The purpose of cutting the comm is to get it round again. Take the motor out and run it on a four cell pack. Put a jewels screwdriver on one the back of the brush where the shunt comes out. If you can see the end of the handle jumoing and vibrating badly, it is time to cut. You dont need new brushes every time you cut the comm if they are not discolored. Breaking in new brushes is very hard on the comm. I recut my comm after breaking in brand new brushes. For me running a TC3, I cut my comms about every 5 runs or so and replace brushes about every 15 to 20.