Wood shrinks and swells with changes in the relative humidity (the amount of moisture in the air relative to the temperature). When the relative humidity goes up, the wood absorbs some of this moisture and swells. When the relative humidity goes down, the wood loses moisture and shrinks. Since the average relative humidity in much of the world is lower in the winter than it is in the summer, wood tends to shrink each winter and swell each summer.
For your information
On the average, the moisture content of wood changes l percent for every 5 percent change in the relative humidity.
This movement, although it may seem slight, is extremely important to woodworkers. To see why, try this experiment: Using waterproof glue, attach a small, narrow board to a wide one so the grain directions are perpendicular. Set this assembly outside on a rainy day and the boards will separate, despite the waterproof glue. As the wide board expands in the opposite direction of the narrow one, the joint is subjected to an increasing amount of shear stress. Eventually, it breaks. More joints fail from wood movement due to changes in moisture than from abuse and neglect. You must take this movement into account and accommodate it in your joinery. Wood moves in three planes, and it moves differently in each plane (SEE picture 1-9.)
All three types of motion are relative to the direction of the wood grain and annual rings:
• . Longitudinal movement is parallel to the wood gram.
• Radial movement is perpendicular to the annual rings and to the wood grain.
• Tangential movement is tangent to the annual rings and perpendicular to the wood grain. Wood is fairly stable longitudinally. An 8-foot-long spruce board will shrink less than 1 /16 inch along its entire length, from the time it’s cut “green” (and about as saturated with moisture as it will ever be) to the time it’s dried to 7 or 8 percent moisture content (dry enough for cabinetmaking and furniture making). Consequently, most woodworkers treat wood as if it were motionless along the grain. Across the grain, its a different story: Some woods may move up to 1/4 inch for every 1 foot of width or thickness.
Furthermore, there is a big difference between radial and tangential movements. Most wood species will shrink or swell about twice as much tangent to the annual rings as perpendicular to them. “Tangential/Radial Movement of Common Wood Species” on the facing page compares the movement of several species along these different planes. As the ratio of tangential movement to radial movement becomes greater, it becomes increasingly important that you properly align the tangential and radial planes of adjoining parts. The disparity between radial and tangential movement causes yet another type of movement to consider as you choose the joinery Depending on how a board is sawed from a tree, it may deform as it shrinks and swells. For example, if the annual rings run from side to side in a square table leg, the leg may become rectangular as the wood shrinks faster from side to side than from front to back. If the rings run diagonally from comer to corner, the leg may shrink to a diamond shape. A round dowel becomes an oval as the wood shrinks, and a flat board cups in
1-10 Because the radial and tangential movement of wood is uneven, boards tend to deform as they go through an annual moisture cycle. The way a board will deform depends on how it is cut from the tree.
the opposite direction of the annual rings. (SEE FIGURE 1-10.) Sometimes you can use joinery to help control this deformation; other times you must simply plan for it. This is a lot to think about. Joinery would be far simpler if wood were the relatively stable building material that many beginning woodworkers take it to be. But it’s attention to details such as wood movement that marks the difference between a true craftsman and a novice. To properly join wood, not only must you plan a joint system that allows the wood to move, but you must also “read” the wood figure as you make each joint. Study each board, then orient the grain and the rings so the anticipated movement creates the least possible stress on the joint. There are several simple joinery techniques that help reduce stress and/or control deformation caused by wood movement. Use those techniques that apply to the structure of your project.
Try this trick.
To help visualize the wood movement in a joint, sketch the boards as they will be assembled, showing the wood grain and annual rings. Mark each board with a small arrow to indicate radial movement and a large arrow to indicate tangential movement. Try to orient the wood figure so the large arrows are all parallel.
Orient the wood figure to make each part as stable as possible. Since the longitudinal plane of a board is the most stable, align this plane with the longest dimension (the length). Align the radial plane with the next longest dimension (the width), and the tangential plane with the shortest dimension (the thickness). This may not always be possible, since most boards are “plain-sawn” from logs so the tangential
plane is aligned with the width. If the alignment of the tangential and radial planes is critical, you may have to pay a premium price for “quarter-sawn” lumber, in which the radial plane is aligned with the width. Or you can rip a board into narrow strips and glue it back together with the rings properly aligned. (SEE FIGURE 1-11.)
I -II Usually, the larger the board, the more critical it is that the longitudinal, radial, and tangential planes all be aligned for maximum stability. This is why furnituremakers glue up “butcher block” table tops from narrow strips. Notice that each strip has been turned so the annual rings run top to bottom. The radial plane of each strip is aligned with the width of the table top. The tangential plane – the most unstable dimension of each strip- is aligned with the thickness, where stability matters least
Orient the wood figure so the parts move m unison Whenever poss1ble, join the boards so the wood swells and shrinks in the same direction When the wood gram must cross at right angles, align the tangential planes. (SI·E FIGURES 1-12 AND 1-13.) Cut large boards into smaller parts. When you must glue or otherwise fasten two boards With opposing wood gram, make sure they are as narrow as possible Without compromising the strength of the structure. (SI·E FIGURJ·S 1-14 AND 1-15.)
1-12 On the comer butt joint shown at the left, both the wood grain and the annual rings are opposed to one another. The joint will soon fat I. On the middle joint, the wood grain is aligned, but the annual rings are not – the tangential planes are perpendicular to one another. This joint wlll fail too, though not as quickly as the first. On the joint at the right, both the wood grain and the annual rings are properly aligned. This joint will last for a long time.
1-13 The wood grain on all three of these mortise-and-tenon joints is properly aligned. But on the joint at the left, the tangential planes are directly opposed on the broadest possible surface- where the cheeks of the tenon meet the sides of the mortise. This greatly diminishes the useful life of the joint. On the joint in the middle, the planes are in some-Couto Be BETTER planes are perpendicular to one another. This joint will fail too, though not as quickly as the first. On the joint at the right, both the wood grain and the annual rings are properly aligned. This joint will last for a long time. what better alignment. The tenon moves radially at right angles to the tangential movement of the mortise. But the joint at the right shows the best possible arrangement- the tenon moves radially at right angles to the radial movement of the mortise, and the tangential planes are aligned.