I didn't know they had single-phase variable frequency drives (VFD) for induction motors on the market until just now. That might work great on a drill press, buffer, etc. There are endless programming options, if you can figure it all out. I am going to start thinking of one to make better use of my drill press. Changing belt/pulley positions all the time on my 15 speed Jet drill press is a PIA. That could be very handy just changing speeds with a dial.
https://www.ato.com/single-phase-vfd