Occasionally I've had a need to mill 3 or 6 sides on some round piece.
Always been a hassle to set the part for the 2nd and 3rd cuts.......keep fumbling around with 30-60-90 gauge, trying to hold all the stuff and tighten vise at the same time; got rather frustrating.
The last need was for a drill extension. You all know how the chucks on cordless drills lack good holding power, so I wanted to mill 3 sides of a hardened 1/2-20 threaded rod that was to become the extension for a 13/16 4-flute drill.
Finally made a fixture that allows indexing at 3 or 6 sides. These were made in pairs; dowel pins were fitted to the smaller holes to keep the 2 parts together while milling.
Started with 2.25 round stock (happen to have some SS). Roughed out the center. Milled to 2" hex.
Bored the opening by offsetting from zero. IIRC it was like + .035 in X and Y. Milled the "V". Drilled and tapped 5/16-18 holes for set screws.
Some different size round pieces will require a different length set screw.
Granted, when the fixture with the piece in place are rotated to the next index, the "touch-off" height will be different, but it's not a big deal.