Spiral grain on logs under bark reveals twist-prone raw material
2001
Kliger, I.R.
Twist has been documented as the most common and crucial defect in sawn lumber. Being able to grade trees in the forest or grade logs prior to conversion to avoid using material with a large propensity to twist ought to be very important for the sawmill industry. Spiral grain can have a significant effect on the technical behavior of wood and on twist in particular. The aim is to demonstrate the assumption that avoiding the conversion of logs with a certain large spirality value measured on the log surface would reduce the number of twist-prone studs significantly. The methodologies applied to measure spiral grain under bark on the trunks of trees and on logs are described. The spiral grain angle was measured on at least three occasions: on some growing trees at breast height prior to harvesting, on all the logs after the trees were harvested and crosscut, and on discs from each log along the diameter and on the tangential faces of the sawn lumber. The number of straight studs after drying can be easily increased by avoiding converting logs with a fairly large left-hand spiral grain measured under bark. Only 11 percent of the trees exceeded a spiral grain angle of 3 degrees measured under bark and was based on 335 logs from conifer trees (178 Norway spruces, Picea abies) harvested in 4 European countries. This 11 percent of the trees yielded around 86 percent of the studs with an unacceptable amount of twist.
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