The frequency and level of sweep in mixed hardwood saw logs in the eastern United States

Hardwood sawmills traditionally saw logs in a manner that either orients sawlines parallel to the log central axis (straight sawing) or the log surface (allowing for taper). Sweep is characterized as uniform curvature along the entire length of a log. For logs with sweep, lumber yield losses from straight and taper sawing increase with increasing levels of sweep. Curve sawing logs with sweep will increase yields by allowing logs with sweep to be sawn along their natural line of curvature. To better understand the potential for the utilization of curve sawing technology in hardwood sawmills of the eastern United States, it was necessary to determine the frequency and degree of sweep that occurs in hardwood sawlogs. In this study, 1,700 random hardwood logs from 17 sawmills located across eight eastern states were measured, analyzed, and classified for level of sweep based on the log scale deduction percentage attributed to sweep. Whereas the majority of eastern hardwood logs are considered straight, our results show that nearly one third of hardwood sawmill inventories have enough curvature to incur sweep scale deductions of 5 percent or more. For these logs, sweep scale deduction percentages increase as small-end diameter decreases and as length increases. Of the primary species sampled in this study, oak logs exhibited the highest average sweep scale deduction—5.9 percent. The eastern region of the United States contains the vast majority of the nation’s hardwood timber resource and is responsible for 97 percent of the U.S. annual production of hardwood lumber (The Timber Tax 2002). In recent years the softwood lumber industry has widely adopted curve sawing technology to increase lumber yield from sawlogs that exhibit sweep (Woodward 2003, Corino et al. 2006). Sweep is traditionally characterized as uniform curvature along the entire log length. At present, curve sawing is generally not utilized in the hardwood lumber industry. Hardwood sawmills traditionally straight saw logs allowing only for taper with sawlines parallel to the log central axis and/or straight surface. However, when straight sawing, yield loss increases as the level of sweep increases in a log (Ronnqvist et al. 2000). Curve sawing generally incorporates gang sawing two sided cants with sweep (curvature) to be cut along the natural line of curvature (Petree 1998). There are two basic curve sawing techniques. One method manipulates a curved cant through stationary saws while the other method uses articulating saws that follow the contour of a log (Nelson 2001). This technology has the potential to increase both the yield and quality of lumber from hardwood logs exhibiting various levels of sweep. A recent study evaluated the effectiveness of curve sawing vs. straight sawing two-sided cants from small diameter hardwood logs (Hamner et al. 2006). In this study, the lumber yield from curved logs increased significantly when curve sawn. The results indicate that, based on a 12-foot log length scale, hardwood logs with average sweep greater than 1.6 inches can produce lumber yields as much as 12 percent higher when curve sawing. The authors are, respectively, Research Associate and Professor, Virginia Tech, Thomas M. Brooks Forest Prod. Center, Blacksburg, Virginia ([email protected]; [email protected]) and Research Project Leader, USDA Forest Serv., Southern Research Sta., Blacksburg, Virginia ([email protected]). The authors wish to acknowledge the following contributors to this study: The USDA Forest Serv., Wood Education and Research Center (WERC) for funding this study. This paper was received for publication in November 2005. Article No. 10137. ✳Forest Products Society Member. ©Forest Products Society 2007. Forest Prod. J. 57(9):23-27. FOREST PRODUCTS JOURNAL VOL. 57, NO. 9 23 Previous studies have evaluated the curve sawing of softwood sawlogs (Wang et al. 1992, Wagner et al. 2002). The research concluded that, when compared to straight sawing, curve sawing logs with sweep increases the value of lumber sawn from curved logs in terms of both volume recovery and lumber quality. Since curve sawn boards are cut along the grain, the potential for producing higher quality lumber increases by reducing slope of grain. Straight grain lumber has less of a tendency to warp than does lumber with sloped grain (Petree 1998). Also, curve sawn boards from sweepy softwood logs are slightly wider and longer than straight sawn lumber from curved logs. Log diameter affects lumber yield from logs that have sweep. The impact of curve sawing on yield is greater for smaller diameter logs. Research by Wang et al. (1992) showed that when curve sawing 16-foot logs with similar sweep characteristics (ranging from 1.01 to 1.60 inches), yield improved by 4 to 16 percent as top end diameters declined from 8 inches to less than 5 inches, respectively. Wagner et al. (2002) established three categories of sweep (for 16-foot logs) as follows: logs with 1-inch sweep are straight, logs with 1.5 to 2.5 inches have moderate sweep, and logs with 3 inches have high sweep. For hardwood logs, it will be helpful to categorize the level of sweep to determine costeffectiveness criteria for curve sawing applications.