Troubleshooting Bow in Kiln-Dried Cherry

      A discussion of the causes and possible cures for bowing of lumber. November 14, 2009

I have a load of 4/4 KD 1com cherry that I'm surfacing to 15/16" and almost every board is bowed. The normal stress test and even the stress test for tension and reaction wood look great. The MC is 7 1/2% and below (haven't done an oven test yet). The rough stock doesn't show a variation in thickness either that would lead me to believe that it bowed when drying. What can be done to salvage this lumber? Would sticking it and a few more hours of conditioning help?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
It is not clear to me - are the pieces showing bow before or after planing?

From contributor S:
Is the lumber all red or somewhat sappy, or sappy on the ends or in the middle? Or sappy on one side and red on the other?

From the original questioner:
The lumber is bowed before and after surfacing. The boards are mostly 90/50 red or better and it's difficult to say how much the sap is affecting it since almost every board is bowed regardless of the sap content. If only a few were bowed I could give you a better answer.

From contributor J:
It sounds like the problem may be with your expectations rather than with the lumber. If the lumber is bowed to begin with, simply running it through a planer isn't going to fix it. If you need large, flat planks finished at 15/16", you'll need to start with thicker lumber. Joint one side flat, then use a surface planer to machine the second side parallel to the first. If you start with 1" lumber then you don't have enough extra material to correct the distortions.

From Gene Wengert, forum technical advisor:
If bowed before surfacing, then it is occurring in drying (poor stacking, but you would have seen this, or longitudinal shrinkage, which is unusual in cherry but can occur in smaller trees) or due to mixed sapwood and heartwood which indicates smaller trees and more chance of growth stress and uneven shrinkage. Are the pieces all fairly narrow? If so, then we can expect smaller trees which do have growth stresses that cause bow.

From Gene Wengert, forum technical advisor:
If the lumber bows immediately while planing, then it is drying stress which could have been fixed in the kiln, using steam for stress relief. Does the kiln drying facility use steam for stress relief? If the lumber bows a few days after planing, then the problem is moisture content change along with some wood from smaller trees.

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