The plans for this grinder are beautifully drawn, and the rest of the website is quite interesting as well.

Warning, don't get the impression that you need a machine like this for grinding a mirror -- it would simply make the job go by faster. The only real tool you need to make a mirror is a good steady table of the right height (they used to always use a barrel), and 2 pieces of glass. Well, and several sizes of grit too... But you get the idea... :)