Hi,
have you done any turning on an ordinary lathe? If you have then you'll know that speed needs to be right but its not supercritical.
For threading on a lathe accurate speed is not that critical but a CONSTANT speed is important. Any and every time you take a cut on a lathe the spindle slows a little
bit. Trying to minimize the speed variation is the key to threading with Mach3/4.
Rigid tapping is a term more commonly applied to mills than lathes but the idea is that a spindle turns a tap, usually fairly low speed, say a few hundred rpm, and the feed
axis advances the pitch of the thread with each revolution of the tap. When the bottom of the thread is reached the spindle decelerates, the reverses and the feed axis
follows suit and backs out. Whats critical is not the speed but the angular accuracy of the synchronization of the spindle/tap and the feed axis. Ordinary DC and 3 phase
induction motors with a VFD are not really suitable, for rigid tapping you really need a servo spindle, that is to say a spindle which can maintain angular position.
Rigid tapping is pretty ambitious for a hobby machine. Single point lathe threading is achievable.