Can anyone lead me in a direction for properly machining grey cast for rotors. Our supplier won't cut any thiner than 200 thou and our machinist assures us he can't surface grind them to 140 thou, last year's thickness, let alone the 125 thou we want to test to run this year. Any suggestions?

I don't see why you wouldn't be able to machine them down pretty thin, using a magnetic table or even double-sided tape (and light depths of cut). Surface grinding should only be to take off a a very light cut, not 0.060"

That said, my usual suggestion with brake rotors for FSAE is that you don't have to use cast iron. You can use steel plate, buy it at 1/8" or 3/16" thickness or whatever and surface/blanchard grind off a few thou for a good surface.