Originally Posted by WizardOfBoz

Bearing race grinding and polishing is done on really precise, very rigid machines. Especially for the higher tolerance classes. One can probably smooth out some rust or corrosion to improve a bad bearing, but don't think that doing so will take your corroded bearing back to grade 10 or 25. If your cup is not totally knackered, clean it up gently (If I was using a die grinder I'd probably use a brass brush) wipe out any remaining rust and dust, regrease and use new balls and cones. Won't be super, but will probably be pretty rideable and will last a bit. I did this with my Schwinn Superior - fingers crossed.

Also, there seems to be some confusion about through-hardened or case-hardened bearing races. Steel that has low carbon cannot be heat-treated to be as hard as higher carbon steels. Heat-treated high carbon steels are brittle. Tempering hardened high-carbon steel lessens its brittle nature, but also lowers the hardness a bit. One can use a lower carbon steel (more ductile, less likely to crack) steel and treat it in some way to increase it's carbon content at the surface. Then when you heat-treat the part, the surface is hard but the core is tough and you have the best of both worlds. You can also harden a case by heating it very quickly and then quenching it to just harden the surface. This avoids making the core brittle. Files are made this way (using induction heating).

Chrome bearing steels are alloys that combine very good wear properties with some toughness. They are through-hardened (at least in the sizes that bicycle bearings are made in). Most higher end bicycle bearings are, I think, chrome bearing steel. This would give you a little bet better corrosion resistance. But the point is, you probably could polish a lighly corroded bearing race to reduce that crunchy feeling, and give lower friction for a while, by polishing.

I guess one can grind bearings with a die grinder, but in all but the most desperate vintage cases - just buy a new bearing! I realize that for a PX-10, that's easier said than done, though.

BTW, back in the day my dad was VP of Sales and Marketing for DuMore, a company that made die grinders and flex shaft tools. They are useful, but having grown up in a machine-shop environment where the products had to run well and last a long time, taking a die grinder to a bearing race... ughhh. It sends shivers up my spine.