Bending Wood

November 25, 2011 Comments Off on Bending Wood

Last weekend I attended a one-day introduction to bending wood at the Northwest Woodworking Studio (Studio). Jeff O’Brien, a graduate of the resident mastery program at the Studio, taught the class. Jeff, in addition to teaching classes at the Studio, builds custom furniture out of his home in West Linn, OR, through his company Dogwood Design. In the class we explored four different methods of bending wood – laminate, steam, vacuum press, and hot-pipe. While eight hours isn’t enough time to fully explore each technique, it’s more than adequate to get the basics down – and to decide which method would be most appealing for your own work.

Laminate bending was something new to me. Jeff is drawn to modern design, and free-flowing, organic curves are a signature of his work. Jeff’s coffee table below shows an example of bent laminations. The stretchers, in this case douglas fir, are comprised of several laminated strips that are glued together and then clamped into a curved mold, or form, and allowed to dry to that shape. You could just bandsaw that curve from a wide piece of stock, but it’s much more elegant and visually appealing to have continuous grain along the entire length of the stretcher rather than being interrupted at each point in the curve. To do this type of lamination you need a bandsaw, preferably a good one with a sharp blade. Choosing your stock is also important. While almost any wood will work – anything will bend if thin enough – something straight grained will result in the nicest finished piece. The type of glue is also important. You need a stiff glue, not one that retains a little elasticity after curing. Aliphatic resin fits this description (Titebond I), while PVA glues (Titebond II and III) do not. Hide glue and polyurethane glue also cure too flexible (like everyone else in the world Jeff hates Gorilla Glue). A quick experiment will bear this out. Place a little bit of each type on flat plastic lids and spread thin. When dry, peel each from the plastic and bend them. Titebond I will break in two, whereas the more flexible Titebond II and III will bend without breaking. The stiffer glue is advantageous in bent laminations because it reduces the chance for springback after drying. A specialty two-part veneer glue that Jeff uses is Unibond 800. Like an aliphatic resin glue, it cures very stiff. Jeff also discussed his methods for building his forms, including the one pictured below. A flush trim router bit and careful layout are your best friends here.

Most woodworkers have seen vacuum presses, either on tv or the internet. This was the first time I had actually seen one used in person. Jeff gave a quick demo on how a door panel might be pressed into a curved shape using an old form he constructed for a piece in his own home. The form was simply some 2x material with foam installation stuffed between the wood ribs to keep the vacuum bag from collapsing into the empty space. He noted that if he knew then what he does today, he would have made it with several improvements, but even this simple form works well. The vacuum press itself – see picture below – is more involved and will take a little more time and effort (and dollars). The panel material is comprised of a flexible plywood material that will bend along one plane but not the other. The outer show pieces are veneers of your choice.

Steam bending is a method we are all familiar with, and it is undoubtedly the oldest technique out there for bending wood. The setup is rather simple. Steam is collected in a small closed chamber, where the wood is allowed to cook for a while. The combination of heat and moisture make the lignins in the wood more elastic. The general rule of thumb is one hour per inch of wood. After which time, you remove the wood and then frantically get it into your form – you have about a minute before the wood no longer wants to bend. It’s usually about this time where the phone rings or someone needs something urgently. We also discussed compression straps, and how they function to ease the stress on the wood at the outside arc of the bend. In the picture below we are wedging the bend into a form, but a simple board with pegs located at critical points would also work. There are rules to steam bending. Straight grained hardwoods work best. Any runout in the grain will likely result in a split – riven wood is your best bet. The wood must be green or air-dried, not kiln dried. Softwoods, such as pine, fir, and cedar, are usually not good candidates for steam bending.

The fourth method we tried was hot-pipe bending. This was based on an article by Michael Fortune in Fine Woodworking a few years ago. This offers limited bending of wood, but it’s perhaps the simplest to set up. A propane torch is lit to heat up black gas pipe that is affixed to a vertical board. Insulation is placed between the board and the pipe to protect the wood. The pipe heats up to the point where a few drops of water will bead off its surface. Then you can place a thin piece of wood (3/8” max) over the pipe and slowly bend the wood over the pipe. The wood is constantly kept wet with a sponge to avoid charring. In the article Fortune makes simple salad tongs using this technique. We tried the same with two different types of wood – maple and honey locust. The honey locust bent nicely, while the maple did not.

For a one-day class, we covered a lot of ground. I had never bent any wood (intentionally) before, so it was all new to me. Even after only one day, though, I came away with a good understanding of how curved furniture is made. Steam bending and bent laminations are the two methods I’ll most likely pursue should I take the leap into non-linear pieces.  I don’t see curved panels in my near future, so the vacuum press doesn’t appeal to me as much. That method also requires a fair bit of equipment and money. The hot-pipe method was peculiar, to say the least. Unless I see myself doing lots of very small pieces, I probably won’t try that technique – and I try to avoid open flames in the shop. Overall, it was a fun class. I’m lucky to have the Studio less than 2 miles away. If a picture is worth a thousand words, direct one-on-one instruction is worth a million.

More:
Jeff O’Brien was featured in the November-December 2011 Woodworker West magazine – that’s his table on the cover.

Joe Woodworker has lots of information about working with veneers, including how to build your own vacuum press.

Lee Valley has a steam bending instruction booklet that is free to download here.

If you need a bite to eat when visiting the Studio, check out Bunk Sandwiches just a few blocks away. It’s a great alternative to Subway (which is also a few blocks away).

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