Veneering Methods for a Torsion-Box Table Top

      When you build a torsion box with veneered sheets of MDF over a central core, do you have to balance the veneer on each sheet of MDF? May 4, 2011

I am making a coffee table something like the one in the picture. The top and sides will be Macassar ebony veneer, done in a vacuum press. Substrate will either be MDF or Baltic birch ply. I want to make the top as a torsion box - 4' x 4' x 2.5".

Do I need to apply backer to the inside surface of each skin? Or do I simply need to apply backer to the underside of the completed torsion box? Some folks are saying that if I do not put backer on each skin, it will warp from the face veneer. This makes intuitive sense to me, but I hear of lots of folks making torsion boxes, putting face veneer on top, backer on the bottom, and it all works out.

I'd like to apply the face veneer after the sides, so it overlaps. Hence, I cannot veneer both sides of the skins first, then build the torsion box. The veneer is simple solid wood (1/28" I think, maybe 1/32") and I'll be using urea formaldehyde glue. What to do?

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Forum Responses
(Veneer Forum)
From contributor J:
Make your torsion box with unfaced MDF, then treat the complete torsion box as if it were a slab of anything else for edging and veneering purposes.

From contributor C:
I have to disagree with what contributor J is suggesting. If you balance the skins on both sides, the box will not deflect between supports. We make lots of similar table tops and have found this to be true. It's less true when you use thicker skin, but in time it will warp. Our method involves pressing the veneer to the skins, then building the box. If you are using 1/4" skins over a frame, I would say it's essential. The bigger issue is, how can you get enough pressure on a panel after you build the box? Any panel will deflect.

From contributor J:
Good points! I was assuming this would be built with honeycomb core or grid spaced closer than the thickness of the skins.

From the original questioner:
I was planning to make a grid of 1/4" ply and use either 1/2" Baltic birch or 1/2" MDF for skins. Is the honeycomb stuff that much better (less work for sure, but more money)?

The only problem with balancing each skin is this: I'd like to put the face veneer on last, overlapping the veneer on the sides. If I veneer the skins on their own, I have to put the side veneer on afterward, and you will see that thin edge of the side veneer. Not a big deal I guess, but less than ideal.

So - there is a real controversy here. Some are saying that you can treat the torsion box as a single panel, and others are saying you need to balance each skin. I guess it depends on the skin thickness and grid material.

By the way, I was planning on a grid spacing of 2" or so. Maybe 3. I can't imagine that a 1/2" skin would sag in between 2" spaced ribs. But my bigger concern is whether the skins will separate at the edges from the pull of the outside veneers, if there is no balancing veneer on the inside.

From contributor C:
When you build the box, how will you be pressing the veneer on?

From the original questioner:
Current plan is this. Build torsion box with these layers:
face veneer
1/2 MDF
3/4" honeycomb
1/4" plywood
3/4" honeycomb
1/2" MDF
backer veneer

The periphery of the torsion box will be 3/4" MDF. I plan to glue up the torsion box in the vacuum press, with the vac turned down to 15" of hg. I'll veneer the sides of the box using clamps and cauls. Then, I'll veneer the backer and the face veneer in the vac press (with the vac set to 21" of hg).

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