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Trouble with Torsion box door1/18/20
I am building some 7' X 10' X 1-3/4" flush doors using a torsion box design. 1-1/4" wide X 3/4" poplar grid with 1/4" MDF epoxied to each side. Building them on dead flat stable table. I can't press them due to their size so I am stacking about 1,000 lbs of 8/4 lumber all over and waiting 24 hrs to cure. Huge bow at the stile/edges when I remove from the table. I would have preferred to make them 2-1/4" but that wasn't an option. Other than thickness does anyone see any immediate red flags ?
The problem is the poplar grid work. Use maple instead and I think the problem will go away.
I built a number of large (up to 5.5' x 10') torsion box doors a few years back and used Baltic Birch for the grid and Okoume for the skins and they did very well, but they were also 2.25" thick. The BB grid was less stiff than I would have liked, but they were pocket doors, so it wasn't a structural concern. I question that you'd get any solid wood to lie flat enough, esp. at your 1.75" thickness - that was my primary impetus for going to plywood.
Yea. What that guy said
It amazes me that people try to use poplar for projects that require stability. Poplar is renowned for being unstable. It warps, it twists. Other than interior trim, occasional face frames. Maybe a couple of cabinet doors if you ran out of soft maple.
Change the gridwork species as noted. Decrease its thickness so the weight does not become an issue. 1/2" or 3/8" is fine. Increase to 2-1/4".
Epoxy does not require lots of (any?) pressure. Placing one piece on top of another bonds as well as clamping the bejeezus out of the two parts. I think the MDF is OK
Now for the quiz:
Are you able to pin/staple thru the MDF into the grid?
How much is "huge bow at stile and edged..." 1/2"? 3"?
Is the bow across the width or length or both - bowl like?
Were all the materials stored in the same shop, same environment for a week or more before using?
Has the structure changed over time? How much in how long? If it still exists, that is.
To all of you torsion box builders, to me the boxes are similar to a hollow core door, I was taught to cut an air relief somewhere on the frame under the skins .Not sure if this has anything to do with the bowing ? Good luck
Thank you David; Here are some answers and some changes I have made:
My guess is gravity is getting you and the skins are causing the bow especially given the ply core being less but still bowed.
Loose lap joints is good. I don't know anything about LVL's, but I would refrain from adding more and disparate materials.
KISS ain't the band, as they say.
Materials coming in from suppliers or other environments put with materials that are shop conditioned will exchange MC to reach an equilibrium. This will cause things to move.
I have seen tight Plastic laminate seams open to .090" when put onto particle board, The P L came in a truck from an unheated warehouse (high MC), and the PB was shop conditioned, had been in there for a month or more (lower MC than the P Lam).
Is your dead flat table still flat after loading the 1000lbs of wood on top?
Virtually any shop vac will pull -2 psi, or 288# psf, As has already been pointed out, epoxy doesn't need a lot of pressure, only that both sides meet. But just in case your interested, your door would have 20,160 perfectly even pounds of pressure from all sides.
Epoxy doesn't contain any water to change the MC of the wood.. If your shop is somewhere that is cool now, you can take as long as you like spreading epoxy and putting it all together, getting the bag sealed, then pull the vacuum and turn on the heat to get it to cure.
I designed some tortion panels awhile back for a tall cabinet project, the sides had to be 16' tall and 3" thick. your grid material should be 3/4" 9 ply or greater straight as hell and standing on edge. cut notchs so your grid members can overlap each other and fit tightly together. the grid cannot be attached to the face in any way as in nails or anything during the glueing process. the grid has to be able to float while its drying. if your unable to pull it off or require ultra lite panels you can always use the company called sing core and have the made to spec