OK, so when one adds CNC to a mill or lathe, it's a good idea to upgrade to
ballscrews in order to eliminate backlash.....sounds reasonable to me, but
I'm a little confused about what this does for the leadscrew of a lathe. Is
there a special ballscrew that can be engaged/disengaged like a T-nut? I
can't quite figure out how anyone would thread with a lathe after replacing
the normal leadscrew with a ballscrew....seems like manual threading would
be completely out after that.

I suppose the CNC controller could perform the threading operation, but it
would need to know the spindle speed and phase...how is this normally done?

Todd