One ring to rule them all?

[Jonathan]

Using HIWIN HG25 linear rails http://cnc4you.co.uk/resources/Hiwin%20General.pdf
basic dynamic load rating (lowest rating)
26kn / 2600kg. At a separation distance of 300mm and 1m Z that would be 780kg or 7800nm (but there is 2 of them so this times two really). Clearly the linear guides are adequate.

Adequate in terms of load rating, yes, but adequate in terms of stiffness ... you need to work that one out. There is stiffness data available from THK if you dig around.

[JAZZCNC]

Completely do-able but just beware if you dream Big then be prepared to Fail Big.!! . . . . The Chances of failure are very high.

I also Agree plane vanilla 5mm box section without any special treatment will be Big fat Fail.!! . . . . It's not just deflection you need to think about. Think about resonance and quality of cut etc. Box section without being filled will ring and large section box with only 5mm walls at over 2mtr will chime like Holy Trinity bells.
I've got a large router with gantry 2mtr wide with 10mm thick walls and that rings like a bell, doesn't deflect much and for router the resonance from it is more than acceptable but for milling Steels, even mild steels then forget it. It would sing like an. .

The point being you'll seriously need to Re-think your material choice and look hard at design methods if this is to come close to working at an accepatable level. Milling steels takes you another level of building compared to routers and cutting exotic steels takes you several levels higher.!!!

But like I say still do-able but with massive chance of failure.!! . . . . Good luck thou.

Edit: I'll revise that to Massive chance of dissapointment.!! . . . . Not to try would be a massive failure...