The RotoZip bit was insufficient: too slow and it burns the wood, too fast and it breaks.  I’ve ordered some endmills from Enco, which I’m much more optimisitc about.

The gear below was cut out of 1/4″ plywood, with and endmill too shallow to do the MDF.  The partial gear in the background is made of 3/4″ MDF; the RotoZip broke just before completion, despite taking passes only 1 mm deep.