All that's needed to control the motor is a single potentiometer. But I wanted to be able to set limits on both the highest and lowest speeds: I didn't want it possible for the saw to appear to be "off" with the speed control alone, nor did I want it to be possible to exceed the 1000 RPM limit cautioned by Shopsmith.

I added an extra "trim pot" to each end of my potentiometer. Think of it as simply a higher valued potentiometer that can't be turned all the way up or all the way down. It takes a little fiddling to get the limits set correctly, since changing one limit affects the other. Using this I was able to restrict the low end to about 180 RPM, and the high end to about 3000.

I'll use bad ASCII art for a schematic because it amuses me to do so on an otherwise perfectly nice web page:

                   to MC60 "high" -------o
                                         |
                                         |
                                         Z
                                    10K  Z<-----------o
                                trimpot  Z            |
                                         Z            |
                                                      |
                                                      Z
                   to MC60 "W" ---------------------->Z  10K
                                                      Z  speed control
                                                      Z
                                                      |
                                         Z            |
                                         Z            |
                                    10k  Z<-----------o
                                trimpot  Z
                                         |
                                         |
                   to MC60 "low" --------o