I spent the entire morning doing some tests. Just my table at the sides with a 48" span will flex 0.008". So, I put mid-span supports across the outer edge. Now it moves about 0.003" Then put the forward support rail right over the top of the outer edge and got 0.020" when pressing straight down with my hands at mid span. I put shims under this rail at two points and now have that down to about 0.005" ish.
And still the middle moves about 0.025" or more. So I pulled off the T-track and wasteboard, removed half of the main mdf board. I put about 15 sets of shims under the formed edges of the supports…staggered between all three. I can still get 0.025" flex when pushing straight down with my hands.
So in my frustrated mode, I left the dial indicator in the middle. I pushed down on the Right Y axis rail and got about 0.0015" flex. Same story on the Left Y axis rail. Then I pushed down on the X axis rail just to the left of center, and got 0.010" flex. Then I pulled forward on the Z Axis carriage and with minimal effort, got almost 0.030"
So I think I am chasing minutiae in worrying about the bed flex. There are other issues to solve. I am going to shim the heck out of the rest of the bed as best I can, add some serious supports in the CENTER of my table. Then I am going to cut some “stuff.”
As I think of torsion boxes, I do not think this is going to help. You will still have to shim as the unit is not dead flat anyway. We are supporting in points. The only way to fix this is grout!!! j/k but not really. There are lots of tolerances built up in the frame when made of bent sheet as someone (Richard?) has pointed out already.
Now the forces at the cutting bit are what I need to focus on. Left to right and front to back, the XXL seems stable. The real worry is the flex/torsion about the X axis and we can control that with liberal roughing tolerances and making final passes very clean and light. At least that is how I am thinking at the moment.
Should get everything back together tonight and supported. Then I will surface the wasteboard and get my tramming set.