Woodworker's Journal 2007-Winter, страница 77

• Use exterior-grade plywood for the box since its waterproof glue won't degrade from the steam.

• Paint the box's interior to protect it.

• Space a line of dowels across the box's width to elevate the wood above the condensation runoff and to promote the flow of steam.

• Seal one end of the box, and hinge the other end for easy access.

• Drill a small hole through the bottom at one end to drain condensed steam.

• Drill a second hole, at the other end, for the steam hose entrance.

The heat source for your steamer can be an electric burner, a wood fire, a camp stove or whatever you have that will boil water. For safety's sake, remember to use your equipment outside if it generates a flame or has exposed hot coils.

The water container should hold several gallons and have an access hole small enough to be plugged with a large cork or rubber stopper. If the container is made of iron, make sure it's galvanized or enameled, otherwise the steam will probably stain your wood. By drilling a hole through the stopper you can hold the connecting hose in place. The hose should be made of rubber, plastic or copper of at least 1/2" in interior diameter. Thin-walled plastic tubing is not recommended as it collapses when the steam runs through. In use, be sure to set the steam box at a slight incline so condensation flows out the drip hole.

Steaming Your Wood

The first step in steaming your wood is to fire up the burner and

Figure 2: Make a banding strap using metal between 1/16" and 3/32" thick, and bolt angle irons to its ends so the steamed wood fits tightly between them.

boil the water. Once steam begins filling the chamber, put the wood in the box and close the lid. Steam the wood until it's pliable, then, wearing gloves for protection, remove the hot wood and rapidly bend it to the form.

How long should wood be steamed? There is no precise answer, so it's a good idea to include a couple of test pieces in the box for experimentation. Species, moisture content, and the intensity of the steam environment are all factors. Here are three rules of thumb for steaming air-dried lumber:

• Keep wood with a moisture content below 20% in the steam box for about an hour and a quarter per inch of thickness.

• Steam wood with 20-30% moisture content for 45 minutes per inch of thickness.

• Wood with a moisture content in excess of 30% needs only about 1/2 hour of steaming per inch of thickness.

Figure 3: A banding strap is often used to force the entire piece of wood into compression, decreasing the possibility of a split on the tension side of the bend.

When pulling hot wood from the box, work rapidly to place it in the bending form. The wood begins to cool instantly, and it's amazing how quickly flexibility diminishes. If possible, leave the wood clamped in the form for several days.

There are a variety of designs for bending forms. The most common is a one piece form, usually built with layers of plywood, to which the steamed wood is clamped (see Figure 2). Another type of form consists of two mating parts between which the work is sandwiched. Since steamed wood almost always has some springback, cut the form to a slightly more severe bend than you want to end up with. However, springback is unpredictable, so until actual test bends are made, the exact amount of over-bend required is anyone's guess.

When you bend a piece of wood, an imaginary line up the center stays constant in length (see Figure 3), while the outside of the curve gets longer (tension) and the inside becomes shorter (compression). Wood is fairly good at compressing, but splits apart readily under tension. It's the tension factor that limits the degree of curvature in a simple bend. Many woodworkers reduce the chance of splitting on the tension side of the wood by using a bending strap — a flexible steel strap about 1/16" thick and as wide as the bending stock, with end stops spaced to enclose the exact length of the workpiece. As soon as the wood is pulled from the steam box, the strap is fitted to the tension side of the stock. With the outside curve unable to expand, the entire piece of wood is forced into compression, minimizing the chance of splitting. @ _/

Winter 2007

77