I am running my step-pulley J-Head Bridgeport mill using variable frequency drive (VFD). Basically, VFD converts one-phase power into three-phase required for Bridgeport mill. It connects directly to the Bridgeport motor bypassing the Bridgeport reversing switch. I went with WEG CFW-10 VFD model that has potentiometer allowing me to adjust the speed of the motor very quickly. So far I’m happy with my choice, however, I haven’t used it long enough to recommend this particuar brand. The set up of the drive is pretty straight forward. The link above provides good information on how to choose the drive as well as has links to you-tube videos, which explain the drive set up, programing and speed control. Good luck with VFD selection!