DJ Delorie replies: Wood moves because of stress and strain within the wood. Changes in humidity cause the types of movement we’re used to, but also the tree is pre-stressed when it grows – that keeps it from falling over. Until all this stress is relieved, wood has the potential to move.
When a piece of wood is milled and dried, the drying process changes the stresses in the wood, causing movement. Even after kiln drying, there is still some moisture in the wood, plus the moisture content is not consistent throughout the wood – usually there is less moisture near the surfaces than towards the center, since moisture on the surface can more readily escape. These differences are minor, but enough to cause movement.
When the lumberyard surfaces the wood, two things happen. First, some of the stressed wood which is helping hold the board’s shape is removed. Think of this like removing clamps – if uneven clamping pressure was required to hold the board flat, removing the clamps results in uneven spring-back.
Second, fresh surfaces are exposed to the air. Since it’s likely that those surfaces will not release additional moisture at the same rate, again, this may cause wood movement.
When milling wood to final width and thickness, it’s important to keep that in mind. If it’s critical that the surface be as straight or flat as possible, schedule some time to let the wood move. Perform most of the surfacing, taking the same amount off each side to help balance the internal moisture, stopping before you reach final dimension. Then, let the wood rest and acclimate for at least a day, but as much as a week or two, to let it move as much as it can. Then do the final surfacing, which hopefully will result in only a minimum amount of additional movement.
Marty Milkovits replies: Warped lumber is most often the result of a change in the moisture content of the wood. Even if the wood was surfaced at 7% moisture and then subjected to high humidity for even a short period of time, it is going to move. How much it moves depends of the grain orientation, compression or tension in the board, and if the board was allowed to free float during this moisture change.
Dave Emerson replies: The fact that the lumber is S2S has nothing to do with it’s warping or twisting. There is no sure prevention for warping or twisting other than quarter sawing. However, grain irregularities, runout, etc. will indicate a propensity to warp or twist. Lumber that is flat will sometimes change when subject to humidity change or milling which releases stress from poor drying.
To insure stability in my cherry, I dry it myself. Maple – I buy from dealers I trust. The cherry I buy right off the saw in early spring and sticker it every two feet in a barn with good air movement. Stickers must be dry. It will dry slowly at first in early spring – very important. I like two seasons for general use, pull during low humidity. For furniture one winter per inch in dry heated conditions is needed.
Guy Senneville replies: Because it’s junk! All kidding aside, it was flat and square when it left the planer. This problem could have occurred for a few reasons. It may have not been stored properly, it may have not been completely dry when it was milled, or humidity changes occurred before you obtained it. It is always best to bring lumber into your shop to acclimate a while before you use it. If you do not have the ability to mill lumber to size, I would suggest purchasing enough extra thickness to square it up by hand before use.