a lathe tool or centre punch will crack instantly... too hard/brittle

I second what gary sais- bearings are what ive always used and ive never had a problem, there also much more suitable for high speeds. If you can get one 20mm bearing either side that would be ideal.

NST has bearings in two bearing holders that clamp to the bar, probably the best way of doing it as you get a lot of "meat" around the bearing. He also kept the shaft as short as possible for maximum strength.