ok, first attempt using sketchup 8

[Jonathan]

I also can't fault your logic and argument on the cost side because that is very important for a home build.

Same here. Thought that it was obvious that you just make it as strong as you can afford. I made mine as strong as I could afford at the time, but not as well as I could and I've been waiting for the funds to improve it for some time.

One last thought:



You could weld a steel sheet (red) on to the back of the RSJ (blue). Then you've got the torsional stiffness of box section and the convenience of RSJ. It needn't be very thick, I think 3-4mm sheet would be fine. Adding several of the green triangular pieces might help, but it's a lot of effort to cut them out without a guillotine. Only problem with all this is the welding might distort the top surfaces, so it's best to do it from the start.
Other option is use box section but add plates on the top and bottom so that you can put the linear bearings off center to get the ballscrew in a good position. Not sure which would cost more...

The only thing that worries me slightly is the weight of the gantry, i know weight is a good thing, but would this be to much to get decent speeds. Now i'm not looking to break any speed records, just to get a reasonable feed etc. The motors i have at the moment are nema23-3NM and MSD542 drivers with 50v power supply.

Dean