I use air, got a valve in the line to keep max speed under control easier and I run 2 air compressors, my big 80 gallon one will get behind after a while so I just piggy back my little 10 gallon one into another shop hose and they keep up great together.

For aluminum I use the aluminum bits with wd-40 and finish em with the iron cutter and lots o wd-40. I buy bits from mac tools guy, he warrantys them when they chip a tooth. Expensive but worth it.

I am not causing global warming, I am just trying to hold off a impending Ice Age!