The purpose of this study was to determine the effects of tool wear, tool sharpening and temperature on lumber surface and chip quality in the conversion of black spruce [Picea mariana (Mill.) B.S.P.] and balsam fir logs [Abies Balsamea (L.) Mill.] with chipper-canters equipped with disposable knives. Lumber quality is determined by losses due to trimming necessitated by large knife marks and wood tear-out, which cause boards to bedowngraded. Chip quality is determined by the proportion of fine particles in chip classification tests. Tests were conducted with black spruce logs in summer (unfrozen conditions) (ave. temp. 9.2°C) and with black spruce and balsam fir logs in winter (frozen conditions) (ave. temp. -14.7°C). As the test progressed, knife wear, chip quality and log surface quality were measured.
During the unfrozen tests, the knives operated during 153 hours and they were sharpened after 51 and 102 hours of operation. The fines content increased by 0.6% (1.8 to 2.4%) over the 153-hour operation, which represents a chip revenue loss of 0.3%. Total value losses (based on current lumber prices) were 5.1% with new knives, and reached 9.3% at the end of the 153 hours of operation.
During the frozen tests, the knives operated during 48 hours and they were sharpened after 16 and 32 hours of operation. The fines content increased by 1.8% (3.2 to 4.8%) for balsam fir for a 32-hour period; the mean fines content proportion was 4%, which represents a 1.3% increase in fines content. The fines content decreased slightly, by 0.2 % (3.4 to 3.2%) for black spruce over a 48-hour period; the mean fines content proportion was 3.3%, which represents a 0.2% loss in chip revenue. Total losses for black spruce were 5.3% with new knives, and reached 8% at the end of the 48-hour operation period. For balsam fir, total losses were 8.7% with new knives, and reached 9.5% at the end of the 48 hours.
The proportion of boards qualifying as Premium grade decreased after each knife sharpening operation under summer and winter conditions. For example, the proportion of boards qualifying for Premium grade with less than 1/32” wood tear-out was 6.8% for new knives, 4.2% for knives sharpened once, and 3.4% for knives sharpened twice (24.5%, 11.9% and 8.4% respectively under winter conditions). Under winter conditions, the proportion of boards qualifying for Premium was lower for balsam fir than for black Spruce. In addition, the proportion of boards qualifying for Premium grade was lower in summer than in winter: those results are difficult to explain, given that knife wear in summer and winter, as were log moisture conditions. The counter-knives were changed between the summer and the winter study. Better fibre separation at the counter-knife can reduce wood tear-out.
Other general observations made during this study:
1- Mean chip length decreased with knife wear.
2- Mean chip length decreased after each knife sharpening operation.
3- Balsam fir mean chip length was lower than black spruce logs under frozen conditions.
4- Mean chip length was higher for unfrozen logs than for frozen logs.
5- Fines content proportions increased with knife wear.
6- Fines content increased after each knife sharpening operation.
7- Fines content was lower in unfrozen logs than in frozen logs.
8- Fines content was higher in balsam fir logs than in black spruce logs under frozen conditions.
9- Tear-out volume per surface unit decreased with knife wear.
10- Tear-out volume per surface unit decreased after each knife sharpening operation.
11- Tear-out volume per surface unit was lower for balsam fir than for black spruce under frozen conditions.
12- Tear-out volume per surface unit was higher under frozen conditions than under unfrozen conditions.
13- Tear-out volume per surface unit was proportional to the affected surface area.
14- With black spruce monetary losses incurred under frozen conditions were comparable to those observed after only a third of the operating time under unfrozen conditions.
15- Under frozen conditions, monetary losses were higher for balsam fir than for black spruce.
16- Monetary losses increased after each knife sharpening operation.
17- Mean tear-out depth was higher in balsam fir than in black spruce.
18- Mean tear-out depth increased after each knife sharpening operation.
19- Knife wear reduction (for long and finishing knife) was about 0.020” in summer and in winter.
20- Total tip recession during 16-hour shifts under frozen conditions was slightly lower than or similar to the wear over 51 hours of operation under unfrozen conditions. In general, we can state that, for new knives, tool wear occurs about 3 times faster under frozen conditions than under unfrozen conditions. For sharpened knives, tool wear occurred slightly faster in frozen wood than in unfrozen wood.
21- Long knife edges wore faster than finishing knife edges under both frozen and unfrozen conditions
22- Knife wear rates tended to decrease slightly after each knife sharpening operation.
23- The difference between chip and fines prices was an invariable loss indicator for the fines proportion. For example if the price difference was $110, then 1% fines content corresponded to an invariable value loss of $17,864 (based on a 100,000 m3 log consumption). If the fines content was 3%, then the total fines loss was $53,592 (3 x 17,864).
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.