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.