Component manufacturers have long been in search of “any practical technology that can lessen rejects” when fabricating trusses from our imperfect supply of lumber. New machines and processes have increased material throughput, but they have also increased the time lost when substandard material reaches the assembly table and needs to be culled there. The commonly held QC principles of “do it right the first time” and “minimize touches” suggest that lumber should be culled as soon as a lumber bunk is opened. However, that is only possible if a single set of QC standards is applied, which may not be optimal for all end uses. [For all photos and images, See PDF or View in Full Issue.]
So which “defects” require culling? Crook is much more critical in roof chords than floor chords, but wane is more critical in floor chords. Bow is rarely detrimental to truss quality, but its incidence has been increasing due to the use of curved sawing at lumber mills, a practice that was begun by Weyerhaeuser in the late 1980s to optimize yields when processing faster growth timber. Logs from these smaller diameter trees aren’t as straight as older, slower growth timber, so with this technology, the saw blades can pivot to roughly follow the curvature of a log. The natural bow that results will sometimes be immediately apparent when the bands of the bundle are broken, and then it can be flattened during fabrication.
Crook and wane are definitely the critical QC issues in truss and wall panel manufacturing, but their treatment is still not straightforward. Linear saws complicate the culling process, because several pieces may be cut from a single board. While a 1” crook is not acceptable in a 16’ #2 roof chord, when that board is cut into smaller pieces it may be acceptable, especially if the crook occurs in a small portion of the board. Likewise, a 1” crook may be acceptable in a 12’ #3 web member.
So where in the process should we cull lumber? To minimize touches, it can be done when the bands are broken on a fresh bunk of lumber. Culling lumber then will rely on the picker’s frame of reference, which is the adjacent boards in the pack or the sightline along the edge of the board. This provides a way to “eyeball” the lumber, but it is not the most accurate. A better method may be for a linear saw operator to observe each board on the live deck. The operator has the best vantage point from which to cull lumber, and he can do it while boards are cycling into the saw’s infeed. He can rotate a piece end-to-end to relocate a defect to the trailing end of a board, which may fall in the cut-off piece. And he can keep a small stock of substitutes close at hand.
As shown in the photos of culling lumber, this saw operator is generally better qualified than your lumber picker would be. While having the saw operator manage your culling may run contrary to the “right the first time” principle, it has the potential to provide a more targeted and effective culling methodology.
The final decision on where to cull lumber will come down to your expectations and your end uses. The significance of lumber defects will vary, so your processes will need to be adaptable yet standardized, so you can maximize efficiency without sacrificing quality.