Bending wood to create circular window frames, arched doorways, drum hoops or other curved pieces is easy to do and a lot of fun. For DIYers, there are two basic techniques which yield good results and require simple equipment.
The first method is dry-bent lamination consisting of strips of wood bent around a curved form made of three-quarter-inch plywood, stacked and glued together. The thickness of the form depends on the width of the laminates. For example, four pieces of three-quarter-inch plywood are required to create a form capable of bending three-inch wide laminates, a useful width for many projects. A three-inch slice from a polymer gas or sewage pipe with a large outside diameter can also be used as a ready-made bending form.
Readily available woods with good bending classifications include ash, red oak, yellow birch (not paper), cherry and beech. Provided the boards are knot-free and have been carefully machined (no tear-out), any of these species planed to 3/16-inch can be dry bent around a form with a two foot or greater diameter.
As the word lamination suggests, a curved member is comprised of two or more laminates bent around a form and glued together. To make a hoop or circular frame, bend the first laminate around the form, leaving about an inch of extra length to cut off. The formula pi x diameter can also be used to calculate the length or, if you are mathematically challenged like me, place a rope around the form and mark the length plus an inch with a black felt pen.
Cut the laminate to exact length with a hand or power saw and then bend it around the form; the ends should fit together to form a butt joint. The second laminate will be slightly longer; the length should be calculated with the first laminate held in place. (This is where an extra pair of hands can help.) Continue to cut laminates until you reach the desired thickness of your frame — four pieces of 3/16-inch laminate equal three-quarter-inch, a standard thickness for most cabinetry work.
With the first laminate held in place around the form by a helper, coat one side of the second laminate with a paint brush with white or yellow, and then bend it around the first laminate so that the butt joint meets on the opposite side of the first joint. At this point, I clamp the laminates together by wrapping and tightening ratchet straps around the top and bottom of the outside laminate. The straps are about 1 1/2-inches wide, so together they put clamping pressure on all areas of the three-inch wide lamination.
It is possible to clamp all the laminates together in one go, but my experience suggests that this approach can lead to an unwieldy mess created by panic as the glue begins to set.
After an hour of setting time at room temperature, it is safe to remove the straps and to glue the next laminate in place. When all the laminates are glued, I leave the straps on the form for 24 hours to ensure the adhesive is completely set.
Lining the outside of the bending form with wax paper or coating it with paste wax facilitates the removal of the completed circular frame. During glue-up, excess adhesive can ooze between the bending form and the first laminate.
The edges of the frame can be cleaned up and made flush with a hand plane or a flush-mount router bit.
A second bending method uses steam or boiling water to make quarter-inch or thicker boards sufficiently pliable to bend around a tight radius. Ash is the best choice for this technique as it has an excellent steam or water bending classification.
The equipment consists of a 10-foot or longer length of four-inch ABS sewer pipe and an electric kettle to produce steam or a 20-litre canning pot in which to boil water.
For steam bending, glue a cap on one end of the ABS pipe with approved adhesive. Place the lengths on wood you want to bend inside the pipe and attach an electric kettle to the open end.
I modified my kettle for this purpose by fitting a one-foot length of copper pipe into the spout — the copper slides inside the ABS pipe and the gap between the two is sealed with a cloth rag. The rag acts like a safety valve as it allows enough steam to escape to prevent a possible, though unlikely, break in the ABS. Depending on the thickness and type of wood, it can take anywhere from 20 minutes to several hours of steaming to make the wood pliable enough to bend without breaking it. The best way to ascertain how long it will take the wood to reach this flexible state is to conduct 20-minute interval tests.
Be careful when you remove the test pieces as they will be very hot.
Try bending them around a pre-made form such as a doorway arch. If the outside face of the wood begins to splinter under bending pressure, you know it requires more time in the steaming tube.
When the fibres have absorbed enough moist heat, the test piece should bend around the form like a piece of heavy rubber, with no cracking or splintering. The wood should be clamped to the form until it has completely dried, about a day at room temperature. It may exhibit some spring-back when the clamps are released but this won’t be sufficient to cause a problem when the arch is joined to the rest of the door frame.
Water bending is similar to that of steam bending, the only difference is that a 24-litre or larger canning pot is required in which to boil water on a stove. Pour the boiling water into the open end of the ABS pipe and place the wood pieces to be bent inside, and then close the open end with a friction fit ABS cap. Wrapping the pipe in some form of water repellent insulation such as Roxul will prevent heat loss.
Similar to steam bending, this technique requires experience to learn how long to leave the wood in the ABS tube. Experiment with different woods and lengths until you are confident that you are ready to proceed to the final project.
Never use light plastic or iron pipe as the former will melt and the latter will stain the wood.
Have fun!