To avoid leaving abrasive particles in the motor, filing is preferable to sanding. If you must sand, use old style, "flint" sandpaper; the? abrasive is not electrically conductive. The mica (insulation) between commutator segments should be lower than the copper segments. On larger? machines, a hacksaw blade is the appropriate tool; on small motors, an X-acto razor saw blade (without the handle) is the right size. Copious amounts of cheapest brake parts cleaner from an auto supply store help to get rid of debris.
Roy
