I've drilled rotors before and am aware that the cutting oil makes a big difference. I was 12- 3/8" holes from being done and ran out of one type of cutting oil, as soon as I changed oil the bit went dull and it was trouble after that. I quickly ruined a second bit and stopped there.
I got a new cobalt drill and it drilled well but kept catching on the edges as it went through, I think it had to do with the split point and angle. Anyway, I ended up with three work hardened holes that nothing would touch.
I went out this morning and punched through them with a 1/4" masonry bit, then I used the best 3/8" cobalt bit from the opposite side with a new cutting fluid (blue gel from Lowes) and what I think is the oil I started with, to re-fill the squirt bottle and I used both. At any rate, that worked well enough to finish it.
I reamed the holes with a 1/2" masonry bit and that worked great.
Next up was a wheel on an axle that could spin to slip the rotors on to sand on a nice finish with an air sander. A used 80 grit with a hard scotchbrite type pad to finish them.
