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Sawmill & Treating Insights: Curve Sawing Low-Grade Hardwoods
Approximately one-third of hardwood saw logs have a significant amount of sweep and that 7% to almost 40% of the yield is lost from logs that have greater than 1 inch of sweep.
By Brian Bond, Virginia Tech
Date Posted: 4/1/2008
I would like to follow up an earlier column with some results for a re-cently completed study of hardwood curve sawing. If you consider that approximately one-third of hardwood saw logs have a significant amount of sweep and that 7% to almost 40% of the yield is lost from logs that have greater than 1 inch of sweep, the potential for yield increases with curve sawing equipment can be significant.
I recently conducted a curve sawing recovery study with Phil Araman of the U.S. Forest Service and with the cooperation of a hardwood sawmill in the Appalachian hardwood region. Low-grade cherry logs with sweep were processed with both straight sawing and curve sawing techniques.
One hundred sixty-two U.S. Forest Service Grade 2 and 3 cherry logs with small-end diameters of 12-14 inches and lengths of 8-10 feet were processed through the mill. Logs were separated into two categories, those with 1-3 inches of sweep (low-sweep) and those with 3 or more inches of sweep (high-sweep).
Processing consisted of scanning each log for scaling information, debarking, and processing into a two-sided cant at the bandsaw head rig. The logs were processed such that the cants were sawn with the faces being perpendicular to the sweep. Cants were then processed by a curve sawing gang with an arbor that could move +/- 4 degrees and could handle a maximum sweep of 1-3/4 inches in 10 feet. All lumber produced was 4/4.
Overrun is the difference between log scale estimate and actual board footage sawn. When comparing overrun between curve sawn and straight sawn cants, the yield was 18% greater for cants produced from low-sweep Grade 2 logs (that were curve sawn) and 6% greater for cants that were curve sawn from low-sweep Grade 3 logs (Figure 1). Figure 2 illustrates that overrun was 21% greater for the low-sweep curve sawn logs than the high-sweep curve sawn logs, indicating the limitation of the sweep parameter that the sawing machine is capable of utilizing. Keep in mind these results are for 8-10 foot long logs; the impact of curve sawing on yield would be much greater on longer logs.
Low-sweep curve sawn cants produced 7-9% more volume of FAS lumber than straight sawn cants, regardless of the original log grade. The lumber value per board foot of log input was 0.11-0.18 ($/bf) greater when 8-foot logs containing 1-3 inches of sweep were curve sawn rather than straight sawn. The increased value indicates that the ability to curve saw logs (with sweep up to 3-inches) significantly increases lumber value output for Grade 2 and 3 cherry saw logs.
One concern with curve sawing is that the boards produced will have significant amounts of warp since the logs are curved. However, since the boards are sawn with the grain, warp should be minimized.
Measurements of warp after drying indicated that the boards produced by curve sawing contained more bow and crook than boards produced by straight sawing (Figure 3). While these differences are statistically significant, the actual amount of bow (less than ½-inch) and crook (1/8-inch) would not likely significantly impact rough mill yield when processed. Twist was not significantly different between the two groups and did not exceed ¼-inch.
The results of the study clearly indicate that volume recovery is greater when curve sawing hardwood cants produced from logs containing sweep, even when those logs are relatively short (predominantly 8 feet in length). Greater increases would likely be possible with longer and/or smaller diameter logs.
Given that sawmills often spend hundreds of thousands of dollars on new equipment to obtain a 2% increase in yield, it is surprising that more hardwood sawmills have not adopted this technology based on the results obtained since the minimum gain was 2%.
Not only does this technology result in greater lumber recovery for the species and grade of logs sawn, it resulted in significantly increased value output.
The study was funded in whole or in part through a grant awarded by the U.S. Forest Service Wood Education and Resource Center and Northeastern area state and private forestry.