When it comes to working with wood, one of the most pernicious problems is the risk of expansion or shrinkage in the wood you’re working with. After all, the best woodwork machinery in the land won’t save you once wood starts cracking or cupping. As such, it can be handy for woodworkers to get a handle on what exactly causes shrinkage and expansion, and how they can work to avoid these problems.
The factor that drives expansion and shrinkage is the amount of moisture. Wood is an organic material, designed for the purpose of taking in and releasing moisture – and this process continues even after it has been cut. If a piece of wood has less moisture in it than the surrounding environment, it’ll take in water from the atmosphere – which leads to expansion. When wood takes in enough moisture, it starts to lift at the edges – this is called cupping.
If a piece of wood has more moisture in it than its surroundings, it’ll release water – which causes shrinkage. If wood shrinks enough, it can cause quite serious cracking. Either one of these phenomena can render a piece of wood unusable – so how can woodworkers prevent this?
One of the ways that disastrous cupping or cracking can be avoided is through some clever placement of wood grains. If you’re gluing boards of wood together, check the end grains to see which direction they’re running in. When it comes time to glue them, place the pieces on top of each other such that the grains are moving in opposite directions.
Wood shrinks and expands across the grain, not along it, so if you glue pieces together with the end grains going against each other, the shrinkage or expansion can balance out.
If you’re building something out of wood, one of the ways that you can mitigate damage from cupping or cracking is by aligning pieces so that the grains face the same direction. If you’re building something with several sides – like a cabinet – aligning the carcass this way will mean that the wood shrinks or expands in a more even manner. While this should help to avoid serious breakage or misshaping, it can make it hard to use any drawers you might insert into your piece.
One of the simpler ways to avoid excessive shrinkage or expansion is to use plywood rather than dimensional wood where possible. Plywood is comprised of several smaller pieces of wood that are laid in alternating directions, and it tends to be laminated. This means that it is less susceptible to change based on moisture levels, and then any changes are stymied by the arrangement of the wood.