I am very interested in this thread. I too broke 2 x 3mm cutters at only 1mm DOC - More than likley my feed & speed was wrong as the cutters was BRAND NEW.

My Speed and feed(off the top of my head) was approximately 640 and 800mm/m and speed was around 200hz and also tried 250-280hz on 1.5kw spindle(not sure what that relates to in RPM but i guess about 12k - 15k)

Craig