Effect of Curve Sawing on Lumber Recovery and Warp of Short Cherry Logs Containing Sweep

—It has been estimated that approximately one-third of hardwood sawlogs have a significant amount of sweep and that 7 to nearly 40 percent of the yield is lost from logs that have greater than 1 inch of sweep. While decreased yield is important, for hardwood logs the loss of lumber value is likely more significant. A method that produced lumber while accounting for log curvature (sweep) would allow greater volumes and higher value material that would also contain less warp when dried. While this technology is being utilized for softwood processing, it has not been accepted by the hardwood industry. A lumber recovery study conducted on mostly 8-foot-long grade 2 and 3 cherry logs at a mill utilizing a curve sawing gang, produced greater lumber volumes from cants that were curve sawn than from cants that were straight sawn. Increases in overrun ranged from 6 to 18 percent while lumber recovery improvements ranged from 0.5 to 1.3 for 8-foot logs containing 1-3 inches of sweep. Since the curve sawing gang used in the recovery study was limited in the maximum amount of sweep that could be handled during sawing (13⁄4inches per 10 feet), simulation software was used to predict the potential increase in volume recovery of the logs sawn if the machine had been able to handle the maximum amount of sweep. Results indicated that lumber recovery increases proportionally with the amount of sweep in the log assuming that the machine could accommodate maximum sweep. Measurements of warp after drying indicated that the boards produced by curve sawing contained significantly more bow and crook than boards produced by straight sawing. While these differences are significant, the actual amount of bow (less than 1⁄2 of an inch) and crook (1/8th of an inch) would not likely significantly impact rough mill yield when processed. Twist was not significantly different between the two groups and did not exceed 1⁄4 inch. INTRODUCTION Approximately one-third of hardwood saw logs have a significant amount of sweep, enough to incur sweep deductions of 5 percent or more (Hamner and others 2007). Common hardwood log grading scale deduction estimators predict that approximately 13 percent (range: 7 to almost 40 percent) of the yield from logs that have over 1 inch of sweep is lost (Hamner and others 2007). These yield losses are likely more significant for smaller-diameter logs. Although decreased yield is of importance, the loss of lumber value is probably more significant for hardwood logs. When curved logs are sawn using straight sawing methods, the highest valued material is removed from the outer portions of the log in order to straighten the log for further processing. In so doing, more of the highest quality wood is removed as slab wood destined for the chipper. The remaining lumber will likely be lower in value as it contains portions of the lower grade interior of the log as well as sections from the higher grade exterior. In addition to volume and value losses, the lumber produced from logs sawn using traditional methods often contains greater amounts of warp when dried due to the severe grain angles produced. A method that produces lumber while accounting for log curvature and sweep would allow for increased volumes and Associate Professor (BHB), Department of Wood Science and Forest Products, Virginia Tech, 1650 Ramble Road, Blacksburg, VA 24061; and Research Team Leader (PA), USDA Forest Service, Southern Research Station, Blacksburg, VA 24060. BHB is corresponding author: to contact, call (540)231-8752 or email at [email protected].