two torsion box questions

[James Baker SD]

I am building a torsion box that is 2' x 13' as a platform for a miter saw and fences. It is all 3/4" baltic birch, surfaces and braces (ripped into 2" strips).

First question, I will glue the inside braces to the inner sides of the surfaces. The braces will have their laminated edges glued to the outer lamination of the surfaces. Would it help to put in screws from the surface into the braces, or will that just splits the laminations apart and not add to the holding power?

Second question, I will finish the outside surfaces, probably with shellac. Would it be a big problem if the inside is not finished? I could glue the strips to one surface, then shellac the exposed wood before continuing, but that would be leave the final surface unfinished on the inside. Alternatively, I could shellac the parts before assembly, but fear that might interfere with the holding power of the glue.

Thanks for insights.
James

[John TenEyck]

It's the inner matrix glued to the skins that makes a torsion box rigid. Mechanical fasteners are needed if/when necessary only to get the glued surfaces together. With the torsion boxes I've made, I stapled the inner "rails" to the "stiles" at each intersection, straight down from the top, flipped the whole thing over, and then all the intersections on the other side. Now that I have a pneumatic nailer, I might just shoot brads through the stiles into the rails, add another stile, repeat. BTW, no glue is required between the inner matrix members. Anyway, once the matrix is made, I just ran a really heavy bead of glue on one side and flopped it onto the bottom skin, put another big bead on the top of the matrix and put on the top skin. Since they were too big for my vacuum bag, I put a 3/4" sheet of chipboard on top and then a whole bunch of bags of sand and let it sit for 24 hours. I didn't put any finish on the inside, and don't think you need to either. Whatever difference in moisture content that will occur will be balanced by the opposing skins. My torsion boxes are as flat today as when they were made, and some are about 20 years old.

[david brum]

I'd be interested in how you're joining the sections of outer skin to get the 13', or whether you're making it in sections. Torsion boxes are great.

[Jeremy Greiner]

Just a note, you should be able to use 1/2'' material (maybe even 1/4'') for the inside I'm not an expert on torsion boxes, but i've seen long torsion boxes made out of 1/4'' hardboard that had almost no deflection with hundreds of pounds added in the center.

-jeremy

[Alan Schaffter]

There is another torsion box thread here that you should read, but the only reasons to use mechanical fasteners are (1) if the skins are not flat and must be forced down onto the "web" (that is the name of the interior structure- actual the full name is "shear web") while the glue sets up. (2) You don't have a vacuum bag, cauls, or weights to put on the skins during glue up, or (3) the edges of the web will not hold glue well or the edges of the ply have a lot of voids, etc.

I see no need to finish the interior.

You are over-building the mitersaw bench using 3/4" birch ply for skins and web. Since it will likely have legs to support the weight, all you need the torsion box to do is stay flat. You can make one that will be sturdy enough using just yellow glue and 1/4" hardboard or 1/2" MDF which are flatter and more uniform in composition than ply. As an example here is one I built for an American Woodworker article from 3/16" hardboard and 1/4" pegboard. It measures 18" X 8' X 2.5" thick with 6" web spacing. It weighs less than 30 pounds. In the second pic it is supported at each end by saw horses and loaded with over 300 pounds of bricks (two deep) at center span- the deflection there was slightly less than 1/2"!! I think that is strong enough for your application. Since I planned to use it as a scaffold, I never loaded it to failure, but suspect it could have supported much more than 300 pounds!!!!

http://www.ncwoodworker.net/pp/data/1421/medium/P1040010.JPG

http://www.ncwoodworker.net/pp/data/1421/medium/P1080028.JPG

[keith micinski]

Just to be contrarian I built a 27" by 27" top for my router table and put a lift in it. I made it out of 3/4 mdf with 2 3/4 stringer spaced 5 inches apart. it has already sagged noticeably in the middle where the lift is at and I added extra support in that area. I wouldn't suggest trying to under build it.

[Alan Schaffter]

Just to be contrarian I built a 27" by 27" top for my router table and put a lift in it. I made it out of 3/4 mdf with 2 3/4 stringer spaced 5 inches apart. it has already sagged noticeably in the middle where the lift is at and I added extra support in that area. I wouldn't suggest trying to under build it.

Torsion boxes need to be engineered for the intended application.

A number of things can cause what happened to your router table. Without seeing it I can't say what other problems you may have, but the biggest one is that torsion boxes get most of their strength from the skins. By cutting a hole in the skin for your lift you compromised the structure. Another issue, is that torsion boxes aren't especially good at resisting point loads- take a look at airplane fuselages and wings- they are torsion boxes. Despite thin skins the wings are strong and able to support aircraft weighing thousands of pounds. But if you look at the wings, especially older aircraft the skins often exhibit deformation between the ribs, called oil canning, from maintenance workers walking on the wings, point aerodynamic loads, etc.

[James Baker SD]

Thanks all for the info. I already have the parts cut, so I will probably stick with the 3/4 ply (plus I need a thick substrate to screw the Biesemeyer miter tables onto so that was the reason for the 3/4"). The internal parts are made from the left overs. It will be fabricated as an 8' section and a 5' section. The long pieces of the shear web are 8' strips and some with be under the 8' section of the skin, but most will be connecting the 5' skin to the 8' skin, with another 5' piece continuing on to support the rest of the 8' skin. The finished box will sit on 4 gladiator storage cabinets, 3 close together at one end (supporting the saw, a 3' fence and one end of an 8' fence) and the fourth at the other end of the box (supporting the other end of the 8' fence); the gap between is where my shop vac will sit when it is connected to the miter saw.

[rick carpenter]

I built a torsion box 18 in by 5 feet for my cms with 1x3 poplar webbing. I figured the mounting points where the base would be screwed down, and drilled locator holes through the top before assembly. Then I filled the areas about 3-4 in square over the holes with poplar scraps. After assembly, I finished drilling the locator holes through then came in with a spade bit through the bottom to the fill so that the bolts holding the cms could be recessed for a completely flat bottom. I hold the base on with wingnuts if I ever need to remove it and I can crank 'em down with no fear of deflection. Mine is much smaller than yours on purpose, and I figured the balance point along the side with the saw base left on and put a screen door handle there. Carries good. I can flip it over and use it for a temp workbench on sawhorses on a worksite.