ENCYCLOPEDIA OF WOOD

In the sawmill

There are two main types of sawmills: those that use a band saw, and those that use a circular saw. A sawmill is often described according to the type of wood it cuts and the type of saw it employs, such as a softwood band mill or a hard­wood circular mill. Large band mills are often required for the larger-sized logs that are common in the softwood indus­try in western North America. Circular sawmills, more common in smaller hardwood operations in the East, have a smaller capacity, but are far less expen­sive than band mills.

The sawing process generates a great deal of “waste”—almost one-third of the bulk of each log—but every possible bit of wood is chipped up and used. Some is sold to paper pulp mills or wood-fired utilities. (The volume of wood-burned fuel has increased substantially since the energy crunch of the early 1970s. Today wood supplies about 3 percent of the United States’ energy consumption.) Even the bark, which is immediately stripped off the logs, frequently powers the sawmill’s drying kilns.

The bark is stripped from the log with large grinding cutterheads or blasted off by high-pressure water jets. The log is then mounted on a log carriage, posi­tioned so that the first cuts slice off the widest, clearest, most valuable boards.

Подпись: A device known as a “slot machine” sorts freshly sawn boards into the right widths and lengths.In the sawmill

Подпись: In less than 2 seconds, this band saw blade, driven by a 150-horsepower engine, can slice through a 16-foot log. The red line—a laser beam—shows the operator where the blade will cut.
In the sawmill

In the mill, the sawyer may rotate the log to “read” the log’s hidden defects. While in the past this might have been done by hand, it is not uncommon to see today’s sawyers work in a glass - enclosed booth, forming judgments with the help of advanced electronic equip­ment. In such a mill, the sawyer uses joysticks—like those of a computer game—to twirl the log almost a full turn in a matter of seconds, firing a beam of laser light down its length to visualize the effect of a particular cut before it is

made. In the most efficient mills, sophis­ticated computers are used to select the best position to obtain the maximum production from each log.

First, the four outer slabs of the log are removed, giving the sawyer a clean plane from which to make his next cut—the so-called “opening face”—to give the widest, clearest board available. Once this face is cut, the log is rotated, and three additional boards are cut— one from each remaining face. Large mills handling big logs send the remain­
ing square timber—called a cant—to a resawing area for cutting into var­ious sizes of dimension lumber. Here again, this sawyer must deter­mine the optimum cutting pattern that will yield the most valuable lum­ber. All the boards are edged, trimmed to length and graded.

Smaller mills, and those handling smaller logs, may use a different sawing strategy. After removing the outer slabs, the boards are cut from the opening face until defects interfere. Then the log is rotated to the next clearest face. As with the first method, the remaining cant is resawn into lower grade lumber. Finally, the boards are sorted, stacked and stick- ered—separated by thin strips to allow air to circulate between them—for their trip to the drying kiln, where they will remain for up to 50 days.

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ENCYCLOPEDIA OF WOOD

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