Woodworking Tips
Woodworking Tips Index
Wood Movement, Part 2
This is the second part of a two-part series on Wood Movement. Last week, I mentioned that wood "moves" because it acts like a sponge -- when the surrounding air is damp, wood absorbs moisture and expands. When the air is dry, it releases moisture and contracts. This movement can be as much as 1/8" across a 12"-wide hardwood board. And there's nothing you can do to stop it. What you can do is design and build projects to allow for this movement. Here are a couple examples:

A wide, solid-wood kitchen table top can expand or contract up to 3/8" across its width. To deal with this, think of a table as two separate units -- a leg assembly and a top. If the top were fastened so it couldn't expand or contract, serious damage would result. If the top expanded, it could push the leg assembly apart. Or if it contracted, it could crack and split.

When building a table, the top must be allowed to move freely. But it also must be fastened in a way that the top is held down tight against the legs and aprons.

There are a number of ways to fasten a table top to a leg assembly. One method we use is to cut little "L" shaped blocks. Each block is like a small hand with a finger. The block is screwed to the bottom side of the table top. And the finger fits into a groove cut in the inside of the table's aprons. As the top expands and contracts, the block holds the top down, but the top is still free to move because the fingers aren't glued in the groove. The fingers can move in the groove and still hold the top in place.

Another method we like is to use a figure-8 metal fastener that's made just for this purpose. These fasteners are available through many woodworking mail order catalogs. (One loop of the figure-8 is screwed to the top edge of the apron, the other loop is screwed to the bottom of the table top.) When attached to the aprons at the ends of the table, they can pivot back and forth with changes in the table's width.

The air inside a lidded compartment (such as the inside of a blanket chest or hope chest) usually contains a different amount of moisture than the air outside the compartment. The relative humidity outside the compartment is almost always changing. While the relative humidity inside changes very little because the top is rarely opened. This can cause a problem.

When the relative humidity is low (for example, in a heated house during the winter), the wood cells on the outside of the lid will start to dry out and shrink. Since the air inside the closed compartment contains more moisture, the cells on the inside face won't shrink.

If the moisture imbalance is allowed to continue, the lid will cup. And if it remains cupped for a long time (like all winter), it's unlikely that it will ever completely return to normal. One way to avoid this is to open the lid every few days. But for most of us, that's not very practical.

Another way is to design the chest so air can flow through it. But that defeats the purpose of trying to keep the moths out.

The best method we've found to prevent cupping is to screw (don't glue) a pair of cleats across the inside face of the lid. Pre-drill over-size screw holes in the cleats to allow some movement. This way, the cleats hold the lid flat while allowing it to expand and contract.

Go to Tip #59
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