Q: Can you tell me what is causing us to see a very bumpy, wavy surface when planing? We are running at about 12 knife cuts per inch, but we see a lot of waves anyway.

A: In most planers, directly behind the knife is the pressure bar. Unlike the chip breakers, which are movable and spring loaded, the pressure bar, once adjusted properly, is in a fixed position. Typically, the pressure bar is set a few thousandths of an inch wider than the thickness that the knife cuts the wood. This slightly wider opening is required because the cells in the wood will spring back slightly after being cut, making the piece a little thicker than it was actually cut.

If the pressure bar is set in perfect alignment with the knife's deepest cut, the wood will spring back and then will be too thick and will not feed well. So, the pressure bar is usually set a few 1/1000ths of an inch more open than the knives.

Bed plate wear

Where we sometimes see problems is that the bed plate of the planer will wear in the center. (Don't most people feed a planer right down the middle rather than varying from side to side like they are supposed to?) So the opening at the sides without the wear is smaller than the opening in the middle. Sometimes the pressure bar will also be worn in the center section.

This wider opening in the center is not a problem with wider pieces of wood, as the tight opening at the edges will hold the lumber tightly. However, a narrow piece of wood sent through the center of the planer has a larger opening than required and so will not be held tightly against the bed plate. It will chatter. This is your problem, so check the flatness of the bed plate and the flatness of the pressure bar, side to side.

Are you old enough to know what carbon paper is? If so, find a sheet of it and rub it gently on the wood surface after planing. This will highlight the surface marks. Now, are the marks spaced identically and possibly in a repeating pattern, or are they random? Chatter is random, but problems with setting the knives or other rotating object will have a repeating pattern.

Note that many planer manufacturers have excellent field maintenance people, and there are a few consultants who can check your machine over and make suggestions on what needs attention. In my experience, the cost of such service is well worth the benefit.

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