Bending Wood the Wright Way
In my mind, there are three classifications of woodworking techniques. There are many that I classify as “useful,” a smaller number that I think of as “indispensable,” and then a very few that represent a true breakthrough in woodworking technology. Bending wood is one of the latter.
The ability to alter the grain direction as our imagination dictates while preserving the strength inherent in a straight piece of wood allows us to create the elegant beauty of a continuous-arm Windsor chair and the inspiring sweep of a vaulted ceiling. We first explored our world in sailing ships with bent wood hulls, then left it in airplanes with bent wood wings. Our world would be much less beautiful and much less exciting without this simple woodworking technique.
I’m currently engaged in a woodworking project designed to create a little excitement, and bending wood is at the very heart of it. I’m part of a group of historians and aviators who are recreating the six experimental airplanes of the Wright brothers, beginning with their model glider of 1899 and ending with the 1905 Wright Flyer 3, the first practical airplane. The frames of these primitive aircraft are a collection of bent wood parts — ribs, wing ends, braces and skids — ingeniously arranged to catch the wind and lift a man into the air.
True Geniuses Prefer Cold Bending
When most of us hear the words “bending wood,” we think of steam bending. The wood is heated briefly in low-pressure steam to soften the lignin (a glue-like protein that holds the cellulose fibers together). While the wood is still hot, it’s clamped into a bending form. The cellulose fibers telescope to conform to the curve, and the lignin cools to hold them in place. Or almost. In actual practice, the fibers never quite conform, and when you remove the wood from the bending form, there is a great deal of springback — the wood loses some of its curve. If the wood is not attached to the other parts in the project so as to hold the curve, it may continue to relax and it will spring back even more. This problem plagued the Wright brothers while they were doing their glider experiments — they calculated precise curves for the ribs to fly as efficiently as possible, only to have the ribs relax and lose a good deal of curvature before they could get their gliders in the air.
To solve this problem, they eventually abandoned steam bending for an early form of cold bending. They arranged the parts of the ribs for their Flyers in a bending form, then nailed them together with brads. They could not use glue — the adhesives 100 years ago were not weatherproof. A good rain and the wings would have come apart.
Fortunately, we have a much larger and more reliable selection of adhesives to choose from than the Wrights. We decided to make the bent wood ribs of our replica Wright gliders by laminating the parts with a water-resistant aliphatic resin (yellow) glue. You could also use Resorcinol, epoxy or polyurethane glue for an application like ours. If your project won’t be exposed to the weather, you can use almost any good wood glue.
To cold-bend wood, first resaw your stock into thin strips and plane it so the thickness is even. The thickness of the strips depends to a large extent on the radius of the curve. The tighter the radius, the thinner the strips. I use this chart as a jumping-off point:
• 2″ to 4″ radius — 3/32″ thick
• 4″ to 8″ radius — 1/8″ thick
• 8″ to 12″ radius — 3/16″ thick
• 12″ radius or larger — 1/4″ thick
There are other factors to consider: the species of wood, the slope of the grain (as it runs between the faces of the strips), the strength you want, and the amount of springback you can tolerate. For maximum strength and minimum springback, we decided to glue up the ribs from 1/8″-thick strips, although the radius of the curve was nowhere near 8″.
Stack the strips as you will glue them together. If you use strips that were all resawn from the same board, flip every other strip end for end to reverse the grain slope. Spread a thin layer of glue on the face of one strip, lay the next strip on top of it, spread more glue and repeat. If you’re laminating a large number of strips, you may want to choose an adhesive with an extended working time.
Before the glue sets, clamp the laminated strip in the bending form. Let the glue set up for its full clamp time. If you’re not sure of the clamp time, wait a full day before you remove the assembly from the bending form. As you release the clamps, there will be a small amount of springback. If the curve is critical (as it was for our glider ribs) make the curves in the bending form slightly tighter to compensate.
Making a Cold Bending Form
Pretty simple, huh? The only real trick to cold bending is in making a form that will apply an even clamping pressure all along the laminated assembly. Traditional bending forms consist of two parts, the form (the positive shape) and the press (the negative shape). Both of these parts are normally cut from the same stock. Begin by drawing the curve you want on the face of the stock. Cut the curve with a band saw, separating the stock into two parts. On the negative part, mark the thickness of the bent wood part. (Tip: Use a compass like a calipers, set it to the desired thickness. Follow the curve with the point of the compass, marking the thickness with the scribe.) Cut away the thickness on the band saw — this will create the press.
The trouble with this traditional bending form is that the press doesn’t compensate for small variations in the thickness of the laminated stock or a band saw blade that wanders a hair off the line. Consequently when you apply the clamps, the clamping pressure may not be completely even all along the form. This may result in weak laminations or even gaps between the laminations when the glue dries.
To ensure that this didn’t happen to our glider ribs, I designed a compensating press. After cutting away the thickness of the bent wood part, use the compass to mark yet another curve on the negative part, this one 1″ larger in radius than the curve you just cut. Saw this curve then cut the 1″-thick piece into 3″-long segments. Adhere the segments back to the negative part temporarily with double-face carpet tape. Glue a strip of canvas to the inside curve of the segments and cover the canvas with 6-mil plastic.
When you separate the segments from the negative part and discard the tape, they should be held together by the canvas like the tambours of a rolltop desk. This is your press. When you squeeze the laminated stock to the form, arrange the clamps in the middle of each segment; this will compensate for any variation in stock thickness or inaccuracies in the bending form and keep the clamping pressure relatively even.
Note: The plastic on the press will keep any glue that squeezes out from between the laminations from sticking to the canvas. To prevent the squeeze-out from sticking to the form, apply paste wax to the form before each glue-up.
Spreading the Glue
Just as uneven clamping pressure will reduce the strength of the lamination, so will an uneven application of glue. You must spread it as evenly as possible, and I’ve got just the ticket. This little trick was shown to me by the good folks at Franklin International (makers of Titebond glue). Get rid of your glue brushes and spread the glue with the teeth of a 3/8″ x 32 threaded rod. The threads spread the glue to just the right thickness (about 0.005″) for a strong joint with a minimum of squeeze-out. For this particular project, I mounted a short length of threaded rod in a wooden handle. Between glue-ups, I keep the rod submersed in water to prevent the glue from drying on the threads. PW
Nick Engler is a contributing editor for Popular Woodworking.