Finally cleared enough space on the patio to fit the table. Still needs leg braces (extremely wobbly atm).
Now it is officially outdoors.
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My mental simulation evidently had an error and this is NOT stiff in torsion like I thought it would be.
It still follows the contour of the floor with minimal resistance.
When I lift it up and twist it, the feet have a very noticeable racking in the X-Y plane, so I think if I add diagonal cross braces connecting the feet it should make the entire frame including the top surface stiff in torsion.
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I think the only way not to fight non even floors is float the table top on a leg base?
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Do you have the diagonals pinned to each other where they cross? That might tighten things up.
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Challenge accepted!
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Eliminate the uneven floor and hang it from the ceiling? 
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It’s going to need leveling feet. Even the big machines have to have leveling feet to stay flat and not warp.
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We poured self levelling concrete this weekend, forthebathroomfloor. Fiber-reinforced. I was surprised with how easy it was to work with. Made me wonder it could be used to make a leveled and reasonably flat bottom layer for a cnc.
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Shoot ween need a self leveling material that can be used as the top and not worry about the bottom at all. When your spoil board gets beat up just add another layer! Thin epoxy with a ton of foam filler (surf repair we used Qcell) to make it less dense…Dang that could be amazing. Pour the whole tabletop.
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That might really be a good idea. you would end up with a super rigid table as well. You could embed regular cheap nuts in it as well if you made it thick enough
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Many possibilities! Just having a reasonably flat surface is a great in it self. Embedding nuts for the corner feet, spoil board etc is also very neat. Or aluminum t slot rails? The options are endless! This product was dust-reduced as well, not very messy to work with, and the finished surface wasn’t dusty.
I don’t think that will work well. Because level to the earth doesn’t matter, trammed to the gantry does.
My favorite idea (which I still haven’t tried) is surfacing some supports to make the supports for the spoil board trammed, and then just install a constant thickness spoil board (do those exist?).
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If you poured the whole surface, it would be level and flat, and then you put the machine on top of the perfect surface. All in one not just the work surface.
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But if there is any sag in the gantry in the middle, you could compensate for that if you trammed the surface, but not if it was just flat and level.
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I see what your saying. Even if you poured a perfect surface you still need to machine it a touch to compensate for any sag.
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A torsion box with four feet should have leveling legs because even a torsion box is not infinitely stiff.
A torsion box with three feet doesn’t need leveling legs.
I have added stainless lag screws to all four corners, but the purpose is to prevent water wicking up the wooden legs if the floor gets wet, not for leveling.
What I’m aiming for is a torsion frame that’s stiff in torsion while still being open in the middle for large or vertical objects, which a normal torsion box can’t accommodate. Then prop it up on three legs by supporting in the middle of the back.
I’m making a device to measure the twist between the two rails (basically a couple of sticks). If I can measure the twist with some precision, then I’ll use the feet-screws or shims to adjust it flat one time only, and then add the cross braces to hold it in that position. Then I can prop it up on two opposite corners so it’s rocking, and see how much the top surface twists, to see how well I did.
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You could also compensate the sag in software, using a straight reference to measure against. Could be a poured table top or a straight edge. Then if you have a workpiece that doesn’t conform to the table, you can still cut it flat.
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Epoxy granite is what is used in a lot of machine tools.
It requires a lot of prep work to make sure you’re starting square/level/planar…
I had started building a Gingery Lathe some time back and I was thinking about changing the bed from an aluminum casting to epoxy granite but I had to put that project on hold. This reminds me that I need to add it back to the list now that I have a functional CNC.
I believe it could be a good solution for a really flat and stable bed but may become unmanageable in a hobby/home environment at a LR size/scale unless you had something like an engine hoist or overhead crane to move it around.
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I was more thinking of a skim coat that you could cut. That Granite would not be tool friendly.
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