Bypassing side bend
September 9, 2010 | 7:00 pm CDT

Q: I have a rip-first mill with a Comp-U-Rip input. We have received advice and suggestions from several people about how to handle lumber that has side bend. What do you suggest?

A:  Your question is an important question, as side bend (which is not controlled in the drying operation, but is a result of naturally occurring stress in the tree) has a tremendous, unappreciated effect on yield.

I am giving you a general answer that has to be fine-tuned for your specific case.

If you have a salvage operation (rip and crosscut that can make acceptable parts from strips with wane) that runs in addition to your main cut-up line, then I would suggest that you rip lumber that has up to about 1-1/2 inches of side bend. You would also need to instruct your Comp-U-Rip operator to place the laser lines wide enough so that the two edge-ripped strips will have about 50 percent wane and 50 percent wood (lengthwise) on the outside edge. Lumber with more than 1-1/2 inches of crook would need to be crosscut first into two shorter pieces, thereby minimizing the side bend.

Without a salvage operation, you need to produce strips that are probably about 85 percent clean on the edge (rather than 50 percent as above). This would involve instructions and skill from the laser operator. Further, I think that any piece with over one-half inch of side bend for every 8 feet of length would benefit from being crosscut first into two shorter pieces.

Without a salvage operation, the effect of side bend on yield will be unbelievably large. With today's lumber quality, I think that a salvage operation and ripping as described above is essential.

I do have a concern, however. If the rip saws are very thin blades, then as the edge saw blade gets close to the edge of a piece of lumber, it is possible that the saw will bend slightly and travel along the outside edge of the lumber rather than in the piece. The end result is that the edge may not be flat enough to be an acceptable surface for gluing.

Now a question for you. What part of the saw tooth on a rip saw blade actually cuts the wood surface that you will be gluing? Answer: The edges or sides of the tooth and not the top. With a rip saw, remember that "side dressing" of the blades is critical for good gluing. Resharpening should include sharpening the edges of the tooth as well as the top.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.