Q. We are noticing that our plant yield seems to be gradually decreasing, year after year. What comments or direction can you give please?
A. The best “answer” to this issue is to have an experienced person (like the Wood Dr., but there are others) spend a day at your plant analyzing your production. Without an inspection, here a some ideas that you can possibly use to direct your own investigation.
GRADE. It should be appreciated that rough mill yield is greatly affected by lumber grade. For example, when cutting clear 2-face strips, we could see around 70 percent yield for FAS, 59 percent for No.1 Common, 43 percent for No.2 Common and 30 percent for No.3A Common. Therefore, changes in yield over time might reflect changes in the incoming lumber grades. (Note that the basic grade rules have not changed substantially for decades.)
Sometimes a mixed grade can be an issue; with No.1 Common and Better, we do not see much “better.” Another possibility is that the lumber within one grade is being sorted into two groups and you are getting the poorer group.
END SPLITS. Although they do not affect the lumber grade very often, end splits can realist in a loss of yield of around 4 percent or so. It will be more with shorter lumber -- about twice as much for 8’ versus 16’ lumber.
LUMBER WIDTH. Lumber width also affects yield with narrower pieces having lower yield. It is really an issue when the sawmill makes certain width pieces and the rough mill has only a few widths at the rip saw. (See figure for a mill cutting fixed width strips. Note the huge affect of width on yield.)
PART SIZE. Yield is also affected by part width and length; wider and longer greatly affects overall yield.
DRYING QUALITY. Drying can also affect yield, especially with respect to surface checks for oak and stain is some other species. The climate that last few years has been drier and warmer than we are used to, so modification of air yards may be required in order to maintain quality drying. Note that warp is not a controllable factor in drying, except for some cup. Smaller trees generally mean more warp.
SHAKE AND PITH. Shake (splits running parallel to the rings) is a grading factor, as well as pith (the bullseye center of the growth rings). Both drop yield very badly, even though the lumber might have a reasonable grade. That is, a piece that is No.2 Common because of the location of the knots will have higher yield than a No.2 Common piece with pith or shake. You might consider discounting pieces with pith or shake, as such pieces are expensive for you as their yield is low.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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