End cracks in rough lumber
November 18, 2020 | 3:19 pm CST

Q. When we receive rough lumber, there are end cracks, end checks and checking in the rough state. Further processing, drying, rough planning and gang ripping will expose more of these characteristics. Help please.

Answer. There are three causes of end splits. First is stress in the tree. Such cracks in rough lumber are ¼ inch or wider after drying. This stress, called growth stress, often also causes warped lumber, lengthwise. We can do little about these stresses.

It is worth remembering that the crack maybe 10 inches long, but the stress actually goes beyond this 10 inches but the stress is not high enough to crack the lumber further.

However, pounding from planer knives or other knives, or even rough handling, will add to this stress level, causing, during machining, the split to grow by a foot or more. This is the second cause. We can minimize this added stress by using very sharp knives, avoiding excessive feed speeds.

It is also possible in the winter that a little end drying and shrinkage will occur when the MC of the wood is several percent higher than the EMC of the air. This end drying and shrinkage adds to the existing growth stress to cause a failure, even though the MC is not that much different.

Without growth stresses, or a preexisting crack, this end drying alone cannot crack wood. Sometimes the heat in the finishing oven can dry the end and add stress, too. Again, this growth stress often causes ripped cutting to warp, especially when machining.

Finally, we can cause end splits due to drying without end coating. It is not uncommon for such drying splits to be hard to detect visually, as often they are internal.

Hence, for this reason, in the past we had the defect saw operators take an extra inch and then look at the end to make sure there was no honeycomb. Sometimes a 2 inch end trim was automatic. The automatic scanning systems do not have this ability to detect small splits so often do not trim enough.

Likewise, even people that mark with crayons miss this. Note that end shrinkage alone with small MC changes is not strong enuf to Initiate a new crack in dry wood. We need another source of stress or a preexisting crack, even if very small.



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Gene Wengert

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.