You've been there. You pick out the perfect lumber from the hardwood dealer for your weekend project—right size, right color, right figure. But when the weekend rolls around and you cut into that lumber, the board that rolls off is anything but straight and square. To minimize your stress in the shop learn to recognize, avoid, and counter the three main causes of stress in wood.
Fence-row trees lean toward the sunlight on their exposed side. Hillside trees struggle to grow in line with gravity. Windblown trees fight to stay upright. Though these awkwardly growing trees may look normal from the outside, they strain internally to do so. In your shop, boards milled from trees like these release their pent-up stresses immediately as you machine them, as shown above, bending in often unpredictable and frustrating ways.
Most lumber from high-volume hardwood dealers comes from managed forests containing few growth-stressed trees. If you do encounter stress, save the wood for small parts. The less wood in the part, the less pent-up stress and warpage potential. Cut oversize; then joint, plane, and sand to final shape.
Drying nearly always introduces some amount of stress in wood. In the kiln, the fast-drying outside of the board tries to shrink, but the slower-drying core prevents it, creating tension in the wood called case hardening. Most professional kiln operators introduce moisture (usually in the form of steam) toward the end of the drying process to relieve the stress on the outer portion of the wood. Unrelieved case-hardening acts much like growth stress in your shop, warping immediately after cutting, and should be handled the same.
Severe case-hardening results in the fibers of the wood pulling apart, creating gaps called checking, as shown below. Report any checking you encounter to your hardwood dealer. They'll want to know about improper drying, and will likely allow you to return or exchange the affected wood.
Even the most stress-free, perfectly dried wood suffers from movement caused by changes in moisture. As moisture is absorbed or expelled from wood, the cells grow or shrink. Improperly stored wood and poorly designed projects suffer from warping or even breakage due to moisture-induced changes.
These changes happen over time, so give your newly purchased wood a fighting chance. Cut and machine it to rough size as soon as you bring it into the shop, milling both faces evenly to prevent an uneven moisture gradient—where wood is wetter on one side of the board than the other. Then sticker and stack the wood on raised runners for a few days before you use it.
Also, build your project with movement in mind. Avoid gluing or screwing project parts cross-grained in a manner that restricts movement. Leave space in door stiles for raised panels to expand in higher humidity. And finish all sides of a project part evenly so changes in moisture happen at a consistent rate.