Automatic measurement of compression wood and spiral grain for the prediction of distortion in sawn wood products

Abstract: Distortion is one of the major obstacles for effective utilisation of wood, especially in the building process, where wood has lost market shares to other materials largely because of the lack of straightness. Bow and spring are often caused by compression wood, while twist is the cause of spiral grain. Compression wood is an anomaly in wood formed by the living softwood tree to compensate for excessive, asymmetrical loads by developing special fibres. Its large longitudinal shrinkage causes distortion in sawn wood, but compression wood also makes the wood brittle. It is therefore desirable to detect and reject compression wood early in the production chain. Four different methods for detection of compression wood are described: spectral imaging, colour scanning, x-ray scanning and tracheid-effect scanning. A combination of colour and tracheid-effect scanning was assessed to be the most promising method for industrial real-time detection of compression wood. Spiral grain is normal in all trees but becomes a problem when the grain angle is large all the way through wood sawn close to the pith. Most Scandinavian softwoods have a left-handed spiral grain close to the pith, but turn right-handed at mature age. Some trees, however, seem to stay left handed all their life and just increase their left-handed spiral grain with age. Lumber sawn close to the pith from such trees is often subject to severe twist after drying. A nondestructive and contact-free method that utilises the tracheid effect for detecting spiral grain is described and evaluated for industrial use with excellent results. A prototype for industrial use was set up measuring spiral grain on debarked sawlogs, and the measurements were used for prediction of twist with a coefficient of determination of 0.76. This system was set up at three sawmills in different regions of Sweden, revealing rather large differences in the distribution of spiral grain angle.