Im having the same problem only Im cutting 25mm mdf. 110mm alphabet A to Z. Im using a 6mm spiral Leitz tool. Spindle speed set at 5000rpm and feed at 7000 although I have this slowed down to 40% as strain on tooling . Im cutting this in 2 steps but its taking over 2 hrs to cut 144 letters..boss thinks Im swinging the lead. If I speed the rpm and feed up it breaks the tool. Are my speeds correct?