It is generally acknowledged that tree-stem taper can significantly affect lumber recovery during breakdown, and that rapid tree growth, particularly after stand thinning, results in stems with more pronounced taper when harvested. Taper has always been a challenge for sawmills and many have debated which log breakdown method to choose so as to minimize its impact on lumber recovery. The introduction of optimized positioning tables has quieted these debates, since this technology has facilitated the optimal positioning of each two-sided cant (hereafter “cant”) prior to breakdown. However, positioning optimization is limited when secondary breakdown equipment consists of a canter followed by a bull edger. In fact, the restrictions on cant offsetting and opening of the canter heads have an even greater negative impact on logs with extreme taper, since the fibre in the tapered section is processed into chips.
Some in the industry have developed and implemented a breakdown method called “bottle sawing”. Our study recommends a variation of this method, known as “half-bottle sawing”, which maximizes recovery of lumber from the log’s tapered section by limiting taper to one side only. One of the canter heads remains in the same position during breakdown but at about one third of the log length, the second head opens up to a width equivalent to a nominal 1 or 2 inch piece. The Optitek breakdown simulator was used to determine whether this method results in greater lumber recovery than current sawmill methods. Two groups of logs were selected on length basis (16 and 12 foot logs), with each group further broken down by taper class (light, average and heavy).
Results show that half-bottle sawing is best for mills processing logs that are 12 foot long, or more, on lines featuring cant positioning tables in front of the canter, regardless of taper class. The half-bottle method yields more lumber pieces from the tapered section of the log by opening up one of the canter heads, so long as the log is adequately secured in the breakdown system. Monetary gains, ranging from 6.5 to 8.9% for 16 foot logs, and ranging from 4.0 to 5.1% for 12 foot logs, were achieved by using the half-bottle sawing method rather than the optimized canter breakdown method. We noted that longer logs generated higher gains, but that heavier taper does not translate into higher gains.
Half-bottle breakdown is therefore an attractive alternative to a breakdown line with a positioning table in front of a canter. Nevertheless, the best breakdown system for lumber recovery is one that includes an optimization table in front of a bull edger, because such a system has fewer positioning restrictions.