I recently purchased an old wood lathe. It has a 2,3,4" step pulley. I plan to use a 1/2 hp motor the runs a 1725 rpm's. What is the best way to get the lathe to run slowly at around 80 - 100 rpm's?

Is the motor single phase or 3 phase? If you are trying to do it with pulleys, divide the motor rpm by the desired rpm and that will give you a ratio of about 17.25 to 1(using 100 as desired speed) You should be able to round this off to 17:1 to make it easier. (85 rpm would give you about 20:1 ration) Are there gears in your lathe or do you have to do it all with pulleys? IF you are doing it with pulleys only, then you need to develop this relationship between the motor and the lathe. In other words 17(17-20) revolutions of the motor for one revolution of the lathe.