Popular Woodworking 2002-02 № 126, страница 69

Dealing with

Wood Movement

Eight ways to ensure the furniture you build will last several lifetimes.

tenons on the ends ■ are slightly narrower than their mortises

A few years ago I built a new kitchen table for our home. It was a classic design — a richly figured curly maple top supported by a stretcher base with turned legs. In my rush to complete the project I forgot to finish the underside of the top.

The next day the relative humidity rose dramatically as a storm front approached; I entered the shop to find the top severely cupped, in spite of the stiff breadboard ends added to keep the top flat. In fact, the breadboard ends were warped, too.

Fortunately, I knew what to do. The moment the sun came out I placed the top outside with the unfinished convex side facing up. After an hour or so out in the warm sunshine, the moisture

evaporated and the top returned to its former flatness. To prevent a repeat of the scenario, I quickly applied a couple of coats of shellac to seal the surface.

It's a fact of life that all woodworkers must face: Wood expands and contracts with changes in seasonal humidity. That's because wood is hygroscopic. What's hygroscopic? Simply a technical term for the concept that the wood's cell structure is similar to a bundle of straws, ready to draw moisture from the surrounding air. As the relative humidity goes up, wood absorbs the water vapor and expands. Days (or even weeks) later, it contracts as the relative humidity drops. To further complicate the prob-

by Lonnie Bird

Lonnie Bird (lonniebird@earthlink.net) builds period furniture and conducts woodworking seminars in Dandridge, Tennessee.

lem, wood only moves across the grain. (Although technically speaking it does expand lengthwise with upswings in humidity, the amount is so minimal that you can safely ignore it.) So when you join two pieces of wood at a right angle, there's potential for problems because seasonal movement occurs in opposite directions. And because wood surfaces absorb and release moisture at different rates, a board will usually warp as it's exposed to changes in humidity. Fortunately, there are a number of steps you can take to minimize the cracking, warping, opened

Rule 6:

Breadboard ends are a good way to keep your tabletops flat and to hide end grain.

joints and other problems associated with seasonal wood movement. Let's take a quick run through each of them.

J Begin With Dry Lumber

You'll avoid many of the potential problems associated with wood movement if you use properly dried lumber. That's because as lumber dries, it also contracts or shrinks. Obviously it's best if it does most of the shrinking before you construct that chest of drawers for your wife's anniversary present. Think Levis jeans: They're pre-shrunk to avoid more shrinking during laundry cycles.

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Popular Woodworking February 2002