Torsion box design

[Bert McMahan]

A point to consider for anyone designing a torsion box. For a given thickness of skin, its effective strength goes up by the cube of the total table height.

In other words, doubling your thickness means an 8x increase in stiffness. This of course ignores local effects like point loads between spans; it's just a general mathematical term.

Doubling your skin thickness will double your stiffness. Doubling your height will give you an 8x increase.

If you have a reference design- say, a 2" table with 1/4" skins that you find "nearly adequate" you can estimate how much more overbuilding you need to do with those scaling factors.

Another way to think about it: a 4" thick table with 1/4" skins has the same overall rigidity as a 2" thick table with 1" thick skins.

(it's Friday, forgive me if I goof a number here or there )

[Dan Chouinard]

I have searched the archives and found mention of gluing both skins at the same time. Anyone here done it this way?

[Kevin Jenness]

That's what I have done.

[Alan Schaffter]

I have searched the archives and found mention of gluing both skins at the same time. Anyone here done it this way?

I did not, but suppose it could be done. I would recommend using glue with an extended set time.

However you approach the glue-up, you will need a flat construction platform. You won't get a flat torsion box unless you start with an absolutely flat construction platform.

I built my platform starting with saw horses and attached the legs to the floor with dabs of bondo so they wouldn't move. Then I made two long stringers from ripped and jointed 2 X 4's and set them between the saw horses. I leveled and aligned the the stringers with winding sticks before attaching them to the saw horses with dabs of bondo. I placed shorter, ripped and jointed 2 X 4s across the stringers. I checked height, level and alignment of the cross pieces before attaching them with bondo. I placed the second skin on the frame and covered it with plastic wrap to prevent glue from sticking then proceeded to build my shear web frame. I added leg pockets and reinforcing for other hardpoints. When the web was done, I carefully marked the location of the web pieces and reinforcing on the skin so I would be sure of hitting them when brad nailing the skin to the web. I loaded the top edges of the web with glue and placed the skin- good alignment is important for gluing and nailing but the skin can be left bigger and trimmed flush with a router afterwards. When the glue had set I flipped it over and attached the second skin. For durability, I wrapped the edges of the table with oak srtips before applying the laminate to the top.

(I used bondo to hold the construction platform stationary, because it can be apply without moving anything and could be easily removed when I was done by knocking it off with a hammer.)

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[Dan Chouinard]

Only one local supplier had any ultralight mdf in stock. 3/4" or 1/2' was the choice. I decided to use 1/2" for the skins and 1/2" x 2 1/2" ribs spaced 5 1/2" apart.

[Alan Schaffter]

That should work just fine.

[Mark Hill]

This is pretty late to the party, but ...

https://www.woodworkingarchive.biz/s.../info-tfz.html

[Dan Chouinard]

You are rather late to the party Mark. In fact the link you gave is to an article written by the gentleman who posted prior to your arrival here.

[Kevin Jenness]

In case you have not yet assembled your torsion box I will describe how I have gone about it.

1.Prepare 10 or 12 clamping cauls about 2"x4"x48". Set half of them out on a flat surface and top them with a sheet of 3/4" mdf.

2. Assemble a perimeter frame and fill it with half-lapped core grid elements- no need for glue at this point. Pin through the outer frame into the core ends so that you can flip the whole thing over once the top sheet is glued on.

3. Run a fat bead of glue along the top edge of all the core elements. I would use Titebond Extend here for a reasonable working time. Set the top sheet on and pin it to the core in a few spots.

4. Flip over this sub-assembly, glue the core edges, pin on the top sheet, add another sheet of 3/4" mdf and the rest of the 2x4 cauls and apply clamps.

This scenario goes best with a helper, but it could be done alone using epoxy for a longer working time. Otherwise, you could do one face at a time.

You may want to insert some blocking into your core for secure fastening to your lift table below, ultralight being what it is.

[Jon Endres]

Am also late to the party but this thread has given me some good ideas - I want to toss another one out there. For years I've had a very lightweight, stiff and flat torsion box bench top in use, as a "temporary" top to throw on a pair of sawhorses. Kind of fell off my radar because I've been using it for so long. It's most recent use was supporting a stack of 3/4" plywood in my shop - 22 full sheets of 3/4" plywood. It's just a 36" x 80" lauan-skin cheap hollow core door. I've been overthinking what to do for my own assembly table/benchtop, and I am simply going to laminate a sheet of plywood to each face of this door and trim out the edges in some hardwood. Stiff enough, flat enough and strong enough to do what I need. Total thickness = 1-3/8" + 1-1/2" = 2-7/8", or I could use 1/2" ply on each side and save myself an inch of thickness.

Only downside that I see is that I can't really do dog holes because the hollow core of the door is inaccessible, but I was planning on using T-track all over it anyway.

[Alan Schaffter]

Don't give up so easily, if you want dog holes!

Once you have added heavier skins to the door, drill oversize holes 1-1/4" or 1 1/2" in diameter all the way through the door and both skins wherever you want dog holes. Then glue 2-7/8" long liners in the hole. For the liners you could use large pieces of dowel, closet pole, round stair handrail, etc. Though it would require more work making the openings, you could even use rectangular pieces of hardwood. You could drill out the center of the liners to make 3/4" or whatever size dog holes you want, before or after you glue in the liners. You might even consider lining the liner because the top opening may wear easily. To use a metal or plastic pipe inner liner, the initial hole would need to be a tad larger. Most of the WW benches I've seen have more dog holes than they will ever need. You won't be compromising the strength of the torsion box (door) you might even be improving it (cheap hollow core doors have a cardboard web.) You could add additional dog holes at any time using this technique.

For a neater appearance, you could install the liners first, glue on one solid skin, drill the dog holes through the skin afterwards from the other side. Or, since you will need a long drill or one with a long shank, you could drill a smaller hole with a spade bit and use a flush trim router to clean up the holes, then install the second skin and drill through it from the other side.

Also, though 36" is plenty wide, if you want a wider bench made from doors, but can't find one that is wide enough, just join two narrower ones together before you add the additional skins. Or . . . . make a hinged table- fold or remove the legs and let one side drop down when not needed. The possibilities are endless!

Whatever floats your boat. Think outside the (torsion) box

[Dan Chouinard]

Just a thank you to all that gave advice on this project, with special shout out for reminder to put blocking inside where it will get fastened to lift table. Although not perfect I am thrilled with the result. It has a perhaps 1/32" crown in the center but I will not loose any sleep over that. I will loose some sleep thinking of the cabinets being built on it. Its overall size of 97 3/4" x 49 3/4 will handle 95% of the cabinets I build. Edged it with poplar and has a sheet of sacrificial 1/4" masonite on it. Very excited to use it.

[Alan Schaffter]

No photos, didn't happen!

[Dan Chouinard]

Someone shows me how to post photos and I wills how them how to make a torsion box. Fair?

[Alan Schaffter]

Start your post. Put your cursor where you want the photo, then click on the "Insert image" icon in the tool bar (3rd from the right). That will open a dialogue box. Select "From computer" then select "browse". Find the photo you want then click on it, then click "Upload file". It may take a second or two. If the image is on the internet (any image), copy the image URL, click "From URL" and paste the URL in the dialogue box. I can get a web image to appear, like my third image below, but I have forgotten how to make the image instead of just the link appear when I upload one from my computer. It could have something to do with my low level membership.

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