Expanding Mandrel for turning large tubes

[MasterMaker]

I Like it. I made a set of internal collets a long time ago to allow me to bore the Id and sometimes thread the ID of long hydraulic cylinder tubing on a relatively short lathe. To do this I used these ranging from 2 1/2" through 5"
click on picture for clearer view the grub screws allowed for tightening to the ID of the tube while serving as an internal 4 jaw

With that design I think I would have just used some sink-head screws with part of the countersink bridging the slot and some extra relief under the head in the main body/threaded part so they would push on the tab with the taper when tightened down.