John, thanks for the reply. For clarity, the wow/flutter effect occurs at all speeds less than full - last night I tested the mill by facing a 50x50x80 length of ali at 10,000 rpm, and the sound was very evident - more so when the cutter touched the metal.

Doesn't occur at 24krpm, as an experiment I've ordered some USB/485 converters, will try driving the demanded spindle speed over digital link.