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Thread: Simple torsion box

  1. #1

    Simple torsion box

    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....uter-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?

  2. #2
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    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.

  3. #3
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    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.



    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

  4. #4
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    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.
    Last edited by David L Morse; 03-01-2020 at 4:54 PM.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  5. #5
    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?
    Last edited by Mark Daily; 03-01-2020 at 5:35 PM.

  6. #6
    Quote Originally Posted by John TenEyck View Post
    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.



    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?

  7. #7
    Quote Originally Posted by John TenEyck View Post
    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.



    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.

  8. #8
    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.Miles' cellar doors 1000px.JPG 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.

  9. #9
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    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.

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    Quote Originally Posted by Mark Daily View Post
    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.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  11. #11
    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.

  12. #12
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    Quote Originally Posted by peter gagliardi View Post
    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.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  13. #13
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    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

  14. #14
    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.

  15. #15
    Quote Originally Posted by Jim Becker View Post
    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.

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