mortise bits with 3 cutting edges,head and 10 mm. side grinded. Particularly recommended for medium depth buttonhole executions up to the total cutting lenght dimension with high feed-speed.The countersink allows the cleaning of the entrance of the buttonhole from possible chips of wood, creates a bevel that makes easier the assembling and also provides a space for the exceeding glue.HS(HSS) for non glued soft and hard wood.