Appearance grading of sawn timber

Sammanfattning: Grading of sawn timber has by tradition been performed by manual ocular inspection of the wood surfaces. Even today, most wood produced is graded by manual inspection. Normally, a grader has approximately 2 seconds to decide the timber's quality, which means that the work situation is monotonous, bound to one place and very demanding, as the grading result has a crucial impact on the financial output. Automation of the grading process has been greatly desired for a long time. Lately, commercial automatic grading equipment has become available. Many sawmills consider investing in systems for automatic appearance grading of sawn timber. The basis for decision before investment is in many cases insufficient due to lack of knowledge about what is really needed, what the existing manual graders actually do and what the automated systems are capable of. Comprehending all of this is of utmost importance as sawmills are facing greater and greater demands on productivity, working environment, precision of delivery, customer adaptability and their own profitability. This thesis consists of four papers. Paper I, which is a Licentiate thesis, discusses a number of models of how to set up and describe grading rules. This gives a good foundation for a changeover to automatic grading systems. A tool for simulation of grading results has been developed, tested and demonstrated. In Paper II the grading accuracy of manual graders is compared to the accuracy of an automatic grading system. The comparison shows that an automatic system in most cases can replace manual grading with higher productivity and higher value yield as results, but that the automatic systems make other mistakes than the manual graders. Today's grading rules are very complex with a large number of influencing parameters. When a parameter in a table in an automatic grading system is changed, it is often difficult to foresee the consequences. In Paper III the grading rules are modelled with the help of multivariate statistics, which makes it possible to move a boundary between grades by just "twisting a knob". This point of action will be a very simple and powerful aid in industrial applications of automatic grading systems. In the Nordic countries, all four sides of the timbers are inspected, according to the traditional grading rules, whereas in Central Europe only the two faces are inspected. In Paper IV it is shown that if a thorough grading is needed, all four sides have to be inspected in order to get a true grading result.