Abstract: Trueing of commutators and slip rings of large industrial motors, generators, convertors and the like without dragover of copper into insulating spaces between commutator segments or loading of surface of the trueing tool is afforded by employing an ultra-high speed grinding wheel mounted on the shaft of a precision portable electric motor mounted for traversing across the width of the contact surface. A grinding wheel in a grit range of 150 to 180 is operated at a peripheral surface speed at the grinding point of at least 7,000 feet per minute (2100 meters per minute). This speed corresponds to a 4,500 rpm rotational speed for a 6-inch. (0.1524 meter) diameter wheel. The motor and wheel are of a size and weight enabling manual mounting on a traversing compound adjacent the surface to be ground.