Technology Roadmap: Lumber and Value-Added Wood Products
4.16 Grading and Trimming

Existing Technology

Mechanical lumber grading and mechanically evaluated lumber (MEL) have been known in Canada for about ten years. Mechanically-graded lumber is mainly intended for engineered wood products markets (I-beams and trusses). Two techniques are currently used to manufacture these products. The first one, certainly the most popular, is a mechanical constraint evaluation of wood by bending; the second uses X-ray technology to measure density, which is used to determine mechanical properties. See Section 7.3.1 for more information on machine stress-rated (MSR) lumber and its production.

Even mechanically-graded lumber needs to be inspected for appearance defects. This is done visually by the same graders who perform visual grading under rules set by the National Lumber Grades Authority (NLGA) or others. In efficient mills, graders are highly trained individuals who optimise value recovery on the basis of grading rules. In the recent past, lumber grading optimisers have been introduced to automate part of the visual grading process. There is no entirely automated grading system in operation in North America that can recognise all the required characteristics for lumber grading. Current optimisers recognise and measure lumber geometry (dimensions, wane, skip, holes) while the operator focuses on other defects (decay, checks, slope of grain, etc.).

The most frequently used scanning technology combination includes vision cameras and laser scanners. Vision cameras, either colour or black and white, are used to acquire information on surface defects such as knots, stains, shake and holes. Laser scanners are used to detect lumber shape characteristics such as wane, thickness, splits, checks, holes, and various types of warping. Some grading optimisers can operate at speeds in excess of 600 m/min (2 000 lineal feet/minute).

To the many sawmills that manufacture an increasingly wide range of products in terms of sizes and grading specifications (including foreign grades and specific client grades), optimisation technology offers flexibility and efficiency, especially when decisions are based on current product mix and product value. Defect measurement can be consistently accurate under a variety of rules, thereby optimising both value recovery and product reliability. Optimisers also make it possible to increase the number of fencing positions in the trimmer, which expands the potential for recovery in mills producing lumber in lengths other than the conventional 600 mm (two-foot) increments.

Like all optimisers, grading machines collect huge amounts of information on the products they process. This information can be analysed and used to further refine the grading process. It can be used to help with decision making (evaluation of scenarios) and for training purposes. This information can also serve as feedback for previous operations (for example, planing, drying, sorting, sawing) and become an essential factor in an integrated process control system (see Section 4.18 on Overall Quality and Process Control). Systems are available for these purposes, but they are not currently used in any significant way.

The integration of multiple sensors using various technologies (laser, cameras, X-rays, microwave radiation, computer tomography or ultrasounds) could make it possible to recognise most geometric and visual characteristics essential for lumber grading, and make the operation entirely, or almost entirely automatic. They have not yet been applied commercially to lumber grading, but could become useful for operations serving re-manufacturing and value-added clients. As was seen earlier in this report, through-the-wood scanning has great potential in sawmills, mostly in the log and lumber breakdown process.

Mills equipped for finger-jointing or supplying material to finger-jointing operations recover trim blocks, and this needs to be taken into account by graders and grading optimisers. These mills would also find it useful to have equipment for sorting trim blocks into size, moisture content and possibly stiffness categories.

Incremental Technological Innovation

• Further develop systems to improve grading accuracy, and help graders deal with a variety of grading specifications and economic inputs.
• Develop low-cost equipment to sort randomly oriented short blocks into size, moisture content and stiffness categories for finger-jointing (see Section 7.3.2 on Finger-jointing).
• Develop and implement optimisers capable of dealing simply and rapidly with frequent and complex grading parameter changes.

Breakthrough Technological Innovation

• Develop and implement an integrated grading system based on input from graders and a variety of sensors in order to increase value recovery and product consistency under a variety of changing specifications.