Popular Woodworking 2002-02 № 126, страница 70

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In the same way, dried lumber is pre-shrunk so that it won't later contract and cause you grief.

So.. .how dry should the lumber be? The best moisture content (commonly referred to as MC) is one that places the lumber in equilibrium with your home's environment. Having the lumber in balance with the relative humidity of your home will dramatically reduce the amount the contraction (or expansion) that occurs. Want a hard number for moisture content? In most areas of the United States this equates to a 6 to 8 percent moisture content.

A common misconception oating around is that only kiln-dried lumber is suitable for fine woodworking. But the process used to dry the lumber is not as important as the final moisture content of the wood. In fact, I dry most of the lumber I use without using a kiln or other sophisticated machines. Here's how I do it: I usually buy my lumber as a great-looking log directly from a commercial logger. Buying the log allows me to get wide and thick boards that would cost a fortune and cost me hours of scrounging around a lumberyard. I have it custom-cut to my specifications, seal the ends of the boards, and stack them in a shed for drying. To promote drying, the boards are separated with sticks during the stacking. This allows air to circulate around the boards. After the lumber has dried in the shed to 15 percent MC (in equilibrium with outside relative humidity in my area), I store it on a rack in my shop to further dry it. Before using the lumber I always check the MC with a moisture meter. This small electronic device works by measuring the electrical resistance and converting it to MC. Lumber measuring higher than 8 percent MC

Quarter Sawn

remains in the rack for more drying time.

a Buying Dry Lumber

If you typically buy your lumber from a dealer, make certain that it is dry. Although most commercially available lumber is kiln dried, it may not be in the 6 to 8 percent MC range that is required to limit wood movement problems. Why? Most storage sheds at lumber yards are not heated. So if kiln-dried lumber has been stored in the shed for more than a few weeks, it's likely to absorb water vapor from the warm, moist, outside air. (Remember, wood is hygroscopic and it seeks equilibrium with its environment.)

So how can you avoid using lumber with a high MC? Always check it first with a moisture meter; for an accurate reading, also check the inside by sawing off a few inches from one end. Additionally, it's a good idea to always store lumber in your shop for a few weeks before working it. This allows it to acclimate to the environment in your shop and avoid potential movement problems.

^ Lumber Selection

Another way to minimize the natural contraction and expansion of wood is to select lumber that is naturally more stable. Because

Flat Sawn

wood moves different amounts in relationship to the annual rings, its tendency to move depends on how it is sawn from the log. Sort through a stack of boards and note the orientation of the annual rings on the ends. On most boards the rings form arcs that are tangent to the board's face. This is called tangentially sawn lumber (also called flat sawn). Lumber with rings that are roughly 90° to the board's face is referred to as radially sawn (often called quarter-sawn).

Radial lumber is much more stable than tangential. Because of this it's sometimes the best choice for broad, flat surfaces. For example, acoustical guitar bodies are made from radial-sawn lumber. However, radial lumber is usually more expensive when and if you can find it. Also, most people find the flame figure in flat-sawn lumber to be more attractive than the straight grain found in quartersawn boards.

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4l Wide Boards Look Best

My old high school woodworking textbook said to never use boards over 6" wide. Wide boards, the author stated, would have a greater tendency to warp. But wide boards have continuity of grain, figure and color that can't be matched by gluing several boards together. Drawer fronts, desk lids, door panels and small tabletops will all turn more heads when constructed from one wide board. In fact, I commonly use boards up to 24" wide. So, how do I keep them flat and prevent surface cracks during the dry winter months?

I begin by allowing them to dry (pre-shrink) and warp before I use them. Afterwards, I flatten them to remove the warp before planing them to thickness. Then I use construction techniques that allow for inevitable wood movement.

Another old saw from woodworking textbooks is to alternate the annual rings when gluing up a tabletop. That is, put one board with the rings facing up, the next with the rings facing down and so on. This does little good. You're better off finding the best face of each board and choosing that side for the top.

^ Constructing for Movement

One of the best ways for dealing with seasonal wood movement is to use construction methods that allow for seasonal di-

MOISTURE METER

Few woodworkers own moisture meters, a tool that can save you from an occasional disaster.With this particular model, you merely set the meter on your wood to get a reading. Less expensive models have pins that you insert into the wood you're checking.

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