Troubleshooting Sinusoidal Waves in Planed Lumber

We just like saying "sinusoidal." Seriously, here's a look at some issues that can affect planer performance. April 30, 2009

Question
I'm planing 4/4 Jatoba in a Powermatic 15" planar and am running into a problem I've never experienced. I can't get the planed surfaces flat, they have a slight sinusoidal waviness in them that won't go away no matter how light or heavy the cut is. This wave is not narrow like knives make but long (besides, I have a Carbide helix cutterhead). I thought that the in or outfeed rollers may be running eccentric but they don't appear to be. The wood is pretty rough at the beginning of the work. Should I adjust the table rollers?

Forum Responses
(Solid Wood Machining Forum)
From contributor C:
When paling rough lumber on a single planer, the correct way is to plane it three times. Run it once. Turn it over and plane the other side. Turn it over again and plane the first side.The reason for this is that the first cut will not be really smooth because the bottom side is so rough that it rocks, bobbles and bumps on the bed. The second cut is pretty good because the first side has been planed, even if not perfectly. The third cut should be perfect because that second cut was really good. Now if you do this and still have a problem with the finish, you have a machine adjustment problem.

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From contributor J:
I had that happen recently with antique heart pine. It was flat sawn and the waves ran along the curvature of the grain. We face planed it first on the jointer, then planed the other side on the planer. We never got the waves out on the planer but got most of it out on the wide belt. The only reason I could come up with was the varying density/hardness between the wood in the individual areas of grain. I normally don't have to use a dictionary in my discussions about planers.

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From the original questioner:
Thanks for your responses. I think I found the source of most of the problem. We had recently replaced (actually, I machined new ones) the support brackets for the infeed and outfeed rollers and the guy that reassembled the planer didn't compress the springs enough. After tightening them most of the problem went away.

Contributor J, you're right about the strategies that need to be employed when planing rough lumber. Contributor C, I have noticed that the grain direction matters too, especially on hardwoods like jatoba. So, there are many factors affecting how wood will plane: machine adjustment, wood properties and planing strategies.

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From contributor L:
It may have nothing to do with your machine. If it's been improperly dried or case hardened, or not kiln dried, especially more dense woods, it can bow as you described and more. If your machine is set improperly, it should be doing about the same thing to each board regardless of species. I've had this happen several times over the years from red oak to South American lacewood. I bought a very good moisture meter. In each case the lumber dealer confirmed my findings and took the wood back for full credit.

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From Gene Wengert, forum technical advisor:
Casehardening refers to a condition of stress in the wood. It cannot be measured with a moisture meter, but requires a piece to be cut into a prong, similar to a clothes pin. Casehardening will cause a piece to warp full length immediately when machining it. It causes no other problems.

Oftentimes the waves can be seen when a bed plate is worn. As the center wears most, this waviness is seen with narrow pieces fed down the center of the machine. The pressure bar can also be misset or worn and cause this same thing. Essentially, the wood is not being held tightly to the bed plate.

Heavy pressure (from feed rolls, too much heel on a knife, dull knife) with a wood that has high density differences within a growth ring (like some pines) can also cause this to happen, as the softer wood is compressed. The compressed wood will expand or springback after a short while. This springback is accentuated if moisture is present (water-based finishes, for example).