Hello Richard,
I was fortunate to work a brand new HLV lathe. for 7 years, but that was 40 years ago now, so my memory is not so good. There is a round knob near the rear of the headstock which will select the threading box. The quadrant on the outside of the box is only used for Imperial and special threads, like DP or Mod or additional Metric not covered by the box.
One point to make when setting up for threading is to set your speed first. Increasing the speed alters the trip point on the screwcutting clutch. It is all to do with moments of inertia.
From what I recall there is a sizeable two speed motor on this lathe. In use it is possible to go from high range directly to low range, this might cause some invertors a problem. I know of one person who used a rotary phase converter.
Is your machine fitted with a handle to alter the variable speed, or does it have a separate motor like the one I worked on. If so I suspect this motor will need to be accounted for with the sizing of any invertor. Plus there is the variable DC power feed to the carriage for normal self acting feeds, this too will be a load on the invertor. No doubt the more knowledgeable electrical Forum members can help there.
Regards
Gray,