Planer splitting
November 3, 2013 | 6:00 pm CST

Q: From time to time we have a rash of splitting at the rough planer. Pieces of rough, just dried, lumber go in with little or no splitting but when they come out of the planer, they have long splits. You can actually hear them splitting in the machine! Have you seen this and, if so, what are the causes and the cures?

A: Your problem is not new; as you state "From time to time, there is a rash of splitting." In fact, at times I have seen lumber that seems so brittle that it even splits when it is dropped on the floor. In short, the wood seems extremely brittle at times.

The good news is that these splits, which most often show up in the planer and are called "planer splits," are 100 percent controllable. (Note: The splits we are discussing here are splits that occur when the piece is handled and not the splits that occur during drying.)

There are five basic causes of planer splits, listed in the order of likelihood of occurrence in most operations.

  • Low MC lumber
  • Dull knives
  • Cupped lumber
  • Casehardened lumber
  • Varying lumber thickness

Often, when looking for the cause in your facility, you need to look for two or more from the list that are out of control. Individually, each one may be just on the border of acceptability, but when two or more are slightly off, the end result is much worse than might be expected from the slight deviation of each one individually.

Low MC lumber

Most strength properties in wood increase as the lumber dries.

However, the splitting strength, called cleavage, may improve as lumber is dried from green to about 10 percent MC, but then the strength falls off again. It seems that in softwoods, the loss in strength (or the increase in splitting) increases substantially under 8.0 percent MC; for hardwoods, under 6.0 percent MC. In fact, from 10 percent MC to 3 percent MC, the loss can be in excess of 18 percent in some pieces. This loss is greatest in the lower grade pieces, in my experience.

The next time you have planer splitting, take an MC meter and check the MC in the incoming stock and of the split pieces. A pin-less type meter is preferred as this meter will measure under 7 percent MC accurately; the pin meter usually cannot measure accurately under 7 percent MC and therefore cannot detect low MCs.

To avoid low MCs, the kiln operator must make sure that equalization begins early enough, when the driest piece is 2 percent MC below the target. In my experience, many kiln operators do not look for the driest piece when sampling and running the kiln, and so some of the lumber in the load is over-dried. The kiln operator is too concerned about the wettest lumber and ignores (incorrectly) the driest pieces. (Note: Over-drying also leads to more cupping. See discussion that follows.)

One additional item: Sometimes that kiln operator will intentionally over-dry the load to avoid any wet lumber and to dry as quickly as possible.

Then, a high-humidity treatment is used to bring the MC of the dry pieces back to a more acceptable level. This process, however, does not restore the machinability of the lumber. Once over-dried, there seems to be a permanent loss in machinability. This sort of over-drying and then restoration of MC is hard to detect; one must work with the kiln operator to avoid this situation.

Dull knives

Splitting requires energy; wood does not normally split while it is sitting still. The energy in some cases is the pounding of the knives. Dull knives would be a problem, especially if they have been jointed too much (over 1/32 inch land, the "flat spot" created when the knives are ground down to get them all protruding the same amount, is probably too much). The next time you have splitting, find out whether the planer operator thinks that the knives are getting dull and need sharpening.

Cupped lumber

As lumber dries, it has a natural tendency to cup toward the face that was closest to the bark (in the log). Further, cupping is worse for pieces cut from nearer the center sections of the log. This means that low-grade lumber and lumber from small-diameter trees (i.e., today's lumber) would have much more cup than higher-grade and lumber from large-diameter trees (lumber from 20 years or more ago). The natural tendency to cup can be accentuated by drying too slowly, by rewetting partially dried lumber (i.e., mixing MCs in the kiln) and by over-drying (the biggest culprit).

When cupped lumber is fed into the planer, the feed rolls and the chip breakers are trying to push the piece flat. If there is a small check or split, this failure can be easily accentuated by this additional pressure from the machine's components. (Note: Do not ease up on feed roll pressure or chip breakers to solve this problem. They are required to be tight to help hold the lumber from moving while machining.)

Casehardened lumber

Casehardening is also called drying stress. It is a stress situation in the lumber that should normally be completely relieved or eliminated at the end of the drying cycle. (The process is called stress relief or conditioning.)

Unfortunately, some drying operations do not have the equipment to properly relieve these casehardening stresses. Casehardening does not refer to a situation where the case or shell of the wood is harder than the core.

When wood is casehardened and one side is planed by the first head of the planer (or jointed), the lumber will immediately warp, or try to warp, toward the face that was just planed. However, the pressure bar holds, or tries to hold, the lumber flat. The net result is a tremendous amount of stress and the increased likelihood of splitting.

Note: Casehardening will also cause immediate warp in other machining operations as well.

Varying lumber thickness

When thicknesses vary, it is likely that there will be excessive cupping and some over-drying, especially with the thinner pieces. Further, the feed roll and chip breaker pressures will increase on the thicker pieces. The thicker pieces will also have more stock removed with the first pass, meaning more energy and chance for splitting. The solution involves working with the sawmill, or, in the extreme case, planing green lumber (called presurfacing) to eliminate thick pieces.


Planer splits are all too common. They are caused usually by over-dried lumber, although other factors can contribute to the problem. Overall, these splits can be 100 percent eliminated without undue effort.

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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.