although i like your three stage method (great tutorial)
i'm not so sure that the WOOD burning is such a bad thing tho
if you let off pressure as you go....because that makes the wood harder
at least that was my experience using a ball rasp
because i used to smoke the cup
on purpose is my point...
it didn't seem to be detrimental for the plug .....prolly bad for the ball rasp though
because the metal would eventually loose it's temper.