Brad, When we undercut a com we cut the insulation from between the bars. This is most of the time a earth element called Mica. But on this com it was a molded very hard epoxy type material. After we turn the commutator we have to lower the mica between the bars so the brushes do not bounce when the motor is running. Brush bounce is very bad with a motor running at speed and under load. I built our undercutting machine many years ago. It consists of like balance waves with two ball slides controlled with oil metered air cylinders, or what we call air over oil control. One slide has a spindle with a motor driving it that we use very small saws. These saws are about 1/2" to 1" diameter with 0.010" to 0.050" wide. The bars slots are different on different motors brands When I have the slot lined up I will press a button and the slide that carries the spindle drive moves down. The other slide moves out. We adjust so the saw drop down into the slot and then the other slide cuts out the slot when it moves out. Release the button and everything goes home. We then turn to the next slot. Cycle time on this com was about 25 seconds per slot. And I had to go around it twice because of the material between the bars.
