Have you tried working out what sort of cutting forces you are expecting? I found some good formulas on the web, and worked out that cutting with a 6mm endmill, 2.5mm deep in Aluminium at 600mm/min, I would have a force of about 200N on the cutter. I then just sized everything to take way more than twice this force
On a 10mm pitch screw, you have a mechanical advantage of about 600:1, so your 470ozin / 3.3Nm motors will be able to resist a backdriving force of about 1900N - equivalent to lifting 190kg/400lb with 1:1 gearing.