90 degree Rose pattern countersink produced in HSS & should not be confused with carbon steel types. They are designed for countersinking mild steel iron & plastics. Ref 49/50 is made from special grade HSS & can be used at high speeds (up to 10000 rpm). It is popular for countersinking aluminium alloy. HSS. 1/4 in. Shank. 12.7 mm.