Bob LaCivita replies: To minimize wood movement when milling rough lumber into parts for a project, I first cut the pieces to length leaving them long by and 1˝ up to 6˝. This way I can cut away any snipe left by the planer. I then cut all my parts to width using a band saw with a fairly wide course blade. I leave the parts 1/4˝ – 3/8˝ wider than the finished dimension. If the rough board has a very bad bow to it, I will remove the bow by jointing or hand planing.
By sizing to a rough dimension, you are relieving the tension in the wood caused by the natural way it has grown or in the drying process. Most of us have experienced a wide board closing in around table saw blade. This will eliminate this for the most part and make cutting much safer. Once the parts are cut to rough size, I joint a reference face and edge and then plan the remaining face and saw the last edge.
Bill Thomas replies: The rule of thumb I learned in school for sizing rough stock was to cut all the parts 1˝ longer than finished dimension, 1/2˝ wider, and round up the thickness to the next standard quarter, ie. 4/4, 5/4, 6/4, 8/4, etc.
The next step is rough milling to an oversize dimension. It is very important to allow the wood to equalize to its new dimensions. Wood can be thought of as a sponge. The outside may be dry, but the inside contains moisture roughly equidistant from the outside. When the wood is cut and milled, some of that moisture is now closer to the surfaces, especially the ends, which creates an imbalance of stresses. That imbalance can cause the wood to go out of flat, sometimes dramatically.
It is critical to allow the stock to settle down to equilibrium before going further with it. How much extra to leave is a judgement call. If the pieces are milled close to their final dimensions, they will be also close to final equilibrium, but run the risk of moving beyond their final tolerance.
Conversely, if too much extra is left, the final milling may need further equalizing. How long the process takes depends on the species of wood, the thickness of the stock and the moisture content of the wood before cutting. There is no magic answer, but as much time as possible should be allowed.