Q: We are having problems with our planer splitting the lumber from time to time. It seems that everyone is blaming everyone else, but no one seems to be able to eliminate the problem. When we have the problem, our folks check the lumber MC with a needle meter and we do not have any wet lumber. Knives are sharp. What should we be looking for?
A: There are five basic, common causes of planer splits: 1) Low MC, 2) dull knives, 3) cupped lumber, 4) casehardened lumber and 5) varying lumber thickness. I have listed these in the order of "most likely cause" based on my experiences. Let's look at each one briefly. Let me know if you require more information.
Low lumber MC
Many properties of wood increase as the wood is dried, but the splitting strength (called cleavage) drops as the lumber MC falls below 6 percent. The loss in strength (or increase in brittleness) between 8 percent and 3 percent MC can be 18 percent! One problem is that the meter you are using does not do a really good job of estimating the MCs under 7 percent. I suggest that for this situation you use a pin-less meter. Also, check the kiln operation. Is the kiln operated at temperatures over 160 F during the main part of the schedule? Does the kiln operator have kiln samples representing the wettest and the driest MCs in the kiln, or just the wettest? Note: If the lumber is over-dried, then cupping is usually worse too. For softwoods, MCs below 8 percent would be over-dried; for hardwoods, under 6.0 percent MC.
When knives are dull or have been jointed excessively, they pound the lumber. Such pounding can be a cause for splitting, especially with over-dried wood.
Cupping of lumber during drying is a natural event, especially in lumber from near the center of the log. Rewetting (incorrect schedule, operator errors) and over-drying accentuates cupping. When cupped lumber enters the planer, the feed rolls and chip breakers are trying to push the lumber flat. If there are pre-existing checks or if the lumber is over-dried, then often these forces will crack the lumber. There will be more severe risk of splitting cupped lumber if stock removal rates or feed rates are high.
Although it is rare to have casehardening (which refers to the amount of stress in the wood and not that the case or shell is harder than the core) cause splitting, it can be a factor in making splitting worse. When one side of the lumber is planed, the resulting unbalanced stress pattern causes the lumber to cup toward that face. The cup is pushed down by the pressure bar, feed rolls, etc., resulting in splits. Casehardening is easily removed in the kiln at the end of drying. Special note: Some species of lumber, mostly non-U.S., have growth stresses in them from the tree, which behave the same way as casehardening stresses.
Varying lumber thickness
When lumber thickness varies, it would be likely that some pieces would have been over-dried in the kiln. Further, the high pressure from the feed rolls and chip breaker on the thick pieces can result in forces large enough to crack the lumber.
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