Ever cut into a tree and watch the board curve up away from the cant as the saw goes through it? Or have a portable sawmill band blade make a dip every cut in exactly the same place? Or wonder why you can’t make a straight cut, even with a new blade and the tension is cranked tight? Even if you get straight cuts with the mill, some boards warp while air-drying. Why is that? Assuming your portable sawmill is in good adjustment and that the blade is sharp with the right amount of set, the only other culprit can be the wood itself.
Wood is an amazingly complex material and the way it grows determines, to a large extent, what it does on the portable sawmill and on the drying stack. Other than some eastern red cedar and one cypress log, my experience is limited to hardwoods—oak, walnut, cherry, hickory, pecan, sycamore, hedge and hackberry. The main features of a growing tree, which can affect how the log cuts are large knots, changes in the grain direction, reaction wood, juvenile wood and internal stresses. The species can also comes into play. Walnut and sycamore, for example, are relatively soft wood and relatively easy to cut straight. On the other end of the spectrum, the general opinion in the various sawmilling forums is that “pecan is the Devil”. Hickory and hackberry are also on the list of least-favorite woods to cut. The wood is tough and there is just something about the grain acting like a magnet, pulling the blade every way but straight. A lot of the sawing properties of the wood result from stress in the tree as it was growing. This article discusses the two most common forms of stress in logs—juvenile wood and reaction wood.
Juvenile wood is a source of stress in all trees. This is the wood the tree put on as a sapling. The juvenile wood cells are thinner and longer than the cells added to the tree as it became larger. As the tree grows, the juvenile wood cells are compressed, creating stress in the wood. As you mill a log, you relieve the stress, and the ends of the boards will bow up. The cant bows in the opposite direction and the center bows up slightly. To minimize sideways bow, saw the cant so the center of the growth rings is in the center of the cant. If a lot of stress is evident, it helps to rotate the cant 180 degrees every board or two, in order to balance out the stresses. Otherwise, as you cut, you’ll wind up with thick and thin boards, even though the blade is cutting perfectly straight. Some species of trees are more prone to this problem than others. The most difficult I have cut is hackberry, which had so much stress, it wound up on the firewood pile, though in retrospect, I could have used the boards for chair rockers. Fast growing trees seem to have the most stress, so noting the distance between growth rings will give you a clue of what to expect. The U.S. Forest Products Lab published a paper in 1987 entitled Juvenile Wood, Tension Wood and Growth Effects on Processing Hardwoods.
Reaction wood is another common cause of problems on a portable bandsaw mill. When a hardwood tree grows with a lean to it, the cells on the uphill side put on tension wood, so called because those cells are under tension as the tree grows. Cells in tension wood have to be stronger than normal wood cells, which normally support the wood under compression. To achieve this, they have a secondary cell wall. This web site has a good scanning electron micrograph of wood cells. This wood is like pre-stressed concrete. You can remove some of the load by cutting it (the log, not the concrete—I’ve tried to cut concrete embedded in a log and the blade did not fare well) and both the cant and the boards will bow.
Tension wood is fairly easy to identify, even without a microscope. Off-center tree rings give it away. The wood on the side with the wider ring spacing is the tension wood. According to the U.S. Forest Products Lab, the problem is worse in logs between 16” and 20” diameter. I have seen the end of a board lift three or four inches off the end of the cant as I cut it. Tree branches have so much reaction wood that most sawmills will not accept them.
There are, however, some sawing strategies to accommodate tension wood. First, the log should be cut into a cant with the center balanced. This will keep the boards from bowing sideways. Cut boards from the tension side first and rotate the cant 180 degrees, if you notice it starting to bow up in the middle. The boards will probably bend upwards from the cant as you relieve the tension as you saw, but the important thing is to keep the cant lying flat on the mill. You may want to keep the boards with the reaction wood separate from the boards with the normal wood as you dry it. Put the boards with the reaction wood on the bottom of the stack, so that the weight of the boards on top will help straighten them out as they dry.
As you gain experience with a portable sawmill, you will learn to identify which logs have reaction wood, which species are most prone to moving as you release the tension and how to mill the logs accordingly. Every now and then though, Mother Nature will throw you a curve. A log just won’t do what it is supposed to. A good last piece of advice is to take your time. Try to figure out whether it is the log or the mill and adjust your sawing accordingly. Hopefully this will lead to less stressful sawmilling—for you and for the log!