Hi Evilness,
I really hope you get a good reply, because I have been trying to get an answer to this for about six months now, without much luck :-(
It apparently all depends on things that's very difficult to define, like cutter shapness, rigidity of machine, cutter material, what kind of alloy you're cutting, what kind of lubrication you use and so on.
After doing some manual milling of aluminum, my advice is to really get the machine as rigid as possible. This is if you want long tool life and a smooth surface. That is, zero backlash components, and make use of U channel and the like, no plate material without reinforcement! (I'm building out of MDF, and making tubular structures for rigidity... not sure if MDF is going to be good enough, but it's what I've got!)
I guessed and set my goal to 1000 N of axial force for each axis, but I'm beginning to think this is much more than needed. Well, I hope someone else knows better!
Perhaps we can ask balsaman or kong, who have both cut aluminum with their machines, to measure their machine's axial thrust for us?
Also, the optimal spindle speed is much lower for aluminum than for wood. But you need about the same spindle power (more torque).
// Arvid