Woodworker's Journal 2007-Winter, страница 76

Steam Bending Basics

With a steam box, a bending form and the right technique, you'll be on your way to creating curved shapes. Here's how.

A door hasp provides a tight seal.

Paint the inside of the box to protect it from moisture.

Dowels held in sidewall holes elevate the bending stock so it's exposed to steam on all four sides.

a basic worker

lthough it may seem like a skill that only professionals can master, steam bending really is technique that any wood-learn. A shop-built steam box and a bending form can be made from common materials.

Steam-bent wood is generally strong since its grain follows the length of the curve. By comparison, curved components cut from solid planks tend to contain weak sections of short grain (see Figure 1, below). Avoid lumber with severe grain runout, however, as it will usually crack.

Figure 1: A curve bandsawn from solid wood (left) usually has significant grain runout, which weakens the piece. The grain lines on a steam-bent piece (right), on the other hand, are continuous, making it the stronger alternative.

Some Basic Wood Technology

Wood consists of cellulose fibers bound together by a natural adhesive called lignin. The first time a piece of wood is heated and cooled, the lignin permanently loses elasticity. For this reason, kiln-dried wood, which has already lost much of its pliability in the kiln, is not your best choice for steam bending. Air-dried wood is the ticket: when it's steamed and bent, the lignin helps lock the new curve in place.

Wood subjected to steam in an enclosed container heats up and absorbs moisture, dramatically increasing the flexibility of its fibers. When it reaches sufficient pliability, the wood should be rapidly bent onto a sturdy form. After cooling and drying the wood will retain its new shape, although varying degrees of springback can occur, depending on the character of the wood, the amount of steaming and the rapidity

A drain hole allows the condensed water to drip from the steam box into a bucket below.

with which the hot wood was bent to the form.

Some species of wood lend themselves to bending more readily than others. For example, white oak, red oak and hackberry are particularly good, while mahogany and hard maple are unsuitable. Select straight-grained pieces to reduce the likelihood of fracturing.

Building a Steam Box

Making your own steam box is easy, as shown in the illustration above. For most bending needs, an exterior dimension of about 7" x 7" x 60" will do nicely. Follow these additional guidelines:

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