The three smaller countersinks have a 1/4" shaft, and the two large ones have a 1/2" shaft.  I've used them on soft & hard wood, on brass, and on mild steel, with good results.  My only concern is, the two larger cutters will eventually wear out and Grizzly doesn't sell them individually.  The three smaller bits are sold by Grizzly (or Amazon) as set H3024, for about thirteen dollars.  They are made in India, if that means anything...  The bits appear to be tool-steel, and are excellent quality.