Here's one to get the brain cells working.

If you have an aluminium part, approximately 15mm diameter mounted on a spindle (we'll call it spindle one, or S1), and you want to machine said part with a 6mm cutter mounted on a seperately powered spindle (spindle two /S2), how many Nm will S1 need to ensure that the cutter in S2 won't spin it in ways you don't want it.
And assume that S2 would have enough power to snap the cutter long before it stalled.

Answers on a postcard please.