Effects of knife velocity, knife bite and number of knives on lumber surface and chip quality in chipper-canters using bent knives and disposable knives (Phase III)
The purpose of this study was to determine the effects on lumber surface and chip quality of knife bite, knife velocity and the number of knives in the conversion of black spruce logs [Picea mariana (Mill.) B.S.P.] with chipper-canters equipped with bent knives and disposable knives. Lumber quality is determined by losses due to trimming necessitated by large knife marks and wood tear-out, which cause board downgrading. Chip quality is determined by the proportion of fine particles in chip classification tests (length and thickness). Twelve tests were conducted with the following variables: 22.2 mm and 28.6 mm knife bite; 3 and 6 knives; and 762.2, 1219.5, 1676.8 and 2131.1 m/min knife velocity. Preliminary results showed that chip fines contents increased with knife velocity while surface quality was not significantly affected. Mean chip length and mean chip thickness decreased as knife velocity increased and knife bite increased. The tests were performed on logs 2.54 m in length and 15 cm in diameter at temperatures in the 15-25°C range.
The proportion of fines was lower with bent knives but it tended to increase when knife velocity increased. There was no difference in the proportion of fines for a given knife bite but it was affected by the number of knives. The proportion of chip fines increased with knife velocity. Mean chip thickness decreased as knife velocity increased; it also decreased with an increasing number of knives. With all other parameters being equal, mean chip thickness was greater with bent knives. As knife bite increased, so did mean chip thickness. Mean chip length generally decreased as knife velocity increased; it also decreased as the number of knives increased. For a given number of knives, mean chip length was greater with bent knives. As knife bite increased, mean chip length increased too.
Unlike the results obtained with mean chip length, mean chip thickness was always greater with primary cuts than with secondary cuts. Differences between mean chip thickness in primary and secondary cuts were less than 1%. It was therefore considered reasonable to analyze chips from both cuts as a single lot without affecting mean chip thickness. Surface tear-out generally decreased as knife velocity increased. Greater knife bite increased surface tear-out. A greater number of knives decreased surface tear-out. Bent-type knives decreased surface tear-out by comparison with disposable knives under all conditions. The proportion of boards qualifying as Premium grade with less than 1/32” wood tear-out was less than 30% for 1.640” target size and 10% to 40% for 1.700” target size. For Premium grade accepting wood tear-out less than 1/16”, the proportion of acceptable boards was 10%-50% (1.640”) and 20%-60% (1.700”). For Premium grade accepting wood tear-out less than 1/8”, the proportion of acceptable boards was 50%-80% (1.640”) and 60%-90% (1.700”).
Under all conditions, cutting knife and counter-knife rake angles were lower at the entry point into the wood than at the exit point. Cutting knives always had larger angles than counter-knives. Finishing knife rake angles were always greater at the entry point than at the exit point. With bent knives, cutting knife angles were smaller but counter knife angles were always greater than those of disposable knives; finishing bent knife angles were greater than those of disposable knives.
Except at 1200 m/min knife velocity, bent knives were associated with higher monetary losses than disposable knives. A head equipped with six disposable knives seemed to incur lower losses at lower knife velocity, and a 22.2 mm knife bite seemed to result in lower monetary losses than a 28.6 mm knife bite.
Note: The calculations of the monetary losses were based considering that the totality of the lumber pieces can be classified Premium. In reality, around 30% of the pieces can be Premium; natural wane, rot, knots and other defects cause degrade without being damaged by the debarker or the canter. Losses must then be reduced to 30% of their value if the entire production is considered.