Well here goes... Aluminium frame router design

[CharlieRam]

Well here it is, Idea 9 , mainly for woods/plastics but also hoping to do a bit of aluminium or cast iron work when I modify my mill (which I havn't actually got yet! )
All bits in blue are steel, the gantry is built using 80x40 4mm box section aluminium with a 20mm plate across the front, the X axis rails will be 80 x 40 steel with 5mm of resin for leveling but I am not sure whether to go 3mm or 4mm steel thickness (is 3mm enough, I will go 4mm if its weldable??) the frame is 50x50 3mm steel and I plan on putting 4"x1" aluminium box section running the length of the bed with a 10mm alu plate on top of that drilled and tapped.
To manufacture the gantry I plan to machine the front plate, leaving a little for finishing. The aluminium box section will then be bolted together, solid aluminium blocks will be machined to fit in the ends. I am then hoping to lightly skim the X rail seating faces square to the front face and also lightly skim the front face before attaching the 20mm plate. I will then skim the profile rail support faces on the alu plate. What do you reckon?
The frame and X rails are all one component and even though I haven't welded before I am hoping to get the top face for the rails within 5mm so that the epoxy leveling will do its job! I realize that I am going to have to shim the ballscrew mounts so that they run parallel with the rails but is there anything else I should watch out for?
I have included the sketchup file for anyone wanting to tear it apart for scrutiny and as always appreciate any feedback as to why it will/won't work.
Cheers, Charlie