The one I made was about 4 degrees, it is not critical at all. I just thought of the old style bicycle handlebar stem expander.

I used 20 degrees included angle, so that the tapers were not self gripping, and released when the drawbar was slackened.

This has worked well on at least three, if not four lathes.

Howard

As I said, the angle is not critical, you are only turning the spindle by hand, threading, most likely. I used the hand method to cut my first thread, a 1 3/4 X 8W for a chuck backplate. I have not needed it since as even if cutting to a shoulder, I just run the spindle in reverse and cut away from the chuck with a left handed tool.

I used 4 degrees a side.

Edited By old mart on 28/02/2021 17:51:59

I made mine, many years ago, basically to G H Thomas's design (in Model Engineers Workshop Manual) and it uses a taper of 20 degrees included. I only tighten it up with finger pressure and has worked well but occasionally slips if I'm asking too much of it, like tapping a 3/8" hole. I made the tommy bar in the end of the drawbar 1 3/4" log overall but it would be better if it was longer which would allow it to be tighten a little more The slits in mine are just over 2" long

John.