I just unpacked better than a hundred pounds of random surplus bits from 80/20 garage sale. They never cease to amaze me.
They actually forgot four small brackets in the shipment, which they Express shipped out with an apology note, so I recieved them before the main package! These guys seriously rock.

I am using these to assemble sections of some small but pretty serious metal cutting vertical mills, and I wanted to throw in my 2 cents on using 80/20 for mills and metal cutting in general.

I have seen comments about this stuff not being rigid enough to make metal cutting machinery from, and I would agree strongly that it is not anywhere near rigid enough to build a machine from entirely. Some very key parts can't be made from this stuff at all for a number of reasons.
This certainly doesn't mean some kick ass equipment can't be made primarily from it, however - so don't write it off as a very important player in your designs just because you can't use it for everything!!!!

I am using separate, unitary, continously-supported slides of differing lengths as the three main axes on each machine. They are rigid enough for machining even as standalone units - the ballscrews, motor mounts, bearing blocks, and dual 1" linear rails are all aligned relative to one another and rigid enough to park a bus on. Then they are bolted directly to each other. Still no problem there.
A structure holding any of the bearing, support rail, and screw specific parts I of course wouldn't trust any aluminum extrusion to hold true on, but the unitary slides take that issue nicely out of the way.

After that, 80/20 makes for a very flexible, fast, and relatively inexpensive material for most of the rest of the mills far less structurally demanding needs.

I'd be interested in seeing some pics of the slides etc you're using.

I am getting close to assembling my (mostly) aluminium machine - it seems pretty damn solid so far - what rigidity problems have you seen?

Cheers.

I haven't looked into posting pics on here yet, but I can put some up of the whole build perhaps. Its pretty straightforward.
I am using short (9-15" travel, for a 9" x 9" x 15" mill working envelope) Thomson slides as I scored a couple dozen identical ones cheap, but there are a whole lot of different surplus slides that will work to save the hassle of messing with the bearings and getting the rails perfectly parallel.

Primarily my rigidity issue didn't come from obvious deflection or weakness in the joints, it came from vibration and harmonics from microflexure in the beams. On unsupported free spans, you get small oscillations going where it moves immeasurably small amounts under metal-cutting loads, causing the bit to unload, snap back, and do the same thing again - at thousands of Hertz. Sounds like horrible tool chatter under load.
I suppose someone with a lot more time and skill in the black arts of vibrational dynamics could probably engineer a solution using the aluminum extrusions, but why bother? Using it for everything but the slides is just too easy, whether you home-build the slides from solid billets and steel rail or just buy slides pre-made.

As one of the previous posters said, this stuff has great strength and little rigidity. Use it for its strength then, leave the rigidity to steel blocks and 1" rails.