This study focused on the evaluation of sawmill secondary breakdown technology performance. Two recent and operational canter-gang edger machine centres were selected in order to conduct mill trials and to simulate their actual and theoretical efficiency.
Three-dimensional in-mill scans of a sample of 4-inch wide and 12-foot long cants were carried out to this end. Two batches of similar cants were produced to test the equipment and perform the requisite simulations.
The study canter-gang edgers (machine centres A and B) can be described as follows: the machine centres have linear infeed systems and are equipped with 3-D scanners (laser cameras) for breakdown optimization purposes. Machine centre A performs natural curve sawing and incorporates a self-centering pre-positioning mechanism. Positioning optimization is achieved in translation only (off-centering parallel to the cutting axis). This machine centre uses conical canter heads and guided circular saws. Machine centre B performs calculated curve sawing; positioning optimization is achieved with respect to both alignment and translation. It is equipped with cylindrical canter heads and guided circular saws. Natural curve sawing allows for sawing along the sharpest curvature without restriction (R=500 in.), whereas calculated curve sawing is usually restricted (R=1,500 in.) although this parameter is adjustable. The smaller the R-value, the greater the sawing curvature.
In practice, the two technologies achieved the same product yield and generated equivalent earnings. Each system posted an efficiency rate of 98% (in terms of earnings) according to the level of performance simulated with Optitek. The two types of canter heads produced chips of similar quality based on the proportion of undesirable chips (3/16 in. and fine particles) after screening. In general, sawing accuracy was excellent; the standard deviation was constantly below 0.020 in. The conical heads (machine centre A) produced only a few pieces with thin bevelled ends, which affected the sawing accuracy of outside pieces. In all likelihood, this was the result of poor machine alignment.
A theoretical optimal performance level was simulated for both canters. Thus, one assumes that the physical positioning of the cants during the breakdown process should result in an accuracy of 0.200 in. or better. If machine centre A had been equipped with a positioning optimization system in alignment and translation, its volume recovery would have been 407 fbm/m³. This corresponds to a potential for increased earnings of 5.1% compared with actual machine centre performance. Machine centre B (R=1,500 in.) yielded 396 fbm/m3. This corresponds to a potential for increased earnings of 2.8%.
Therefore, the main challenge facing equipment manufacturers consists of increasing cant-positioning accuracy during breakdown operations.