3M unitized wheels are long-wearing with a unique spring-like action. The open-web abrasive surface resists loading, delivers consistent finishing results and creates an air cushion that minimizes undercutting and gouging. The 3M codes below indicate wheel hardness and abrasive type and cut. The abrasive grade notations describe the finish on precious and softer non-precious alloys. This unitized wheel requires a 1/8" threaded mandrel.