I had to go a round about way. ROTOZIP has a 1/4 to 1/8 step down that's pretty accurate. And since I didn't have any 1/4 callets for my large CNC I made a 3/8 to 1/4 spacer from a bronze oil-lite bushing by slitting it down the edge. All available at Lowes under 10 bucks. Runout isn't something to brag about but it does a superb job engraving with cheaper, and more importantly available, Dremmel bits.