Cutting

Cutting Wood



Portable Sawmill

When a tree falls in the forest--whether someone hears it or not--it begins a process of decay which will transform it into new soil, ready for the next year's saplings.

When we interrupt the process to preserve and make use of the tree's wood, it helps to remember that the tree was once a sapling itself.

Looking at the end of a cut log, we can see how the tree grew. The dead cells of that sprout are still there, at the heart of the tree (called the pith). The tree has added more wood every year, forming a ring around the center which tells the story of that year's growth.

Freshly Cut Log

The wood's structure is built upon this expanding circle of living cells, and as the tree grows outward the older cells die, to remain embedded in the trunk, giving the plant physical support as it adds more growth and weight.

As soon as we cut the tree, the wood begins to dry out.

The ends dry out first, since the long, thin wood cells are hollow inside. That's why loggers will coat the ends of their logs with wax or sealer, to prevent their drying out faster than the rest of the log.

Drying Stresses

A useful visualization of the resulting stresses is to imagine a series of rubber bands stretched tightly around the log, each one pulling in a direction parallel to the rings. As you would imagine, if they pull hard enough, somewhere around the circle, something is bound to give. The result is the gaping cracks that result in logs that are left to dry without first being cut into smaller sections.

So the first step in converting the log into usable lumber of any shape is to prevent this from happening. An easy way to do this is to split the log in half, right along the line of the pith or center. In a log intended to be dried in large pieces, this step will be enough to eliminate almost all cracking.

But most logs are then cut up further, into some usable shape, either stick or panel. There are two main ways to do this.

Cutting the log along its length into slabs is the most obvious method, and it is called plainsawing or cutting "through-and-through". It is also the most efficient method: no wood is wasted.

Plainsawn Log

Plainsawing cuts across the tree's annual growth rings in a way that gives the most striking presentation of their annual rings, producing boards with the most interesting patterns of grain and texture. Plainsawn boards are usually the ones chosen for doors or panels that show off the wood's grain.

Their beauty comes at a price, though, because the planks will all tend to shrink by cupping inwards toward the center, or pith, of the tree. The stresses present in the original log remain in the newly cut shapes. These same boards will also tend to cup inward, then outward, then inward over the years as they respond to changes in humidity, even when built into a piece of furniture.

Quartersawing the log is another way of cutting out usable lumber, with the purpose of minimizing wood's tendancy to warp. It is done by cutting as much as possible in a direction straight out from the pith, or radially.

Quartersawn Log

Though most wood cells in a tree grow parallel to one another, a small number--about ten percent--grow straight outward from the pith. They are shaped like thin hollow tubes, just like the others, but they grow at right angles to all the rest. These radially growing cells, or ray cells, interspersed throughout the wood, affect how it shrinks as it dries. Since they pull in entirely different direction as they dry, the wood will shrink more in a direction around the circular rings (tangentially) than in will in a direction straight out from the pith (radially).

A board that is cut radially, or quartersawn, does not cup or deflect very much as it dries. Furthermore, the radial wood cells help reduce wood shrinkage in the radial direction, so it doesn't shrink as much in width as plainswan lumber.

Of the several ways to cut quartersawn lumber, all are wasteful because of the conflicting shapes of circle and line. This is why quartersawn lumber usually commands a higher price.

An unexpected beauty of radially cut lumber lies in the tendancy of some species to grow large, flat sheets of radial cells. The ray cells in red oak, in particular, produce such stark contrasts of light and dark grain and texture that quartersawn oak has become prized for its spectacular displays of grain and texture. Red oak shares that characteristic with several other woods, including white oak, ash, sycamore, maple, hickory and cherry.

A few other things to keep in mind:

  • Logs from large, straight, tall trees make the best lumber, and then usually only the main trunk. Trunks or branches that are eight inches or less in diameter often produce knotty and stress-prone boards.
  • Wood from any portion of the tree that grew slanted or at an angle to the vertical, which includes the lateral branches of most trees, produces what is called reaction wood. Because of tensions created as the growing tree tried to right itself, this wood has internal stresses which make it prone to warping and distorting even after dried.
  • Any plank of wood which contains the very center of the original bud (called the pith) can be expected to crack or distort badly. The plank can be saved by first splitting or resawing into smaller planks to remove the center, then drying the two halves.

A Freshly Cut Log

One last thing: Please never cut down a tree unless you have to. Since trees are part of our common inheritance, their continued presence is needed, now more than ever.

If you do need to cut down a tree, or have lost a tree to disease or weather, consider turning the trunk and any large branches into lumber. Sawyers are available who can cut the tree for you, often using a portable mill on location.