Sawing Imperfect Logs


When I first started running a portable sawmill, I was confident every board was going to be free of defects; I would have a nearly unlimited supply of straight, clear lumber for building projects and to sell to expectant customers. But nature designed trees to survive long enough to reproduce and in surviving their lives and resulting shapes generally take a myriad of twists and turns. The fact they make an ideal building material is totally irrelevant to the tree. The buttress at the base of the tree, branches and bends, which allow it to find sunlight help the tree survive, but can make the sawing process a bit more complicated. Dealing with imperfect material with portable sawmills can be a challenge but, with a little experience, you can find ways to make the most of every log. What’s more, if you’re like me, these challenges can become a source of entertainment and enjoyment.

Large Macrocarpa LogIn general, the best logs are the “butt cut” or the bottom log of a tree. When buying lumber, sawmills often pay a premium for these logs, because the wood is straighter and has fewer defects. The biggest problem in dealing with bottom logs is the flair. In an effort to get the most out of every tree, loggers cut them as low to the ground as possible. Flair butts can create a host of headaches until you learn to deal with them. These logs typically don’t roll straight, which makes them tougher to load onto the bandsaw mill and, once loaded, the log will most likely lie on an angle. It will help to trim off some of the flair with a chainsaw while the log is on the ground. Sawyers have differing opinions on which end should be toward the sawyer. Generally, it is easier to gauge the cuts if the narrow end of the log is facing toward you. But, on logs pushing the diameter limit of the mill, it is better to start with the butt end of the log toward the sawyer because it lets you center the log on the bed. If the log needs more trimming, you will see where and how much to trim before you start the cut. This avoids the frustration and loss of time, which occurs when you get just 2” from the end of the cut and realize you need to back the blade out to trim or reposition the log. The other issue with tapered logs is levelling the log to get the most efficient use of it. Imagine a line running down the center of the log lengthwise. Ideally, the line should be parallel to the track. Some sawmills are equipped with or have available a log levelling device, which are most commonly called toe boards. Even if your mill lacks this feature, you can easily improvise with a hydraulic jack or scissors jack (just be sure to put it back in the car where you found it when you’re finished, especially if it’s not your car). 

There are a number of different cutting patterns you can use.

Grade sawing consists of sawing the best face of the log. As soon as you get down to the point where another face has better quality, turn the log and cut that side. According to the USDA Forest Products Lab, you can increase the value of your lumber by as much as 15{ecddf9adde44d27d9b15adaf662c1dc75823d7a27d1c0a3dca6c68c19970fb5c}, since this removes as many defects as possible from as many boards as possible. 

Another cutting scheme is to determine the width of the board desired and then cutting a cant to that width, with the growth rings centered in the cant. Then saw the cant into boards. The boards will be less likely to warp if you employ this technique.

Upper logs don’t have the flair of their “butt cut” brethren, but they tend to have more knots, since there are more branches in the upper part of the tree. When an upper branch dies and falls off, the tree is less able to heal over before decay sets in, so there is also more chance of rot. Upper logs generally bring a lower price for this very reason. The best cutting strategy for these logs is to start with the clearest side of the log first. Knots can pose an interesting problem for a sawyer. Bandsaw blades are designed for rip sawing, but they often have to cross cut through knots. The teeth of a band saw blade tend to grab the grain of the knot, which can twist the blade just enough to make it want to rise or dive in the cut. Even circle saw blades tend to deflect somewhat when they hit knots. A combination of a slower feed rate, more tension on the blade and, if all else fails, a fresh blade, should straighten out the cut. If that doesn’t work, check the tracking to make sure the blade is riding properly on the band wheels and the blade guides for proper alignment.

Burley Cherry Log
A knotty burley cherry log waiting to be milled.

Curved logs can also present interesting challenges. Use them to your advantage. For example, if you have a customer who makes signs, you may get a premium for custom milled curved planks. If you need straight boards from a curved log, trim them as short as possible to minimize the loss of wood. If your customer is a sign maker or customer woodworker, you might be ahead to “flitch saw” the log into curved planks with the bark left on both edges. You may even get a premium for curved planks, if you happen to get the right customer.

There are a lot of tricks you can learn to make a portable bandmill work better for you. Eventually, you may need to back the blade out of a cut, possibly because the blade took a dive, or you because you need to do a bit of trimming to make it fit. I keep at least three plastic wedges handy (the kind loggers use to persuade a tree to fall in a desired direction). Driving them into the saw kerf opens it up, making it easy to back the blade out of the log without pulling it off the bandwheels. If you don’t have plastic wedges, it is easy to make some out of wood, but you should have some ready to use before you need them.

Always keep your chainsaw sharp and ready to use and have chaps and a logging helmet handy.  Trimming logs with a chain saw is the single most dangerous operation around a mill.  According to Consumer Reports, the average chain saw accident required 110 stitches and cost $12,000 at the Emergency Room. If you come up with a technique that solves a problem, be sure to take the time to share it here or on a sawmilling forum.

Dry Cracked LumberThere is one more type of log to consider; the branches. Some oak and walnut branches are big and straight enough to put on a sawmill, but it is seldom worth the effort. There is generally a lot of stress in a branch. The wood on the top side is “tension wood” and has a different cellular structure to handle the stress of holding the branch out straight. Once the stress is relieved by milling it into boards, it will tend to warp as it dries. In extreme cases, the boards will actually bend as they are cut on the mill.  The best use for branches of any size is usually firewood or the chipper for mulch.

It can be interesting and certainly useful to scale a log before cutting it to understand how much wood and profit can be realized. The International ¼” scale is the most accurate in determining how much lumber will come out of a log. For band mills with their narrow kerf, adding 15{ecddf9adde44d27d9b15adaf662c1dc75823d7a27d1c0a3dca6c68c19970fb5c} yield to the scale gives a pretty good estimate of what to expect from a log. Swing blade mills can typically add only 6{ecddf9adde44d27d9b15adaf662c1dc75823d7a27d1c0a3dca6c68c19970fb5c} to the scale because of their larger kerf. Keeping track of the techniques you use and the scale of the lumber will help you learn to maximize the yield from each log you cut.


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