[LongRat]

Personally unless the servo is brushless, which I doubt for that price, I'd stick with the 3 phase.
I have to say that I never cut aluminium with 4-flute tools. Not only are they terrible at plunging (many are not designed to) but the lack of flute clearance causes chip welding all the time for me. You could try ramping or helical plunging, but honestly I would stick with 2 flute tools regardless for aluminium. It's just safer even if theoretically you can't feed quite as fast. Especially because I don't want to run liquid coolant, I would always lean towards the more reliable but slower method and give up a little on the material removal rate. At the 500mm/min, 5DOC, 5000RPM that I ran on a test there's no way I would leave it unattended. If it jams the tool, a broken tool is the best I can expect. I leave my machine running for several hours at a time and I would go for peace of mind every time!

[digits]

I was looking at an AC servo for £260 odd + driver on Zapp Automation.

I hear what you're saying Re the 4-flutes. I bought a 3-flute long rougher too which is supposed to be good for ally but I haven't used it in anger yet. I might order some long 2-flutes as well - but TBH I don't have any galling or flute jamming issues with my roughers - I suspect the lower stock rpm also helps there.

I have a 3" deep pocket to cut, and on this mill, I don't really think I want to use anything fatter than 12-16mm, so I'm suffering from tool deflection issues.

[LongRat]

On a pocket that deep I would probably step it out with a series of tools. Which could just be the same tool running incrementally greater projection for each operation. Backing off the depth of cut increment with each op to account for the increasing projection. That way you minimise the machining time by not taking it overly easy on the shallower part of the pocket.
The other option is to pre-drill it and then spiral it out at full depth with a very small step over. Avoiding corners of radius less than or equal to the cutter radius.

[LongRat]

LONG TERM UPDATE!
I have been running this mill at the 4000 RPM ceiling for 7 years, it seems.
It still has the same 3 phase motor and VFD and has had next to no maintenance really. Survived a house move which included the mill being stored in a warehouse for a year in dubious conditions. In the last week the spindle started rattling and cuts were terrible. Preload had slackened. I opened up the spindle to find bearings all in good condition but the preloading ring backed off. Grease looked dirty. Cleaned all that out, re-greased and reset the preload. Once running the head gets warm like it used to, I think the preload must have slackened off gradually over time but only recently had it become bad enough for effects to be seen. Back in the game now though, not bad going. This thing has done some serious work and continues on!

[arizonavideo]

Nice to hear, what you making?

[LongRat]

Mostly RC car parts. I build custom cars and make upgrades to kit cars.
Couple of examples below.

Alloy wheel for Kyosho Scorpion:


Heavy duty transmission for Tamiya Monster Beetle:

[dazp1976]

Mostly RC car parts. I build custom cars and make upgrades to kit cars.
Couple of examples below.

Alloy wheel for Kyosho Scorpion:

Heavy duty transmission for Tamiya Monster Beetle:

Nice jobs.
How much you charging for a set of those wheels?
Machine must look pretty smart when it's going. Have you got 4/5 axis capability?

[LongRat]

Cheers. I haven't sold any of these wheels, just made for my own cars. Probably go for around £100/pair if anyone wanted a set.
I have a 4th axis setup, it's a converted Vertex 4" table running a NEMA 23 stepper. I don't use it too often, last job I did was a scaled down dog skull. Multiple 3 axis setups using the 4th to index between locations.