It just dawned on me this morning While I was looking at a solid core door mobile table top bench I built, that would be a perfect surface I will be able to use to lay some beams on and once they are relatively flat, then I will set the grid upside down and then clamp it using those threaded insert corners and then look at How flat it is and if it’s “good enough“ I will glue the bottom skin on.
Pretty sure that would be more than good enough if it’s eyeball flat. Yeah?
I’m willing to bet what you have is good enough.
Trying to change the shape of a torsion box goes against the reason for using a torsion box in the first place, it should not move at all.
It’s not a box until the skins are on.
Once the skins are on, the shape is for life.
Currently it’s a ladder frame, and technically the framing members don’t even need to be glued together - they are just there to keep the skins apart, so it should be no drama to level it.
If I remember right, this one is already glued, it is not going to move in the planer direction much. The skins really lock it in but I am pretty sure a door is not going to pull it flat.
I would love to be wrong here though.
Well, I cleared off my slab workbench and rolled it to the middle of my shop and laid the grid down and clamped it flat. And then I laid the three-quarter inch MDF Y axis strips, and eyeballed it for flatness.
Huge difference just by doing this
Then I turned that MDF one by six on edge to see how much gap if either there was, there was a little in the middle. So then I grabbed the conduit and checked that against the MDF - nine sheets of paper at the largest gap I’m pretty confident that’s gonna be Accurate enough. Now, if I can just glue the skin on and actually have it hold the grid flat in place then we have a winner
I’m thinking my use of the 1/8 hardboard or 3/16 or whatever it ended up being isn’t going to be rigid enough, I may switch over to half-inch ply.
It’s raining now so I’m not gonna do much don’t wanna get the plywood wet
“Nine sheets of paper” has to be the new standard! 
I really wish I had thought of this earlier, and I’m almost tempted just to start from scratch making the exact same half inch ribs.
If I only laid it out like this when I glued it up and clamped it down to the flat slab rather than my makeshift cabinet assembly table, it would’ve come out nearly eyeball perfect That’s my theory anyway
You really don’t need those corner gussets for the next time, they will make the frame harder to square - loose and sloppy is your friend. If you add the perimeter trim it’s easy to just square the overall structure, and that’s all you need to worry about. It really is a simple process!
I like to line a square edge of the skin against two sides, tack those in place (after gluing all the faces you need) then you can go to the other side and tap the whole thing into square.
If you want to make it fancy later you can add a second trim around the perimeter once the skins are on.
Yeah. That squareness comes from the top/bottom sheets of the torsion box. It is amazing how strong even a thin sheet can be.
So I added those after gluing up the ribs (days later) and their sole purpose is to hold the treaded inserts for the M8 nylon bolts that will hold down the 3/4” MDF spoil board. 
I think next time the ribs don’t need to be glued until like you said, one side then the other, then the skins being glued would have been great process. Next time which may be next week. LOL
I think you mean coplanar instead of parallel. This is one of the risks of some torsion box designs, if the rails (whether they’re connected or there’s a gap) are significantly out of plane with each other (one runs high to low, the other low to high) it could lead to some interesting results.
But for the record and any future readers, the LR has been fairly forgiving, and its possible to design the table so the rails can be shimmed if needed.
Once again - as Ryan says, this increased complexity doesn’t really yeild any practical gain. Its all to get the similar performance of a solid sheet.
My goal was to make a lightweight full size tabletop one-man maneuverable that I can store on long edge when not in use… and reasonably flat as sighted by a human eyeball. My issue began when I saw what looked like a 1/4” sag when I sighted the long edge… then I looked at that table I assembled it on… and yup, 1/4” sag on the table. So just trying to work that out before skinning. Reasonable flat is all I’m after but more flat is better and less work to surface…etc etc. 
You are surfacing your spoilboard once anyway, so there’s no change on the amount of work…
I wouldn’t trust th concrete floor, never seen a level floor ever. in nz the standards allow for a 4mm variance over 1m you could run a straight edge or string line over each piece
mark the variances , cut slots top and bottom along , glue and insert wedges clamp to straight edge to straighten it out . do it all the time in old houses to straighten old studs
Something tells me that it’s easier to surface when the surface is only a 16th off of flatness in spots vs being off a 1/4” dip and having to dig into 3/4” MDF spoil board everywhere else to surface. 
Lol, an old framer’s trick for sure, after inspection, of course🤣. Couldn’t tell you how many walls I had to do that to to get them straight.
Got it all clamped down to the solid-core door workbench so it’s as flat as it’s going to ever be, And I checked it for square, surprisingly 113 3/4 on both diagonals so at least it’s square. LOL
Now we’re gonna glue down quarter inch plywood on both sides for skins starting with the top.
How ever this turns out, it should be light enough to be maneuverable and be flat enough to serve as a table top for this LR4 once the three-quarter inch Y rail and removable spoil board are attached.
Is that actual concrete?
It’s a shame you couldn’t have found something heavier! 
