Machine Grading–Theory and Practice, Part I

This article deals with the nondestructive strength testing and grading of lumber. All systems for strength grading lumber of nondestructive nature are considered. Characteristics that are common or required of all such systems are isolated and discussed with particular emphasis on the similarities and differences with respect to visual grading. Essentially visual grading and machine grading are two nondestructive test methods that may be used to grade lumber for strength. The visual grading system was established by using average strength properties of small, clear, green specimens of wood and then modifying the average strength to arrive at safe allowable design stresses for each species and finally for each piece. Modification factors were determined by continuing research on those characteristics known to influence strength. The criteria for machine-grading are based mainly upon the high correlation coefficient between strength and modulus of elasticity of wood specimens that have been established in laboratory tests. The relationship between modulus of elasticity and stiffness and the effect of moisture content and species, are also taken into account in establishing the criteria for machine-grading. Also included is a discussion of present, stress-grading machines, lumber grades, commercial practices, production, certification requirements and efficiency. Visual and machine grading systems are compared and areas requiring further research are suggested.

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