Every woodworker has experienced the problems that accompany uneven shrinkage or swelling of wood stock. When wood dries unevenly, it can warp, cup, bow, twist, kink or check. The key to building quality woodworking projects is to recognize when any of these problems might affect your work and to prepare for the inevitable.
Keep in mind that the following information relates to dimensional wood stock, both hardwoods, and softwoods. Plywood and other manufactured wood stocks are less susceptible to moisture related movement.
Why Does Wood Shrink or Swell?
When it is still alive as part of a tree trunk, wood is basically a series of thin tubes that circulate sap & fluids from the roots of the tree to the upper branches. Stock found closer to the outer portions of the trunk are more active and pass more sap than the heartwood found closer to the center of the trunk.
Your woodworking projects will react accordingly. Projects built from heartwood are going to be far less susceptible to shrinking and swelling than projects made from sapwood, as the heartwood is less likely to retain moisture.
Acclimatization of Wood
Ideally, wood should not only be dried properly before building a project, but it should become acclimatized to the environmental surroundings in which the project will be used permanently. For instance, a project that is built in a humid environment like Florida will likely react quite severely if it is to be put to use in a dry environment like the Arizona desert.
Instead, the wood should be purchased and stored for a few weeks in the location where the final project will be used before beginning the project. This way, the wood will be far less likely to move after the project is completed.
Radial, Longitudinal, and Tangential Shrinkage
All wood stock is three dimensional, and knowing how wood shrinks and swells along these three dimensions will help you prepare for problems.
Longitudinal (along the long axis of the stock) shrinkage is very minimal. In most cases, from freshly-cut green wood to properly oven-dried, you can expect only a very slight amount of movement along the length of a board.
To determine the radial and tangential directions of a piece of stock, you need to look at the end grain. The radial direction is perpendicular to the growth rings, where tangential is parallel to the rings.
Keep in mind that movement along the tangential axis is almost always going to be greater than along the radial direction. By looking at the direction that the growth rings are oriented in the board, you can get an idea of how the board will react as the tangential movement exceeds the radial movement.
Tip: Movement is one of the many reasons why quarter-sawn lumber is so sought after (and why is expensive). Due to the way quarter-sawn wood is hewn from a log, the growth rings are relatively square to the sides of the board. As such, the board will swell or shrink relatively evenly across the entire board.
How Does This Information Help?
Knowing how a board may shrink or swell will help you determine where (or whether) the board should be used on a woodworking project. For instance, if you're gluing up a table-top from a series of tangentially-cut boards, you're likely to experience some cupping (the center rising away from the ends, causing a slight arc in the width of the board) as the wood swells or shrinks.
If you have a number of tangentially-cut boards that may experience similar cupping, flipping every other board upside down (so that one will cup upwards while the next will cup downwards), you can diminish the possibility of a large bow in the table top.