The Basics of Wood Movement

Anyone who's been doing woodworking for a while knows that wood moves. But what exactly does this mean? Carl Hagstrom recently wrote a great, in depth article about why wood moves and it's effects. I'm not going to try and repeat the points made in this excellent article, but I will suggest some ways to design your projects to account for wood movement. Nothing is more frustrating than spending many hours on a projects, only to have it pull itself apart over the years.

One way to think about wood is as a collection of straws that are all bundled together. It is these bundles that form the grain that you see in the wood. These straws are what allows nutrients from the roots to make their way to the top of the tree and into the branches and leaves. However, these straws are not rigid, like your plastic drinking straw. Instead, they can stretch and swell in response to moisture. When moisture is absorbed into the wood, either from direct contact with water or humidity in the air, these straws tend to get bigger in diameter but they don't grow in length all that much.

By knowing how wood absorbs moisture, you can predict how it will expand and contract over time. Since the "straws" making up the wood get wider, but not much longer as they absorb water, wood tends to expand perpendicular to the grain much more than along the grain. This means that a board will have most of it's movement along the width, rather than the length. Of course, this assumes the board absorbs moisture evenly. Often, one side of a board will absorb more moisture than the other. This means that one face of the board gets wider than the other, causing the board to cup toward the dryer side.

With all this in mind, you can account for this movement when you design your projects. Generally, any time you are jointing two pieces where the grain is perpendicular (cross-grain), you need to think about wood movement. What follows are some common ways to handle wood movement in your projects:

Frame and Panel

In situations where a large panel is needed, but the dimension needs to remain relatively stable, such as a cabinet door, woodworkers have come up with the frame and panel. The frame, because it's fairly narrow, doesn't move much. However, the large panel is allowed to expand and contract within the grooves in the frame.

Table Tops

In the case of a table, it is not usually a problem if the top is a bit wider or narrower. However, the way it's attached to the base needs to take this movement into account. There are a few ways to handle this.

Slotted or oversize screw holes
When drilling the pocket screw holes or through any cross members, make the holes a bit oversize or else cut a slot.
Figure 8 fasteners
There is hardware made that looks like the number 8. One screw goes through one hole and into the table base. The other screw goes through the other hole and into the table top. The fastener is able to rotate as the top expands and contracts
Clips
This method involves cutting a slot on the inside of the table apron. Then, a wooden clip or a z-clip is inserted into the slot and screwed to the underside of the table top. The slot can either be a dado run the full length of the apron or shorter slots cut with a biscuit jointer or router.

Moulding

Often, base or crown moulding is attached across the grain of the sides of a case. Depending on how wide the side panels are, you may be able to get away with using just brads without glue. The nails will flex just enough to allow the wood to move a bit. If the side is wider, it is sometimes desireable to cut a dovetail or "T" slot in the moulding and attach a corresponding key to the case side. The moulding then slides onto the key.

These are just some of the ways to account for wood movement in your projects. What are the ways you design projects to handle the expansion and contraction of wood?

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