Hmm - my two table builds are not exactly great for instruction but you may glean something (the links are below). It’s a bit of a reminder - I’ll try to do better next time!
Don’t set up on the floor!
Get two trestles or a table or anything that you can put two “beams” exactly level. By level, I mean sight along the tops and make sure they are exactly the same - you can do this better by eye than by measuring. If you don’t understand the above let me know and I’ll draw a picture.
Now secure the beams using clamps or better still screws if you have a sacrificial surface.
Place the torsion frame spanning the beams - check for square and use chocks on the beams or even a bit of string if have to on the diagonal to pull it square. If the warp is still apparent, use clamps to hold it to the surface of the beams - it should then be level, square and flat.
You might have to use some cleats on the frame to hold the clamps so you can skin the top.
Now fit the top skin. (or the bottom one if you love working upside down)
When the glue has dried you should have a “less warped” top - flip it over fit the remaining skin and clamp it into position.
It’s not as hard as it sounds, and you should end up with a really flat surface unless you have an alignment problem with your joints - best to check them before you start.
Not sure if the links below are helpful, or even entertaining to anyone but me.
Also, you are going to plane the spoilboard anyway so it’s only really important that it does not move later on. If you glue both the top and bottom on, it should be a lot more straight (straight enough).
Gonna try to find some perfectly straight beams to use, and as far as cleats, I will just flip the thing over and skin the bottom first because of the 90° corners I glued in each bay for the threaded inserts & will let those serve as cleats, if I can get it flat upside down, then it should be flat right side up or as near as it’s gonna be to take the ripple out
Again, thanks for the encouragement I will pursue this and rather than abandoning this grid
Perfection is a tough thing to chase. This would work just fine on your lumpy floor, so it will work just fine on a lumpy table. The difference is you can surface your lumpy table to make it true to the rails and the machine.
There will be no noticeable issues unless your build is super crazy out of wack. All flat materials will conform to the warp or bends of your table, all small materials will not notice it because locally that one spot is probably pretty dang perfect.
I can not stress enough how much PERFECT is a dirty word sometimes.
Well, when I can see the 3/4"x 5.5" MDF Y rail platform more than 1/4" sag in the middle when sighting down the long edge, thats when I noticed the sag, then I checked my table and there it was, sag city. So getting it to lay flat then gluing on top and bottom skin should make it flatter… the concrete floor will probably suffice when gluing and weighting the skins on. but using the beams and clamps to get it just a bit closer when I have the ability and items necessary, I’m game for that. however, if I get lazy this afternoon, concrete floor, glue and weights will be the call (depends on how many Woodfords I have between now and then)
I think I read somewhere here, someone suggested an idea of — before adding your skins — shimming the underside of the torsion box, and using the Lowrider to resurface the top of the torsion box for flatness, in other words, shaving a bit off the ribs and spars themselves, then perhaps flip and resurface the bottom side. it gets to a pretty good thing to use as your basis to add the skins to.
You can but surfacing after is just as good and easier. The issue is it can not help the rails, which are the reference surface. Like I said before though twist warp and bend are not as not a deal as you would think.
If super crappy tables caused major issues all of our builds would be in trouble. I made it 8+ years with a 1x1 table frame skinned on one side with 1/2" MDF, sitting on top of another table that frequently got moved.
This is a great idea but it’s still only going to correct within the current constraints and errors in measuring used. It’s going to be subject to a number of additional variables, really too many, the biggest being the measuring device used to remove the sag (“laser” level/sag cards), then the clamping can’t move while the glue sets. Then there are always the fun variables such as how humidity affects wood and then the ever so fun with thermal expansion/contraction… The re-surfacing of the bed might be an easier way to remove most of these but still subject to the flatness of the rails on the X and Y which will still change with seasons from the factors above. I’ve struggled with this as I plan my table but I think I just need to accept that perfect likely can never be achieved.
That’s what so many of us have a problem accepting!
I do think this thread has given me an idea for future tables, in that I’d like to try installing a “shim layer” of 6-12 mm mdf which could be flattened with the LR. Then a more durable 1/4" ply or something laminated on that top.
But it won’t help the rails. That variation in the rails is one of the real risks of trying a torsion box design.
Many of the torsion designs have “independent” riding boxes / rail boxes that are attached like pontoons, which works well. Focus is making sure they’re parallel to avoid twist.
This also makes me think of something similar: one should probably seek avoid putting the wood of the ribs and spars under tension by bending them against the normal resting shape, because then you have that pressure fighting against the straightness you glued in place.
Let’s be clear - I’ve built at least DOZENS of torsion structures, tables and doors (in a hobbyist kind of way). If you want a large warp-free door or a strong lightweight table - it’s super simple to get something that’s “good enough”, but it takes skill and practice to get something close to “perfect”.
“Good enough” means it isn’t bowed or warped by eye! You should be able to achieve that easily, without stress - but by the nature of the construction, you can’t straighten it once the skins are on.
I would encourage you to use use beams every time because it’s a hassle clamp or screw to the floor. I would certainly think about gluing the beams to the floor with construction adhesive though! It’s easy enough to clean up afterwards with a chisel. The floor is a bad idea because you can’t easily sight along it, and for me it just hurts getting up and down!
KEEP THIS IN THE BACK OF YOUR MIND IF YOU BECOME DISCOURAGED -
IT’s just a matter of what is the easiest way of getting a reasonable job with the least stress.
I would not recommend facing the frame with your machine (for starters you need a bigger machine than the table to do that! - that’s not for taking warps out, just minor discrepancies in the height of the laps, but even then a quick rub with a sander or at the worst case a plane will do the trick, as long as you are not digging ditches while you go! Even then, for our purposes a spoil board will take out most of the minor dips if the skins don’t span them.
It’s not relevant here because you’ve used ply, however for reference, the "timber wants to warp" can be overcome almost entirely (assuming you are using rubbish) by keeping the grain vertical as much as possible, and by alternating the direction of the growth rings for each piece. “Perfection” would mean buying quarter sawn timber and you really don’t want to pay for that!
Ply and MDF won’t warp at all and it’s cheap as chips if you want to practice!
