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.