Please review this router gantry idea

[Jonathan]

My first machine uses a 910mm long piece of 80x80x3mm aluminium box section for the gantry, and it's strong enough for hardwood aluminium (but not especially fast), so I'd advise getting something bigger. Your design lends itself nicely to making the rail spacing the same as the diameter of the tube, so if using profile rails you wouldn't want to go below around 160mm spacing. A round tube that diameter would fit well and be plenty strong enough. Edit: However something a bit smaller would be strong enough, just less convenient.

Also, you don't necessarily need a high wall thickness. Use the formula for the moment of area to compare them:
J=k*(od^4-id^4)
Where od is the outer diameter is the inner diameter, and k is some constant we don't care about as we're just comparing. For example suppose you find a 180mm diameter tube with 3mm wall thickness and a 150mm diameter tube with 5mm wall thickness, J for the large tube is 1.3*10^8 compared to 1.4*10^8 for the smaller one, so a tube that's a little larger can still be stronger even though the wall thickness is small. In this example I'd go for the 180mm tube as it's about 40% lighter than the smaller tube.