Thanks for the feedback, much appreciated!

I'd like to be able to mill aluminum, otherwise I'd feel like I failed. The building materials I'm using can handle alu at the precision I'm happy with, as Washout clearly shows in his build videos. It's up to me to make sure it does.

As for the FEA, I'm not really looking at the numbers at this point. I know I've made too many simplifications and false assumtions for that. It was mostly there to get a visual feel of where the weaknesses were.

I've redone large parts of the construction, trying to listen to the advice routercnc gave me. I've also taken some inspiration from Washouts machine.
What kind of precision are you getting with yours when milling alu?
Also, what kind of problems are the round rails giving you? Chatter, instability, binding?

It's out of my league working with steel I think, even alu is stretching it.

I've uploaded another album with some pics of my revised build.
http://imgur.com/a/pM3GU

It was done in a bit of a hurry, I have to admit.
What do you think? Did I improve it? Have I missed anything, apart from the obvious mounting of the ballscrew supports on the y-axis?