I have Turned and under cut many a Comm, both Motor and Gen. The best way I found was to remove the Brushes, install the correct size tool to hold the Stones, put a Cadillac Vac near the work, and Turn the Comm. Then under cut with the correct size rotary cutting wheel, then champher the under cut. Finish up with an eraser stone. Use De-natured alcohol to put a glaze back on the Comm. All the Stones, Jigs, tools etc; can be found at Martindale in Cleveland. They have a great catalog. I am assuming they are still in business.