I want to make some rings but if they are thin, they are difficult to hold. Moreover if you want to make a nice uniform surface the easiest thing is to mount it on a drill and use files, sandpaper to smoothen the outside. So you insert a M4 nut into this mandrel, then attach it to drill, slip on a ring and by tightening M4x30 bolt this mandrel expands and holds your ring firmly.

You can buy it for 30$ but why to spend this money if you can 3D print it. It works with rings sizes 3 to 24 HK which means 13,2 to 20,2 mm in diameter.

I printed it out of PLA, but PETG or ABS will work as well. TPU might provide even better grip but I have not tested it yet.

Printing: 

Material: PLA 

Perimeters: 5

Infill: 20% adaptive cubic

Supports: only on ring_mandrel-01 - organic supports everywhere

ring_mandrel-01 must be printed as shown in the picture to prevent snapping of the moving jaws