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Component Manufacturing dverti$ dverti $ er
Don’t Forget! You Saw it in the
July 2024 #16300 Page #152
Adverti$$er
The Last Word Where to Cull Lumber
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omponent manufacturers have long been in search of “any practical technology that can lessen rejects”1 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.
Joe Kannapell, P.E.
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.
1 For another aspect of this discussion, see The Last Word from October 2023, Can Crooked Lumber Defeat Robotics?. Continued next page
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