Disc Grinding

Going back to Rohan's reply, Cast Iron contains free carbon. If you rub your fingers on it or some white paper you will see the carbon. That carbon lubricates any cutting tool so cutting lube is not required. You wouldn't want free carbon lubricating your disc brakes.

Nether the less, the discs are cast iron.
Having had a spot of braking bother with some guzzi discs,
where the discs actually snapped in half !,
I can vouch that the discs are indeed (brittle) iron.

O, and iron discs have a better coefficient of friction than steel, graphite content not withstanding, or so many auto braking applications wouldn't use it...

Its a strange form of cast iron that would actually mark a sheet of clean white paper. You may get a smear of rust if the disc hasn't been used recently.

Drilling a commando disc produced flakes off the drill, which is a clear sign of cast iron - it CANNOT be anything else - as mentioned previously.

These guys did mine:

http://www.ebay.com/itm/NORTON-DISC-BRA ... 24&vxp=mtr

Nice job job grinding with a non-directional pattern, drilling, lightening, and painting. I contacted him directly, not through Ebay and it was a little less expensive. I lost that contact info. Braking is better without the chrome plating.

need-source-for-rotor-turning-t7143.html?hilit=rotor%20ground

Yep, Gary did mine and a friend's MK3 and they came out great
Cheers

Just got my disc back from my machinist. Took it to him and asked what he recommended to remove minimum of material. He did it using a flywheel grinder. First skimmed the disc to hub mounting surface to true it as reference. Used that to mount the disc and then ground the outer surface. Flipped it and then ground the inner surface.