular tolerance zone of the taped hole .005. That way the shop can't ever measure it.
Sorry for the sarcasm, but, true position is so over used as to be rediculous. Most engineers don't have a clue how to use it. They don't realize that if the clearance hole is just a bit larger, the location tolerance can be larger, hence less expensive.
I wonder how much money is wasted in engineering offices, and their respective shops, trying to use a system that is unnecessary? Not to mention totally applied.
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