Ross, that's an interesting concept using off-the shelf alu sections. What about using one of the slotted alu sections like kev is doing at the moment (ITEM, or similar).

I was buying some router bits the other day from trend, and saw this video. If in doubt, copy the professionals! :) (have you seen the price of these machines from trend!!)

One interesting part of this machine (well, to me anyway!) is the relatively small Z travel, allowing a simple frame.

The vid may also contribute to the bearing discussion. I can't make out the bearing system on this machine (about 1min in is where the machining starts). Are they v bearings?

[ame="http://www.youtube.com/watch?v=Ue5PcqTwihM&feature=player_embedded"]YouTube- Trend CNC Machining Centres[/ame]