Determining Lumber Target Sizes and Monitoring Sawing Accuracy

A sawmill manager who wants to recover as much lumber as possible from a log must measure the lumber and monitor sawing variation to determine minimum target thickness. The smaller the sawing variation, the smaller the lumber target size can be, and the greater the recovery and profits to the mill. A simple yet effective technique, suitable for use at the mill level, is described. Initially, 25 groups of 4 boards each–a total of 100 boards–should be measured for thickness or width. Each of the four boards in a group should be taken one after another from a convenient place behind the machine being studied. For illustration, thickness was measured at four random locations on each board; however, the technique is suitable for any number of measurements per board. After the measurements from the initial 100 boards are collected, within-board thickness variation, between-board thickness variation, and total thickness variation can be calculated. From this, lumber target thickness is determined based on the finished lumber size, shrinkage allowance if applicable, planing allowance, and total sawing or thickness variation. Total sawing variation is determined by relating standard deviation to the range of board thickness. This simplifies the mathematics involved, making the calculation more acceptable to sawmill personnel. Next, the same data collected for determining lumber target size are used to construct statistical control charts for monitoring the target sizes determined earlier and set on a particular machine. These control charts–which monitor average lumber thickness, within board sawing variation, and between board sawing variation– indicate when the thickness of the boards produced by the machine remain in statistical control.

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