Q: In order to increase yields in the rough mill with a rip first operation, we have asked our people that mark the strips for our automatic chop saws to mark as close to the knots as possible. Indeed, this increased yield, but the grain around the knots was especially hard to glue and with the yield emphasis, we are getting more of this rough grain. The bottom line is that we have increased our gluing failures. We want to confirm our observations. Does this make sense to you?
A: You have hit on one very important concept in rough mill yield improvement and that concept involves the definition of what is a defect. It is certainly easy to define knots as defects, but as you noted the cross-grain around the knot is hard to glue (and also hard to machine well). So, I maintain that such cross-grain can and probably should be included in the definition of a defect.
Unfortunately, some people who write about yields, computer simulations, rough mill performance and so on do not have this practical experience factored into their suggestions. In fact, in some species, such as hard and soft maples, in addition to gluing and machining, the color variation around a knot due to cross-grain light reflecting properties could also support calling this grain a defect.
A second consideration when trying to improve rough mill yields is that we encourage the crosscut operations to take a minimal amount of end trim from the rough lumber. This is certainly a great suggestion for lumber that is properly end coated (assuming the coating is thick enough, which is often an issue). End coating can show yield gains of 2 to 5 percent in some cut-up operations. However, if the lumber is not properly end coated, or has no end coating at all, there will be some splits that are very obvious and some very fine end splits that are impossible to see when defecting the lumber. (They are most often impossible to see if the lumber has been stored at over 40 percent RH. This higher RH will swell the ends and close the end checks and splits that occurred in drying. But this moisture only closes them and does not heal them.)
So, there will be a few cut-up pieces leaving the rough mill that have small, invisible hairline-type checks, cracks or splits on the ends. These cracks will re-open whenever the wood is exposed to dry air. In other words, the checks seen later in manufacturing are old checks that are reopening and not new checks. Efforts to stop the manufactured wood from drying using coatings or plastic wrapping or other techniques are ineffective, because eventually the ends will dry and the crack will reopen. Oftentimes this reopening will be after assembly of panels or other products, and may even be after the piece is finished. The bottom line is that yield gains, which look good on paper, are quickly offset by the number of rejected manufactured panels or other products.
The bottom line is that yield improvement is a fantastic objective, but such improvements must be made considering the quality of the manufactured part. Such quality must be considered or evaluated throughout the entire manufacturing operation and not just at the back end of the rough mill.
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