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
Attachment 41451

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.

[Toolmaker51]

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.

Mechanically speaking, a flathead screw could perform that wedging action, counterbored deep enough to apply that about mid point.
The drawbacks?
At least two limitations; no flathead screws are dependably concentric to their threaded body, and a large enough screw thread (and corresponding wrench size) might have too large a conical section to fit between fixture shaft and ID of part.

That said, an expedient inside grip is achieved on short parts, sawing an arbor into 4 segments after tapping center for a NPT pipe plug. Another way turns OD of socket cap screw head to about 7°-10°. With same setting to compound, bore a recess same angle into the plug. Fine threaded fasteners work best.