Think of a bicycle handlebar stem … slightly tapered plug and a screw thread.
It’s the same general idea.
Alternatively you can use a split collet, like on some photographic tripod legs.
If you don’t get it, I will take a photo tomorrow.
MichaelG.
.
Or you could look at this: https://patents.google.com/patent/EP1039149A1/en
Edited By Michael Gilligan on 25/01/2020 19:58:51