As sawmilling machines are becoming more accurate it may be worthwhile to revisit sawn timber size settings by taking a leaf out of modern engineering tolerancing and look at tolerancing in the sawing process. Geometric dimensioning and tolerancing is a technique normally used in engineering where tolerances need to be defined easily, precisely and unambiguously to ensure the assembly will fit. Sawmilling is a disassembly process where a log is broken down into sawn components and it is important to ensure that the required components can be extracted from the logs or flitches even if all stages of the production process are not perfect. In sawmilling tolerances during the manufacturing process are often not clearly specified. Often the product aim (green sawn target size) sizes are specified as the aim sizes at each machine centre, rather than sizes, including required tolerances, which will ensure that the intended aim lumber sizes are produced at the end of the production process. If the green mill aim sizes are specified at the first machine centre there may not be a manufacturing tolerance available to ensure that the required products can be recovered at subsequent machine centres. This can be caused by the scanned log shape, positioning of the log, positioning of the cant, scanning of the slabs, and positioning of the slabs for edging not being perfect. The question is how to specify tolerances along the production process to ensure that the maximum value is extracted from a given log supply. In most modern sawmills it is easy to determine if a dimensioning and tolerancing problem may exist. If the predicted product yield for a shift and the actual production for that shift differs, the yield loss may be due to a log-scanning problem, log movement after scanning or a tolerancing problem. The current sawmill practice of specifying the same product sizes and wane allowances at all machine centres can lead to production losses. By applying the principles of geometric dimensioning and tolerancing it is possible to calculate appropriate target sizes at the different machine centres. A recent study by the author study found that accurate log shape scanning is important as errors get magnified, retaining the log position as scanned is of the utmost importance. Dimensional tolerancing can help to improve the match between the lumber mix predicted at the main breakdown saws and the tally on the tables, in spite of scanning and positioning errors.
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