The equal division 3 x 120° of the flutes offers good results in most materials.
For iron alloys vibrations may happen and generate facets.
These new ANTI VIBRATION countersinks with unequal division of the flutes avoid this phenomenon.
HSS-E Cobalt = hardness 65 HRC, to machine steels up to 1000 N/mm², cast iron up to 180 HB, copper, brass, bronze, aluminium.