320, 400, 600, 800, 1000, 1200 EDM Polishing Stone · Aluminum Oxide · Hard Structure with Slow Breakdown AM-8 is an aluminum oxide finishing stone designed for removing EDM (electrical discharge machining) scale on all types of mold and die steel and is the hardest stone that we manufacture. It is particularly great for use on heat treated steels. Because it holds its shape well and breaks down slowly, it is an excellent tool for polishing sharp corners and small details either by hand with one of our Hand Holders or with the use of reciprocating profilers like our U-Lap and Turbo-Lap Air Profilers. AM-8 is the only conventional polishing stone that can be used in an Ultrasonic Polisher such as our NSK Sheenus Neo or Ultrasonic ECO Polisher. Must be used with a lubricant to avoid stone loading.