Thanks Geof, that was a lot of help.
I slowed the spindle down to 60 RPM, did a 10 TPI thread, 0.500 deep. At that speed I could watch the chuck and ride my hand along on the Z axis handle.
The thread should have been 5 revolutions. Haas feeds three lead-in threads for a total of 8 revolutions. It stopped
every time on the same position. I tried 0.600 deep and as suspected, it did 9 revolutions, reversed and stopped at the end on the same clock position.
Next: I tried 0.550--the chuck stopped 180 degrees out, 0.575, 270 degrees out, etc.
It looks like it does indeed 'rigid tap' but this is where the breakdown occurs. If I pre-rotate the chuck before pressing Cycle Start, it uses the position it started from for its clocking (the new position is also exactly where it stops after the cycle).
So what I think is happening is that the machine will go through the motions but I'm not sure if the spindle encoder is 'allowed' to have absolute control over the Z axis encoder (with R-tap turned off).
My concern is that the energy in the chuck (at higher RPM) might cause the chuck to over-run the position. There's prolly no free lunch here.
I would probably single-point most threads anyway but it's interesting to see what it can/can't do without the option.
I'm now thinking of floating tap holders. Just a thread or two worth of float would allow it to work.