[InFlight]

Center Drilling a 8 mm hole (.315) in stock that's .375 hex won't leave much (.03) wall thickness across the flats. You would then need a 2 mm keyway broached into the corner of the hex. If your robot gets hit hard in a defense game, that's a failure waiting to happen.

I would suggest going with a different miter gear on a larger round shaft. Look at SDP, Martin, & Boston for alternate miter gears. Adding a keyway for a miter on a round shaft is trivial on a milling machine.

Another advantage of round shaft is there are smaller ball bearings available and you can use needle bearings. There are no 3/8 hex bearings smaller than R8 (1.125 inch OD). That will make for a larger steering tube.