I'm in the beginning stages of making one. I have 2 bench grinders I thought about using. 1 is a large one, 3/4 hp, 450 rpms. The other is smaller, 1/4 hp, 3700ish rpms. Is my research correct that you only need 300-400 rpm? Could I run the big one direct-drive? Or should I go about 8:1 ratio on the smaller one?