Ignoramus8473 wrote:
OK, so I have a CNC mill, in which I know the spindle speed only
approximately.

Can I, realistically, use it for threading through holes, perhaps with
tapping head?

Yes, if you have a Procunier or TapMatic head, it has a forward clutch
that engages when forward pressure is applied on the tap. So, what you
need to make sure of is that the spindle rotation stays ahead of the Z
feed. In that case, the clutch will slip a little here and there to
keep the tap synced with the thread. If the spindle is too slow, it
will mash the tap through the hole, leaving horribly mauled threads. I
had the belt in the wrong step once and did this. So, the spindle speed
should be a little higher than the mathematically calculated value for
the Z feed, or the Z feed should be set a little slower than the spindle
RPM would suggest.

I have a program hidden on my web site to do this,
http://pico-systems.com/recttap.c so you can see what the code looks like.
My Procunier head doubles the spindle RPM in reverse.

on