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Going with the Grain Sawing with the Curve
Virginia Tech: Sawmill & Treating INSIGHTS
By Dr. Brian Bond
Date Posted: 10/1/2006
Editor’s Note: Dr. Brian Bond is the newest regular columnist to join TimberLine. He is an assistant professor at Virginia Tech. Dr. Bond is a specialist in improving efficiency in sawmills and lumber drying operations. His columns will regularly cover general topics and the latest technology for treatment and sawing processes. Dr. Bond may be reached at (540) 231-8752 or firstname.lastname@example.org.
Sawmills are constantly trying to improve their competitiveness as markets change and competition increases. Decreases in log availability, size and quality have made processing logs into lumber a challenging task. Adoption of new technology or sawing methods often allows significant financial gain over the competition and can make processing of previously unprofitable material, small diameter logs for example, profitable. One development that has resulted in increased volume and grade recovery for both hardwood and softwood mills is the curve sawing gang. When logs contain significant sweep or curvature, traditional straight sawing methods result in a significant loss of volume. The curve portion must be removed to produce a straight cant which can then be further processed. The potential to saw lumber with the curvature of a cant and produce straight boards would obviously increase lumber recovery and value.
A common, but undesirable attribute in both hardwood and softwood logs is the occurrence of sweep. Sweep is traditionally characterized as curvature along the entire length of the log. Ideally, the majority of sweep is removed when trees are bucked into logs at the landing or at the mill. However, a quick look at the logs going into a mill will clearly show that a fairly large percentage of the typical sawlog still contains significant sweep. A recent study at Virginia Tech indicated that approximately 1/3 of all hardwood saw logs possessed in eastern sawmills contain an average of 4.1 inches of sweep. It is estimated that a 13% yield loss occurs in hardwood saw logs that contain a scaleable sweep deduction over 1 inch.
What is curve sawing?
The idea of curve sawing is to produce a two sided cant using some type of head saw with the main curvature remaining in the cant. The cant is then sawn in such a way that the true form is followed thus producing longer lumber, improved lumber grade and less slope of grain with boards.
Sawing logs with curvature is typically done at a special gang saw. There are two types of gang saws used in curve sawing. One method feeds the cant through stationary saws by special feedworks that is capable of accommodating the curvature. The second method feeds the cant straight through the saw but the saws are mounted on a special system that makes them movable with the curvature of the cant. This method is often referred to as a “wiggle box” system. Most of systems that utilize moving saw heads also incorporate some type of scanning and optimization.
Does sawing a curved cant produce curved boards? Boards up to two inches thick usually straighten out after being sawn. Handling can occasionally be a problem in the mill as some boards may not straighten out completely until the lumber has been stacked and properly dried. How does the lumber dry? If the lumber is properly stacked and stickered the lumber will actually dry flatter since the grain in the board will be straighter than if the lumber had been sawn through a straight gang. The straighter grain also assists in helping pine boards meet higher grade standards since “slope of grain” is one of the factors of the pine lumber grading rules.
Many producers of softwood lumber have already adopted curve sawing technology. It has been estimated that 60% of pine sawmills in the south have curve-sawing equipment. While the idea of sawing with the form of a log has been around by some reports since the 1950’s, curve sawing became prevalent in the southern softwood industry in the 1990’s. Adoption of this technology by the U.S. softwood industry is likely due to the significantly larger and higher production and the greater economies of scale of softwood producers.
The majority of softwood curve sawing installations that I have seen include both scanning and optimization. Softwood mills typically report a 2- 10 percent recovery increase after installing a curve sawing gang.
The increase in recovery is highly dependent on how much optimization was used in the mill prior to installation. Increases in lumber recovery have also been shown to increase as stem diameter decreases, which is a common problem facing many mills. There are many different manufacturers of curve sawing gang saws and scanning/optimization options for the softwood industry.
Only a handful of hardwood sawmills have adopted curve sawing technology. Those that have, claim significant increases in both volume and grade recovery. The main drawback to curve sawing technology for hardwood sawmills is the high cost of the machinery. Curve sawing installations with scanning and optimization can range from 1 to 3 million dollars.
Corely Sawmill Machinery is attempting to bring a lower cost curve sawing gang to the hardwood industry. They have recently developed a curve sawing gang that would cost approximately 25% of what most optimized movable sawhead systems would cost according to Chuck Boaz, the company's vice president.
Corley’s system utilizes technology that feeds the curved two sided cant into fixed thin kerf saws. With approximately 1/3 of hardwood saw logs containing deducible sweep, curve sawing would seem an excellent way to increase grade and volume recovery, if the price were right.
Is curve sawing right for your installation? Typically the increase in grade and volume recovery makes such technology worth consideration. Take a close look at the log quality that is currently processed at your mill.Keep in mind that the smaller the log diameter, typically the greater the recovery gains for softwood. Also, take close look at your current scanning, optimization and processing methods, the greater your current optimization typically the smaller the increase in recovery gains. This information in addition to your total volume produced will help you determine the time required to pay for the investment. Working closely with a manufacturer of curve sawing gangs and using your production information, you should be able to more accurately estimate the increase in recovery.
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