TWO WORKERS WERE CHANGING CUTTING EDGES ON A CAT 633 SCRAPER. INJURED WAS LAYING ON A CREEPER REMOVING BOLTS FROM THE CENTER EDGE. CO-WORKER WAS MOVING A SIDE EDGE OUT OF THE WAY. WHEN HE HEAR D THE EDGE HIT THE GROUND, HE TURNED AND FOUND INJURED'S RIGHT ARM PINNED UNDER THE CUTTING EDGE. HE REMOVED THE EDGE.