Frame materials

As far as I can tell from what I've been reading, the key part to an accurate high resolution cut is a rigid frame. if the frame doesn't move (apart from when it's supposed to, IE the gantry) then there is a fair chance the cutting tool will be able to perform at it's best.

Lets have a look at the options...

Wood

Cheap, plentiful, easy to work with... but the idea of having a frame material that is 'softer' than the material being milled seems like a bad idea. So I'm dismissing this one straightaway.... well, apart from using a sheet for a sacrificial bed, that seems like a good idea.

Aluminium extrusion

More expensive, still plentiful, almost like it was designed for the job. But is this strong enough? And is it cheap enough? Chinese suppliers are around £5 per foot for 20mm, £9 per foot for 30mm. 

Aluminium bar

And the price goes up... aluminium bar at 40mm x 10mm is £16 per foot, and based on my limited knowledge of material strength may not be any stronger than the extrusion. So another no here.

Steel tube

Now this is an interesting one. Price? Almost zero, I have a fair bit of leftover lengths from the car build. Even if I have to buy more, a 19mm square section was £20 for 4 metres. Strength is more than aluminium, and from the car build I am very familiar with triangulation. Construction would be easy as well, the parts could be welded together instead of bolted (although I'd have to be careful about warping). The extra parts would just require drilled and tapped holes, even the adjustable parts could be done with slots in the steel. It would also be a fair bit heavier which I am under the impression is a good thing. The only question mark here would be the gantry, a steel gantry would need a decent amount of torque in the motors. And support in the support bars. Although with only a 400mm span I would expect the rigidity to be fine. 

If I go down the steel tube route, I might also pick up on a tip from This Old Tony. His gantry CNC is filled with sand to give it some extra 'stuff'. Obviously this would only apply to the gantry and base, the moving parts will be left as is.