I use a round nosed lathe tool in mine. The radius on the tip is about 2mm. I am using a brazed carbide tip tool right now but previously I have used plain HSS. Because of the intermittent cutting and the higher toughness of HSS over carbide I suspect HSS might be the more appropriate tool material, but either works well for me. A small amount of GT85/WD40 on the work piece and a single pass gives a very high finish.