Router build

[JAZZCNC]

i want to cut fast ... i cant sit around all day watching the machine scrape about ....

so what would you recommend to cut 2400x1200 MDF minimum ... my initial design was 76x76 tube which was too small ... so 150x100 is too big ...

Ok well if your wanting to cut fast then your concerns are not just strength.? Your main concern will be spindle power and tooling.
You Can build the machine like a tank but ultimately the spindle power and tooling will dictate the speed you can cut at. Any machine using 100x100 steel section for the gantry will more than match most spindles that would be attached to a DIY machine.

Building a successful machine is about balance. No point wasting money on expensive servo's, ballscrews and massive steel structure etc if your going to attach a 1.5Kw spindle at the sharp end.! . . . . . Equally no point attaching 10Kw ATC spindle if the machine is flexing like a jelly fish. The results while differant in practise will still be the same.? . . . Disappointing.!

Balance is required so you need to define what your going to cut with the machine and be realistic regards the feeds/speeds/DOC. Everything needs to match and suit to get a balanced machine and this is where it gets hard and expensive to achieve with a Jack of all trades machine. Esp if your chasing speed.!!

Edit:

so what would you recommend to cut 2400x1200 MDF minimum ... my initial design was 76x76 tube which was too small ... so 150x100 is too big ...

I never said 76x76 would be too small and 80 x80 would make a decent gantry if design was good so I wouldn't be too concerned using that size section provided it had decent wall size. . . BUT . . Your box on box with no bracing and open ends isn't a good design IMO so I'd be looking to change things in this department.