Video - July 3, 2015
Why Bent Lamination?
When it comes to cutting curves, the easiest way to get the job done is to simply cut the curve from a piece of solid wood. But solid wood has significant limitations. Some curves would require a piece of stock so thick that it would not only be incredibly wasteful, but you aren’t likely to find the wood in the first place. The second issue comes down to stability. When you cut a curve in a straight-grained piece of solid wood, you inevitably create weak point where the curve cuts across the grain (a situation known as “short-grain). So your best bet with heavilyt-curved parts is to either steam bed the material to shape or do a bent lamination. Pictured left you can see a closeup of one of the rockers on my Maloof-Inspired Rocker. This is a perfect example of a situation where bent lamination is a good choice.
How it Works
The concept is actually pretty simple. Thick boards don’t bend well but thin ones do. So we simply cut a board into thin strips so that when bundled together, the strips can flex to the desired shape. With glue between each lamination, a press is used to bend the bundle of laminations around a form. Once the glue dries, the workpiece will hold it’s new shape. So the first order of business is to build the form followed by cutting the wood into lamination strips.
The Bending Form
There are many ways to construct a form and apply pressure. The way I learned to do it leaves little room for error and produces a form that can be re-used time and time again. Using 3/4″ plywood, I cut and finesse the desired shape and then build the stack as tall as I need. A flush trim bit makes sure each layer conforms to the shape perfectly. I then create a negative of the curved profile that will serve as the caul, applying pressure to the laminations and pushing them into the form. This negative takes into consideration the thickness of the bent piece so that the curves match up perfectly with no dead space, once the stack is placed between them. I also like to use cork on the bending surfaces to help distribute the pressure evenly. The entire form is covered in packing tape to prevent the glue from sticking. Wax is a good idea as well for a belt and suspenders approach.
The strips are usually cut at the bandsaw, a tool that excels at resawing. A table saw could also be used. How thin you cut the strips depends on a number of factors, include the wood’s natural bending properties, the severity of the curve, and your tolerance for springback. 1/8 – 1/4″ is common but I usually end up somewhere around 3/16″. Once the strips are cut, they need to be smoothed out and brought down to final desired thickness. I usually use my drum sander for this task but a planer can be used as well.
There are many glues on the market and just as many schools of thought about which glue works best. In my opinion, a good rigid glue line will provide the best long-term results. That would be resin glues like DAP Weldwood and Unibond 800 and also epoxy. But many folks will use standard PVA glues in their laminations with no trouble at all. In fact, that’s what Sam Maloof himself used in his chairs so I decided to try it on mine. I used Titebond Extend for the extra working time. I’ll let you know in a few years if it decides to de-laminate. The glue is applied to each strip using a glue roller for expedience.
Once the laminates are in the press, leave them under pressure overnight. The next day, you might need a mallet or dead blow hammer to pop the workpiece out of the form. At that point I usually sand the excess glue off the surface and then joint and plane as needed.
Once you understand how this process works, it really opens up the playbook of design possibilities! Give it a shot!