I run a Mitsui Seiki HU63A, a 4-axis HMC with a Fanuc 15M control. Rigid tapping works great with standard thread pitches using G84.2, but it consistently screws up metric threads done the same way.

I don't even understand how the machine could possibly care whether the thread pitch is standard or metric, but it does. Small taps break unless they're in a floating holder, big taps make really bad, ugly threads. Once, using an M16x2 tap pretty far from the center of the pallet, the machine threw a B axis overload alarm (it does that if I'm using a big drill too far off center and forget the M10 to lock the B axis). So it does get fairly close to the right feed, and it certainly acts like it's rigid tapping, it just gets it wrong by a bit.

Sample code for M16x2 tap:
G84.2 R.1 Z-1.25 S127 F10.0

Working code for 3/4-10 tap:
G84.2 R.1 Z-1.25 S130 F13.0

This makes no sense to me. Any ideas?