A couple questions that I have:
How does the design look so far?
For the main steel tubes, will 1/8" wall thickness be sufficient? Will .5" aluminum for the Z be thick enough?
For the gussets and angles, is it overkill? It seems overkill to me.
For the aluminum box that I made of plate around the gantry. I have easy access to some 4"x8"x.25" aluminum box. Would it be better to replace the box made out of plate with the box tube? In order for me to make it work, I would need to reduce the gantry tube to a 3"3" tube and add a couple small shims between the bearing blocks and the inside of the 4"x8" tube.. However, I think the 4"x8" box tube would be much more sturdy than the one that I could make out of plate.
Lastly, I have it set up so that I have 4 feet of travel in the X direction and 3 feet of travel in the Y direction. In your experience, would it be better to switch the two just for being able to place a whole sheet of wood on the machine? Would there be any disadvantages of this? The only real problem that I see with this is being able to get the part of the sheet hanging off to be flat with the table surface.