....
Excuse my ignorance, but if you are tapping, the tap is going to self feed
once it gets started. Don't you only need to concern yourself with the
depth of the tap prior to the spindle reversing?

Seems to me that that would then be a function of the number of
revolutions
rather than the speed that the tap is turning. (unless you are going too
fast in which case you will hear the snap and then call on your reserve of
magic words!)

I'm setting up the machine for rigid tapping on the large taps. Put the tap
right in the collet. To do this, you slave the Z-axis to the spindle. For
example, for a 3/4 x 10 tap, the spindle will move down 0.100" inches for
every revolution of the tap.

The machine can also hob gears. With a 4rth axis, advance the gear blank one
tooth for every one revolution of the spindle. This is the reason I needed
the extreme accuracy, it takes 1000s of turns of the spindle to make one
gear. For tapping, 20 turns is a lot.

Karl