Simple torsion box

[Mark Daily]

While reading this old thread I noticed post #3 which mentions gluing a piece of rigid foam between two pieces of plywood to make a router table, torsion box, etc.

https://sawmillcreek.org/showthread.php?269663-Combination-of-BB-Plywood-and-MDF-HPL-for-Router-Table-Top

Has anybody ever tried this? If it works it seems like an easier way to make a torsion assembly table top. I think you could glue them together with clamps, cauls, weights, etc. instead of a vacuum bag. Comments?

[Pat Barry]

Not sure how you would glue it up. Not sure how you would hold it flat during glueup. Not sure it wouldn't curl up like a potato chip after a while.

[John TenEyck]

I made a passage door much the same way; a perimeter wood frame with foam panels set into it, then 1/4" MDF skins on both sides. I used Gorilla Glue to glue it all together because it sticks to the foam board.

https://lh3.googleusercontent.com/leAWgwC2ya_OOUF02iMx-G7nqxMcB1DsYEyqU_Utsts4LI3Mix1kKEccMIkgLdNk45ldgOx MTCUStV1_TkyqHUcN6n-_lfVRZlBx2MLhvlgq76k1vBduzcqSzMumpUbNabnOxMbqCHz0i vx8RWLmCb0EfhWt-ClotnWC5jJreos_95c4iz6EkBre5lx1Xrt4IEcPA_78jDiAX6Q ZchO7xvUNjdtN8iDE3lh0-prGgF4-0iZCn0Z-RBHOoBvt73NcDaryCdQ9FHs2aqhLOefFTGuMje45h1CLfKgLd2 wp7fq3ByZ-l1dRpkaLZLTO8WHXBLI_wbwZclyAFE1oMUIZ28T7g_mh_bJGme NimCriejSVA9TOfj09swVN71vt4YHfixL5QMUervqCVWBLMRP4 KNoJzFjiOaI3Hv1X3ZujBjzCpmg3lrLGgBjZwOR0_Sm6ATV0yQ BxyEhECMlEJsbPZs6tdrin8m_cpoIL0oFxlhNACMFfdmJBGqN7 30N9gYYqiIclLO-PN0TGFLDeaD4gYcbKVAvIaiLI76gEc59O12aRCJB7uBdPpdZig j5hxgOLO3zpzN3oy8twV1yOwRCckR0bpPaq5r_8teo_GFNvQEq 5j8o_t5mjO4URGq_e0fUxIoZLmjn60POnMma3ROhBKbsqMwdJV f8feep6IECetpjCH_x7fJlW2LOO8l5Dvo-ORyyI9u-00zhNJmAiOqqdgTQxtm12j7HykDOhKYUSANC06XqEuA=w835-h626-no

It is very flat and very rigid. I wasn't looking for extreme flatness like I would want for a router table. In that case, I would run the foam board through the planer or drum sander to get it dead flat.

John

[David L Morse]

I've done this. As John pointed out the thickness of foamboard is not very constant. I didn't try to plane it but rather used the adhesive as a sort of filler. Note that since the core only needs to offer support in compression the adhesive is not really needed for stiffness except as a filler. The low shear strength of the foamboard negates any structural effect of an adhesive. It's most important function though in many applications is to reduce any rattles caused by the skin losing contact with the core under stress, like a hollow core door.

[Mark Daily]

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?

[Mark Daily]

I made a passage door much the same way; a perimeter wood frame with foam panels set into it, then 1/4" MDF skins on both sides. I used Gorilla Glue to glue it all together because it sticks to the foam board.

https://lh3.googleusercontent.com/leAWgwC2ya_OOUF02iMx-G7nqxMcB1DsYEyqU_Utsts4LI3Mix1kKEccMIkgLdNk45ldgOx MTCUStV1_TkyqHUcN6n-_lfVRZlBx2MLhvlgq76k1vBduzcqSzMumpUbNabnOxMbqCHz0i vx8RWLmCb0EfhWt-ClotnWC5jJreos_95c4iz6EkBre5lx1Xrt4IEcPA_78jDiAX6Q ZchO7xvUNjdtN8iDE3lh0-prGgF4-0iZCn0Z-RBHOoBvt73NcDaryCdQ9FHs2aqhLOefFTGuMje45h1CLfKgLd2 wp7fq3ByZ-l1dRpkaLZLTO8WHXBLI_wbwZclyAFE1oMUIZ28T7g_mh_bJGme NimCriejSVA9TOfj09swVN71vt4YHfixL5QMUervqCVWBLMRP4 KNoJzFjiOaI3Hv1X3ZujBjzCpmg3lrLGgBjZwOR0_Sm6ATV0yQ BxyEhECMlEJsbPZs6tdrin8m_cpoIL0oFxlhNACMFfdmJBGqN7 30N9gYYqiIclLO-PN0TGFLDeaD4gYcbKVAvIaiLI76gEc59O12aRCJB7uBdPpdZig j5hxgOLO3zpzN3oy8twV1yOwRCckR0bpPaq5r_8teo_GFNvQEq 5j8o_t5mjO4URGq_e0fUxIoZLmjn60POnMma3ROhBKbsqMwdJV f8feep6IECetpjCH_x7fJlW2LOO8l5Dvo-ORyyI9u-00zhNJmAiOqqdgTQxtm12j7HykDOhKYUSANC06XqEuA=w835-h626-no

It is very flat and very rigid. I wasn't looking for extreme flatness like I would want for a router table. In that case, I would run the foam board through the planer or drum sander to get it dead flat.

John

John, if the foam board were uniformly thick, would this work as a worktop for something like an assembly table?

[Mark Daily]

I made a passage door much the same way; a perimeter wood frame with foam panels set into it, then 1/4" MDF skins on both sides. I used Gorilla Glue to glue it all together because it sticks to the foam board.

https://lh3.googleusercontent.com/leAWgwC2ya_OOUF02iMx-G7nqxMcB1DsYEyqU_Utsts4LI3Mix1kKEccMIkgLdNk45ldgOx MTCUStV1_TkyqHUcN6n-_lfVRZlBx2MLhvlgq76k1vBduzcqSzMumpUbNabnOxMbqCHz0i vx8RWLmCb0EfhWt-ClotnWC5jJreos_95c4iz6EkBre5lx1Xrt4IEcPA_78jDiAX6Q ZchO7xvUNjdtN8iDE3lh0-prGgF4-0iZCn0Z-RBHOoBvt73NcDaryCdQ9FHs2aqhLOefFTGuMje45h1CLfKgLd2 wp7fq3ByZ-l1dRpkaLZLTO8WHXBLI_wbwZclyAFE1oMUIZ28T7g_mh_bJGme NimCriejSVA9TOfj09swVN71vt4YHfixL5QMUervqCVWBLMRP4 KNoJzFjiOaI3Hv1X3ZujBjzCpmg3lrLGgBjZwOR0_Sm6ATV0yQ BxyEhECMlEJsbPZs6tdrin8m_cpoIL0oFxlhNACMFfdmJBGqN7 30N9gYYqiIclLO-PN0TGFLDeaD4gYcbKVAvIaiLI76gEc59O12aRCJB7uBdPpdZig j5hxgOLO3zpzN3oy8twV1yOwRCckR0bpPaq5r_8teo_GFNvQEq 5j8o_t5mjO4URGq_e0fUxIoZLmjn60POnMma3ROhBKbsqMwdJV f8feep6IECetpjCH_x7fJlW2LOO8l5Dvo-ORyyI9u-00zhNJmAiOqqdgTQxtm12j7HykDOhKYUSANC06XqEuA=w835-h626-no

It is very flat and very rigid. I wasn't looking for extreme flatness like I would want for a router table. In that case, I would run the foam board through the planer or drum sander to get it dead flat.

John

If nothing else, you just gave me some ideas for making some interior doors that are somewhat soundproof compared to a typical hollow core door.

[Kevin Jenness]

It works, for sure. Plenty of cold molded boats are built with "structural" foam cores, higher modulus material than Dow extruded polystyrene, but xps will serve for many purposes. If you have access to a wide belt sander the stuff can be calibrated with a sharp,coarse (50# or less) belt and high feed speed, then laid up with epoxy.427156 These are a pair of 4 1/2" x 48" x90" doors with two layers of 2" blueboard, 6mm marine ply skins and 1/16" red cedar veneer pressed with vacuum in stages.

I still would make a shop bench with an egg crate wood or plywood core just to keep the plastic usage down, but the foam core will certainly make an equally strong panel if well glued. I would not use contact as it is not a rigid glueline. You could use weights, but vacuum is the easiest way to get even pressure.

[peter gagliardi]

As far as that goes, you can get very high density urethane foam board for this application through a sign shop- its called sign foam, and comes in 15 and 18 pounds per cubic foot density.

[David L Morse]

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?

No, you're not really missing anything. The foam core will be neither as strong nor as stiff as a glued timber core. It will however be much lighter. It's a typical design tradeoff problem. Pick your poison.

My guess is that the glued foam would initially test better than no glue but the foam would eventually fail and make it about the same as unglued. I have no actual data to support this.

[Kevin Jenness]

The strength and stiffness of a torsion box are related primarily to the properties of the skins. As long as the core is stout enough to connect the skins together it can be made of anything- in the case of a timber grid core it's 90% air. I don't see why a foam core that is not highly stressed or exposed to weather would be expected to fail spontaneously.

[Jim Becker]

As far as that goes, you can get very high density urethane foam board for this application through a sign shop- its called sign foam, and comes in 15 and 18 pounds per cubic foot density.
It's definitely dense and true to thickness. The downside is...it's very expensive. A full sheet of 1.5" thick 20lb runs about $360.

[John TenEyck]

Mark, the OCF foam board I used is plenty flat enough to use for a work bench. The door I made is very flat and torsionally rigid. It feels a lot more robust than a hollow core door.

Don't use solvent based contact cement as it will dissolve the foam. I know. Gorilla Glue bonds to it really well. I didn't spread it out into a continuous film; rather, I just applied a continuous bead on the wood members and beads about 3" apart on the foam.

I think a foam core will fail before a wooden one because of the lower shear strength of the foam. You could compensate for this, I think, by applying glue to a larger surface area. Epoxy over the whole surface should give the max. strength possible. Using epoxy would eliminate the need for a vacuum bag, too. Just put a sheet or two of plywood on top and a few bags of sand.

John

[Don Stephan]

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.

[Mark Daily]

It's definitely dense and true to thickness. The downside is...it's very expensive. A full sheet of 1.5" thick 20lb runs about $360.
Caramba! Not gonna happen.

[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/products/build/construction-adhesives/loctite_pl_300_vocfoamboardadhesive.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.

[Mark Daily]

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.
Thanks Pete- good to know if nothing else it’s a durable building method.