Woodworker's Journal 1985-9-4, страница 14

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Restoring Antiques

More About Warped Boards

by John W, Olson

As discussed in my two previous columns, wood is made up of hollow cells which expand as they absorb moisture and shrink as they dry out. A board with more moisture on one side than the other will probably warp because the cells on the side with the greater moisture expand more than those on the opposite side. This type of uneven moisture content is the cause of most warp, and it can usually be solved by techniques that involve wetting or drying one side or the other to balance the moisture. Last issue in this column, I talked about several of those techniques.

However, uneven moisture content by itself is not always what causes a board to warp. A tree normally grows reasonably straight up and down, but for various reasons some will grow with a distinct tilt or lean to one side. As a leaning tree grows, stresses develop in the wood fibers of the trunk, and boards cut from that trunk wiil include those stresses. When combined with uneven moisture content, such stresses often create a warping problem that can't be solved by the usual methods. Generally, the answer is to use some sort of mechanical means to physically straighten the board, or to relieve the internal stress by removing some of the stock.

Cleats

One or more wooden cleats screwed to the stock can be an effective way to deal with a stubborn warp problem. They are commonly used to flatten wide tabietops and carcase sides.

There are several things to consider before cutting the cleat stock. Since the warped stock is pulled to the cleat, you will need a flat cleat to make sure that you end up with a flat surface. Also, the cleat must be strong enough to straighten the board without itself bending. Try and locate the cleats where they are least likely to be seen. If one large cleat looks unsightly, try substituting two or more smaller ones as their combined strength can be just as effective. Before assembly, 1 like to chamfer the exposed corners and sand all surfaces thoroughly, then apply a final finish that matches the piece.

Don't use glue if the cleats are installed across the grain (as is usually the case) and be sure to slot the screw holes. The stock must be free to expand and contract with changes in humidity.

Rods

I once encountered an old bench with a seat that refused to remain flat. The seat measured 1 '/₄ in. thick by 14 in. wide by about 32 in. long. Cleats would look unsightly so I fell another technique was in order.

The seat was clamped flat then, using a long ^lA in. diameter drill bit, I bored three holes along the 32 in. seat,

14

The Woodworker's Journal