Adverse Effects wrote: what are you going to do about the rest of the bed if the sides bow even 1 or 2 mm the center will sag 10's of mm just under its own mass then you will have the trouble of any thing you put on there will push it down as well
the bigger you go on the X and Y axis the harder it will be to level and keep level the bed
honestly i would love a mill 1.5 x 1 Mt but it would be a night mare to level and keep level with out going with a slab of 1 inch Ali and adjustable cross bracing in all directions
plus its better for me to play devils advocate now in the planing stages then you finding something bad out once its all built
You would be surprised how strong the 30 x 150 bed is. Even with me pushing reasonably hard in the middle of a single length of beam supported at both ends there is barely any deflection, add 16mm of solid base (4mm alloy 12mm ply) bolted to that (so it now acts as one structure) and bolt that to a steel tube base and it wont sink.
Remember the only thing pushing down is a couple of mm cutting bit. I am not concerned in that direction I am more worried about diagonal twisting when moving. Hopefully boxing in the ends so it becomes a square box and the top and bottom are supported it might be ok.
As I say my design is so much stronger than the typical flat bed with a separate gantry which doesn't add any additional support.
I am glad you are questioning what I am doing it makes me think twice before doing anything and its better to be safer than sorry.