The chart I have from Jancy is based on the 1" diameter cutter. For 1" its 382 rpm, according to the chart for mild steel, you divide the 382 rpm, by the cutter size , so 1/2" would be 764 rpm. I use the cutters all the time, I have 3 mag drills, also an arbor with a MT#3 shank and one with an R8 shank