Simple torsion box

[Mark Daily]

I have made a number of jam chucks, that I use to turn off the tenon on the bottom of once turned bowls after they are dry and sanded, by gluing pink or blue high density foam sold in 4x8 sheets at Home Depot in 1", 1 1/2". and 2" thicknesses. (I think the foam is made to lay under basement concrete floors as it is strong enough not to be crushed by the weight of the wet concrete.) I just use yellow woodworking glue to join the foam to 3/4" baltic birch plywood, and even to laminate two layers of foam for deep bowls. Haven't had any glue joints fail, and some are more than a year old.

That’s good to know- I have plenty of “ole’ yeller! Lol

[Mark Daily]

Thanks all for your input. I was just looking for a quicker, easier, and lighter way to build a torsion box table top for an assembly table.

Because of lack of space, I need something that I can bring out when I need it and store it away when done. Like all of you, I need it to be flat. I also need it to be light and durable enough to assemble things. Doesn’t need to withstand forces like hammering, chiseling, etc.

[John TenEyck]

Why not a hollow core door? Super light, plenty stiff, and pretty flat.

John

[Mark Daily]

Why not a hollow core door? Super light, plenty stiff, and pretty flat.

John

John, that’s what I’m using now! Cut it in half and wrapped it with maple and it worked well for a few months then it began to warp. Don’t know why but the surfaces began looking like a potatoe chip. (Wavy lay’s, lol)

[Lon Crosby]

Hollw core doors frequently use cardboard in an egg crate pattern. Pressure deforms the veneer cover damaging the cardboard and the thin veneer is pulled down because of the compressed core. The solution for a workbench is to add a layer of plywood (or hardboard) over the top of the door.

[Richard Coers]

Hmm- I’ve never paid much attention to foam board so it would be important to ensure the thickness is consistent. Aside from that why not lay a sheet of 1/2” MDF or plywood down, cover with contact cement or whatever will bond foam to wood, add the foam board, cement and another sheet of plywood on top. Contact cement would bond instantly so no clamping needed.

Of course you would use strips of wood on all four sides and glue them in place along with anything else.

David, I understand what your saying about bonding to the foam board but wouldn’t it help the entire “sandwich” in resisting twisting, warping, bowing, etc. the same way sticks of wood do in a traditional torsion box?

I just reread your post and if I understand correctly, gluing the wood to foam board won’t be very strong if some force is applied to it like twisting, etc.. In that case gluing to wood sticks on the interior would be stronger, right?

If this works it would eliminate cutting all the wood that goes inside and would still be lightweight. Or am I missing something here?

Before using any solvent based adhesive, make sure it's compatible with the foam you choose.

[Tom Bender]

Don
I had a different experience with woodworking glue and foam. It worked fine against wood but between two layers of foam it didn't dry due to lack of air.

[Michael Drew]

You might try and experiment with some low expansion foam as your "glue". I used that framing a house, as I wanted to try something faster than construction adhesive (floor sheeting, stair risers, etc). Worked amazingly well, although messy.

[Mark Daily]

Probably something like this:

Loctite PL-300

https://www.loctiteproducts.com/en/p...dadhesive.html

[John TenEyck]

As I said, Gorilla Glue bonds really well to foam, and wood. It became far less of a PITA to work with once I learned that MS wipes it right off.

John

[Jim Becker]

Polyurethane glue (Gorilla brand or otherwise) is kinda good for this application. It's moisture cured and does bond many dissimilar materials nicely. It's a staple for signmakers doing dimensional work with HDU and other foamy/plasticy products. One thing...it's absolutely a best practice to wear gloves when working with it. Get it on your hands and you'll have some wonderful black stains that are a bear to remove.

[Mark Daily]

Thanks for all the replies- I’m not so sure now that this will insure the top & bottom will remain flat. Traditional wood cross pieces inside will probably be more durable but I may make one up with foam and see how it holds up.

[Don Stephan]

Apologies for not remembering resin coated honeycomb. It is stiffer than uncoated paper honeycomb, and although I have not used it in years my experience was it did not crush or reduce in thickness in a vacuum press drawing 22-25 psi. My source always was Vacuum Pressing Systems. I haven't confirmed that VPS still carries the product (I seem to remember they had more than one thickness) and never looked for another source for single sheet quantities.

[Mark Daily]

Apologies for not remembering resin coated honeycomb. It is stiffer than uncoated paper honeycomb, and although I have not used it in years my experience was it did not crush or reduce in thickness in a vacuum press drawing 22-25 psi. My source always was Vacuum Pressing Systems. I haven't confirmed that VPS still carries the product (I seem to remember they had more than one thickness) and never looked for another source for single sheet quantities.

Thanks Don- no worries. I don’t have a vacuum press and don’t intend to get one.

[Pete Staehling]

This may or may not be of any use at all, but i do recall a friend making a table top that way a long time ago. He used it to take to arts/crafts festivals and it took lots of abuse, some exposure to the weather, and showed no obvious damage or warpage. His needs for flatness were not all that demanding so I don't know how flat it was or if it warped at all, but there was no obvious warpage, at least not to the extent that I noticed it. I don't know much about exactly which types of ply, glue, or foam he used so this is probably not super useful info. It was very light and quite sturdy though.