help on constructing torsion box web

[Paulh Tremblay]

I am building a torsion box. I have ripped the narrow pieces for the internal web and would like some advice on how to hold it together. The two plans I have seen call for either using brad nails, or constructing half lap joints. I don't have a nail gun or a compressor, and I don't have a table saw, so I can't use either technique.

Basically, the joints of the inner web only serve to keep the pieces straight until you glue on the top skin. Once glued on, the top and bottom skins provide the strength. So I just need some temporary method for the joints of the inner web.

My choices are:

1. Buy a brad gun and compressor. I don't know how much I will use either in the future, so I risk wasting money and having extra stuff hanging around (which I try to avoid).

2. Buy a jig saw to cut the half lap joints and trim them flush with a router. I would have to buy the jig saw. Although I have a router, I want one with dust collection (that is, probably a Festool), so I would have to spend a lot of money right now, though I would use both tools in the future.

3. Use screws. I worry that even if I drill pilot holes, I won't get the pieces perfectly flush. The web has to be absolutely flat. Also, It might be very difficult to drill pilot holes for the pieces as I get to the end of the web.

4. Use clamps. I would think I would not be able to get pieces perfectly flat and flush with just clamps.

[Michael Kellough]

I am building a torsion box. I have ripped the narrow pieces for the internal web and would like some advice on how to hold it together. The two plans I have seen call for either using brad nails, or constructing half lap joints. I don't have a nail gun or a compressor, and I don't have a table saw, so I can't use either technique.

Basically, the joints of the inner web only serve to keep the pieces straight until you glue on the top skin. Once glued on, the top and bottom skins provide the strength. So I just need some temporary method for the joints of the inner web.

My choices are:

1. Buy a brad gun and compressor. I don't know how much I will use either in the future, so I risk wasting money and having extra stuff hanging around (which I try to avoid).

2. Buy a jig saw to cut the half lap joints and trim them flush with a router. I would have to buy the jig saw. Although I have a router, I want one with dust collection (that is, probably a Festool), so I would have to spend a lot of money right now, though I would use both tools in the future.

3. Use screws. I worry that even if I drill pilot holes, I won't get the pieces perfectly flush. The web has to be absolutely flat. Also, It might be very difficult to drill pilot holes for the pieces as I get to the end of the web.

4. Use clamps. I would think I would not be able to get pieces perfectly flat and flush with just clamps.

How did you rip those strips without a table saw?

People get too hung up on the neat looking grid inside a torsion box. As you noted the grid's prime function is to keep the top and bottom skins separated. You don't need the nice tight grid people show off on the web. You just need a reasonably regularly spaced glued support to keep the skin from either lifting or collapsing. The thicker the skin the farther apart those supports can be.

The long strips may add some strength if they don't have deep notches for the side pieces but they don't add a lot of strength. You don't really need notches if you have another way to keep the short side pieces standing up square to the bottom skin. You can use hot glue for that. Just keep a spot clear of the wet glue for the hot glue. You can do the same for the long pieces too. Spot the wet glue intermittently so there is a dry place for the hot glue.

When the inside supports are all glued in add glue to the tops of everything and lay the skin on. You can clamp, nail, screw, or simply lay a lot of heavy stuff on top until the glue dries.

Of course everything needs to be sitting on a dead flat smooth surface to start with...for best results.
Usually a solid core door is sufficient.

[Paulh Tremblay]

To cut the strips, I used a Festool TS55 track saw with the parallel guides. I thought about using hot glue and almost included it in my options. Now that someone else suggests it will work, I think I'll go with it.

Now that you mention it, I guess a hollow core door might have been easier. though a bit narrow. The door certainly would have been lighter than my box!

Thanks.

[Rick Lizek]

The originator of the torsion box in woodworking is Ian Kirby who wrote the first article in Fine Woodworking many years ago. He used a simple manual staple gun and butt joints for inner grid. Everything written after that made the process more complicated and time consuming. Ian's method is the best and you can make them very fast and accurate. Finding a copy of the original article would be extremely beneficial and enlightening. Using 1/4" luan skins are all you need and you put some blocks of solid wood in key spots to attach legs and whatever. Making the core grid from pine and 1/4" luan skins yields an amazing light and strong structure. I see more bad techniques in torsion box construction in the magazines and the web. Kirby's method is the best you can ever use.

[Rich Enders]

I built a very simple torsion box according to the design of Kieth Rust (FWW #175, Feb 2005 page # 59+). His was 4' long by about 12" wide, but I extended the length to 8'. There are just two outside long divider strips, plus a center one, and the stiffness when everything is glued together is good. I used this as a jointer sled for a cheap Ryobi planer when I did not have a jointer. I screwed the divider strips to the bottom "skin", and then screwed the top "skin" to the divider strips. Most everything can be done by hand if you have some uniform thickness divider strips which you already have.