Slight bow to torsion box? Is this normal?

I assembled my torsion box trying to be careful my measurements were right. I probably went with too dense a grid. I was using rather thin plywood in an attempt to keep the table lighter for a wall mount. Anyway, more measurements induce possible issues and I think I might have one. My torsion box has built in torsion :confused: It is important to mention I have not affixed the table top portion, which will give it the majority of it’s rigidty, so only the bottom and grid has been installed. Currently there is a slight bow to the box when laying flat, similar to how a license plate flexes at opposite corners. If I stand on on corner, the perpendicular sides will raise slightly.

My plan is to have a few friends (that’s a lie, it’s going to be my wife) stand on a pair of corners, and I’ll stand on the other while I fire brads into it, hoping the added rigidity will hold it in that spot. Unfortunately it’s still a gamble and I’d rather it have been flat to begin with.

Has anyone ever encountered this? Does this usually disappear when the top of the box is affixed?

I was planning on using some thicker 3/4" for my table top hoping the thicker material will resist the flex more. I chose plywood for the frame as it should be more stable with humidity, but I am considering using MDF for the table top since it’s more uniform and I would think less likely to flex. Is that logic sound? Also, I see several people adding foam to the individual cells. Is this to reduce the table from being “boomy,” acoustically? Is the xcarve really that loud without it?

Its final strength arrive from two things:
1 - Gluing the 2nd surface
2 - Adding full height side bracing (outside trim), may be omitted since weight is an issue.

Your simplest option is to find the most level surface available to you, glue down the top surface and weight down the crap of it while it cures. Use books, sand bags whatever :slight_smile: Use a straight edge to find a good spot for it to cure.

A thicker MDF top is fine, 3/4" and plenty thick enough to do a final leveling once the machine is rigged up. 3/4" MDF is heavy though.

You can also add insulation foam/expanding glue to the box cavities for added rigidity.

Ah, so the foam is for rigidity? I assumed it was for acoustics.

I just remembered I have like 800lbs of lead I use for reloading, so that will work - except I better take a day off work for recovery. I’ll use the MDF top if it gives me the best chance of getting a flat surface. You mentioned “final leveling.” Is there an additional process after the table is assembled to surface the table?

Acoustics is part of it and the major rigidity player are the “ribs”.
By “Final leveling” I was refering to having the xcarve shim the workware perfectly even (relative to the gantry) as no material is 100% true in every dimension :slight_smile:

When my model airplane wings have torsion I do not apply the top sheet befire that is solved. It is no use of holding it down, glue the top and see it warps back half the distance when the clamps are loosened. You need to get rid of the torsion first.

So mist spray the internals of the torsion box with some water, mist not flood… Put it on 4 blocks, one on each corner and a weight in the middle, let it dry for a day or two, do not rush or put in any additional heat as this may make things worse. Measure the progress and repeat the process untill it is totally flat without any stress in the material. Now you are ready to glue on the top. Make sure the weight applied when glueing does not introduce a warp.

Erik, in order to get the flex out of the box I would have to apply a pretty decent amount of weight. Wouldn’t I be risking a new bow in the middle just as likely as removing the existing bow from the structure? edit: I reread what you posted. I’m assuming you need to balance the weight accordingly.

I’ve used a saturating process with pure wood, but never plywood. Do you think it would work the same. I do like the suggestion and will try it this evening. Just use a spray bottle on every side of the cavities?

I did leave the top off of it for exactly these suggestions, still time to make changes if necessary.

Big thanks for everyone responding, by the way. Defintely helping me figure things out.

alright, that actually relieves a lot of anxiety. I was planning on having to resort to shimming the machine when the table was completed. It’s nice to know that’s how everyone is doing it, then.

As a fellow RC modeller I can relate, for a wing minute details related to AoA and aerodynamics can have great impact on its performance.

Depending on the amount of bow currently (relative to what?) present, I would consider Erik’s “repair” process prior to closing it up :slight_smile:
The main purpose of the torsion box is to remain fixed forever, however wood is a living medium and will be affected by temp./humidity to some extent regardless.

The final leveling will provide the true level surface required for the actual carve.

Yes, that is why I wrote mist not flood. You have to get a feeling for what your material is doing. So start with something you cannot imagine to be enough and see what happens, measure and decide what next.

On a model airplane wing an error of as much as 0.5mm is noticeable in flight, since i make my wing ribs with the machine they fly a lot better.

I used 3/4" marine grade plywood for top and bottom surfaces and 3/4" birch plywood for the stringers (3 in each direction). After glue up it had a slight bow on one side, I put two boards under the torsion box to allow sag and was able to load it up with 5 car batteries and my belt sander to get it to deflect the same amount in the opposite direction while the glue was still wet. Let it dry overnight and when I took the weight off it was near perfectly level. I glued down a sheet of 1/8" rubber on the top surface to reduce any resonance from the torsion box. After bolting the 1000mm down with the Inventables waste board it had a few slight high and low spots, I added another sheet of MDF and planed it flat. It is now as flat and accurate as one could hope for.

That makes me wonder if the bow can be corrected if I force an equal but opposite bend and affix the top.

I placed a few hundred pounds of weights on the table last night just to see if I have any change in it’s flex today.

I should reiterate that the torsion box is flexing/bowing - it’s not that it isn’t flat, per se. When you apply pressure to the surface (with weight) it is flat. So dimensionally, it’s flat. Sanding the ribs, etc isn’t necessary. The problem is it’s bowing up at a corner when at rest, presumably due to pressure in the ribs. If you press that corner down, a perpendicular corner will rise. My concern is if I apply the top to it will it eventually warp because of internal forces.

Honestly, at most it’s out by 1/8-1/16", worst case scenario. I’m going to set some weight on it and see how it responds.

Realistically it seems the only real solution is to affix the top and shim anything out of it.

Since I’m waiting on my x-controller I’m not in any real hurry so I plan on trying a few more things, especially what Steve mentioned above.

In theory, If the offending corner is 1/8" high when placed on the ground I’ll add 1/4" shims to the two perpendicular corners and then apply weight to high corner and the one opposite, which should force the bend in the opposite direction with twice the force and distance. Hopefully the end result when the force is removed is regression to the mean and a perfectly level surface.

I have a sneaking suspicion the cause of my problem is due to a miter saw that wasn’t perfectly square. I did make numerous measurements beforehand to insure it was square, but I took a measurement today (after all of the individual ribs had been cut) and it was very slightly out of square, about 1/32" over 12", which is minimal, but when you have 10 rows of ribs, it adds up. My guess is in a few areas the majority of the ribs aligned in such a way it caused uneven pressure. I was going to use dados and slots to do each rib as a single interlocking piece, but much of our character I backed out at the last minute and went the easy route.

on a related note - Steve - Where did you find the 1/8" rubber mat you mentioned? Is there a reasonably priced option out there?

Lol, yes I’m sure I am over thinking it. I’m not a fan of things being out of spec and I have plenty of time to burn. Good ideas on both counts.

I haven’t set the xcarve down on the table since I don’t have a top yet.

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Don’t mean to steal the thread, but I am having similar problems, Do most people fix their xcarve to the table somehow? I just have mine sitting on the table.

If your torsion box is already glued, then I concur with the folks who recommend mechanically(sanding, planning) flattening the box base before attaching the top. Weighting down the device might flatten it, but you’ll be left with internal stress, which might manifest down the road. What reliable straight edge/flat surface do you depend upon? Eye on the prize. Phil

C.J., a local feed store, for horses and other farm animals, normally stocks it in a roll, used in horse trailers,etc. a 4’x4’ section was about $20. Problem was, I stopped by multiple times over a month and they still were out of stock. Mentioned it to a customer of mine that works at BOEING, he hooked me up that day :wink:

What Robert said, too little to worry about :slight_smile:

Oh okay, awesome info, Steve. I’ll try a Tractor Supply down the street. I wonder if I could use it for seals on my dust collector, also. We don’t have a Boeing here in Fort Worth, but we do have a local Lockheed Martin, so I’ll check there. :slight_smile:

Last night I hit it with my biggest sanding block (about 18" long) so I could get a decent surface. I also knocked down some of the high spots with my block plane. It’s slowly coming into shape. I had some low spots on the perimeter so I glued down some of my veneer and planed it to match.

The weights accomplished precisely nothing, it sprang right back to it’s original form. When I apply pressure with one of my arms, the whole table becomes 100% flat. The amount of pressure required to flex the box has decreased substantially; however, the whole structure feels much, much more rigid than yesterday. I have a feeling when I affix the top it’s going to be fine.

Should I spend the time to foam the cells? Is this worth the trouble by those who have done it? I don’t have any expanding foam, and I’d probably end up using great stuff. Since I have 80 individual cells, I would think this would take several cans and $$$, unless there is a cheaper two part foam that’s reasonably priced.

Saw slice down through the ribs to relieve tension, weight the frame until level, glue splice boards to each side where you sliced???