it is hard to machine i agree, ive done trials on castors which went quite well, but the compound was harder, im gona be speaking to our tool rep next week, hopefully he will help out.
A simular machine in america uses a rotating carbide blade angled about 45 deg to the wheel, this just peels it of with ease. I intended on something simular in my old prototype, but im hoping to simplify it and remove the motor and cost with a single point tool, something very sharp, lots of rake etc and apparently fairy liquid and water is a good cutting fluid ( aside from freezing it)