This is a set of six countersinks starting at 6mm (1/4 inch) and increasing. My set is $8, which is really cheap. REALLY, REALLY cheap. A cut costs $8 at most hardware stores I live at. I'm not a metal worker or a machining expert. I mainly work with wood, some aluminum and some softer steels. So I can't confirm how they handle hard materials. They are good for what I do - no problems so far. They have 5 edges - many C countersinks have 6 but I don't mind. They are mounted on a 1/4" hex quick release shank which is handy. Slow down your drilling speed, use some cutting fluid or other metal lube and I think they will serve you well. Thanks for reading.