Q. With the higher prices for lumber, what can we do to increase yield? Any increases should have tremendous benefits in our profits.
A. You are indeed correct that the yield is a huge factor in processing lumber into parts, as lumber costs are perhaps 75 percent of the manufacturing costs -- not labor or machine costs.
I have some data from a rough mill that should help you direct your attention toward the best areas to look at for improvement.
This mill measured their various losses using weight. They weighed a pile of the rough lumber and then weighed the scrap pieces and sawdust at the various processing stations. They found that rip saw kerf (sawdust) was 4 percent of their waste; rip saw edging scrap was 8 percent; end trim was 4 percent; crosscut or chop sawdust was 0.3 percent; and knots, warp and other “natural” defects were 8 to 10 percent. (They had the lower amount with No.1 Common and the higher amount with No.2 Common.)
Note that if the original volume is based on green lumber footage, there is also 6 to 7 percent loss due to shrinkage. In this study, they did not cut back to specific lengths; any length was OK. When the pieces were trimmed to the usable lengths, another 6 percent loss. So, they “automatically” lose about 38 percent yield.
So, what can we do to reduce losses? Here are a few ideas.
Rip saw kerf? Thinner saws with better guides and stability features, cutting wider pieces for glued up panels will work.
Edging scrap? A lot of scrap results because the lumber is warped slightly. Most, if not all sidebend is because of a sawmill error in positioning the log or due to growth stresses in the tree. It is hard to control the sawmill's behavior or the tree’s growth in a cabinet plant. A second cause of edging strip losses is because we do not rip the lumber to maximize yield; computerized rip saws with scanners offer huge benefits here.
End trim? Some end trim is because of drying splits. Proper use of end coating on green logs and lumber can eliminate all these defects before they form. We can also save on end defect loss by trimming closer to the natural defects, but this can be dangerous, because many defects have swirly grain, so now the piece we want to use has swirly end grain which is hard to glue, machine and finish.
Natural Defects? Knots, stain, streaks and so on increase with lower grades, but it might surprise you to know that their volume increases only slightly. Lower grades have knots and other natural defects positioned in various locations so that large clear areas are hard to find. The smaller the size of the cuttings that we need from lumber, the higher the yield and the less effect of lumber grade.
Length? We often put a premium on longer pieces, scarfing yield. Maybe we can consider using random length pieces that are finer jointed together into long pieces that we then cut to the precise lengths…expensive, but with higher lumber prices and tight lumber supplies, this is beginning to look better and better.
What does yield improvement save? A few years ago, we calculated that a 1 percent yield increase was worth about $50 per 1000 BF of parts.
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