Sawing Curved Cants from the Primary Breakdown Unit
Contrary to popular belief, the amount of curvature processed in a cant is limited by the mechanical actions of saws and less by lumber handling restrictions. When the amount of curvature processed is reduced, lumber recovery, value and grade suffer.
This article addresses the sawing of curved cants from the primary breakdown unit. Curve sawing is defined as following the form of a cant to achieve longer lumber, improved lumber value and reduced cross grain.
The curved path followed by curve sawing is a series of joined points or short lines which form curves. The amount of curvature is defined as the radius measured from a tangent at these points. This can also be the amount of the cant’s curved deflection to length. For instance, a 400-inch radius equals 5 inches of deflection in a 16-foot cant. More than a 5,000-inch radius becomes straight sawing with 0 to 1/2-inch deflection.
The amount of curvature processed is limited by the ability of saws to withstand side pressure from contact with wood in the curved sawing path. This is normally measured as the clearance between the wider saw tooth and the plate along the curved path. It is measured at the front of the saw where the guides are located since the guided area of a thin-kerf saw is rigid while the back can flex.
Factors affecting the clearance include saw diameter, where smaller saws reduce the radius, and wood abnormalities such as flared butts, which force the wood against the saws.
Two current popular methods of curve sawing are ‘form following.’ which is mechanically following the form of a cant, and ‘skew and translate,’ which follows an electronically defined path by pivoting the arbor and shifting the saws laterally against the incoming wood.
Form following has been around for more than 50 years with roots in Scandinavia. It remains popular today due to the ability to follow the natural form of a cant and the low degrade this method produces. Even curved lumber is known to become straight on the green chain with this method.
Form following requires a four-sided cant that has been curve sawn by a chipping canter in a separate operation with curvature controlled by ‘natural’ form following or by electronic means to restrict excessive curvature. This permits a separate canting operation independent of the saws and allows the use of conical chipping heads with superior quality chips and the ability to process flared butts.
Form following uses two sets of rolls placed in close proximity to the leading and rigid edge of the saws (due to guides). They transport the cant through the saws with a very short tangent, reducing the radius to 500-inches irrespective of the cant’s thickness. This equals 9.3 inches of deflection in a 16-foot cant.
Skew and translate is used to process two-sided cants with round sides and is absolutely dependant on electronic controls. The articulating action of the arbor tends to force the saws against the incoming wood, effectively reducing the radius to the 2,000-4,000-inch range or 2.3 inches down to 1.15 inches of deflection in a 6-inch cant 16 feet long. If these amounts are exceeded, under normal circumstances saw abrading from wood contact will occur, causing overheating, saw damage and downtime. This is further aggravated by flared butts where chipping heads force material onto the saws.
The limiting radii which are based on the mechanical action of the saws — and not their electronics systems — are estimated as follows:
Limiting Radii* Theoretical Deflection
in 6-inch, 16-foot cant
Form following curve sawing: 500 inches to 9.30 inches
Skew and translate curve sawing: 3,000 inches 1.50 inches
Straight sawing: 5,000 inches 0.97-inch
*estimates reported in industry
Therefore, we can conclude that contrary to popular belief, the amount of curvature processed is limited more by the mechanical actions of saws and less by material handling equipment. And reduced curvature processing results in reduced lumber recovery, value and grade.
For more information, contact SawQuip at (450) 586-5100, fax (450) 586-5105, e-mail sawquip@sawquip.com, or visit the Web site at www.sawquip.com.
