I have the same Rigid trim router and I'm presently building a CNC system to use it. Most speed controls operate by chopping the AC current at some point on the sine wave and I think thats why Rigid says not to work another speed control with it in series. However I thought this did not rule out running on reduced voltage such as when a variact is used. I happpen to have a large 30 amp variact and I just now tried that method. I was very surprised because there was hardly any change until the voltage was cut down to about half. Then some slowing but NO power. The other strange thing was that when going over the normal 120 volts to about 140 which the variact allows, the Rigid's speed actually dropped somewhat. I would guess that the Rigid uses a micro proccessor to control the speed and soft start feature. Thats the only thing I can think of that would provide that kind of
control with reduced voltage. We are both going to have to figure other ways to cut plastics.