Wavy surface during planing
October 15, 2009 | 7:00 pm CDT

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.


Q: How long do we need to keep the clamps on our panels? We use a clamp carrier and I think it might be a good idea to add a few more sections so that the pressure stays on longer.


A: What you are asking me is how long until the adhesive develops a joint that is as strong as you need it.

First, you need to be concerned about how fast the adhesive cures. This is provided in the technical information about a particular adhesive; ask your supplier. Note that the curing time can be varied by changing the formulation and also by changing the temperature of the wood.

The second consideration is how much strength do you need before the joint is as strong as you need it for subsequent handling and processing? Oftentimes, you may only need 50 percent of the ultimate strength to develop when you remove the clamps and begin to process the panel. The remaining strength will develop after leaving the clamps. Again, this is a good question for your adhesive supplier, as some adhesives do not develop maximum strength if the pressure is removed too soon.

If I read between the lines, your question must have arisen because of some problems. If the problems are related to a short clamping time, then lengthening the carrier is a good idea indeed. But, perhaps the problems that you are having are related to something else, such as high moisture wood that results in open joints at the ends of the panel in the wintertime. Longer clamping will not help this; correcting the moisture will.


Q: We purchase KD hardwood lumber for our operation. Is there an easy way to check the footage? We did a few tests by actually measuring the footage of every piece and once in a while have found that there is often a variation (not in our favor).


A: If you want to be 100 percent accurate, you have only one option and that is to measure every piece, using the correct measurement rules.Oftentimes it's possible to estimate the footage; if the footage is very close to the actual, then accept the sellers' tally.

Measuring every piece

For hardwood lumber, the standard length of the lumber is the last full foot of length. Inches are not counted. So a piece that is 9 feet, 10 inches has a standard length of 9 feet. The width is the actual width, measured in inches and fractions; most of the time, fractions are 1/4 inch or 1/8 inch increments. To get the footage, multiply the width times the length and mark down the answer; this answer is called the surface measure (SM).

Then, note the thickness. After this is done, then go to the next piece and repeat. When all pieces have been measured, then take all the pieces of the same thickness and get a sum of all the SMs. Then multiply the total SM times the thickness to get the board footage. (If you multiply the individual SM by the thickness for each piece of lumber and then total, you will likely not get the correct answer.)

Thickness is the nominal thickness, so 4/4 is 1 inch, 5/4 is 1.25 inch, and so on. Note that the actual thickness can be thinner if dealing with planed and/or kiln-dried lumber, but always use the nominal thickness.

Estimating the footage

One short cut technique to provide a very close estimate of the footage is called block tallying. In this case, the width of an entire layer is measured and treated as one piece of lumber. Then this width is multiplied by the length to get an SM for each layer. If the length of the pieces varies within a layer, this will be a source of error. Total up the SMs for each layer and then multiply by the nominal thickness to find the estimated board footage.

Another technique is to weigh the pile of lumber using a weighing system built into a forklift. Such systems, which are very inexpensive, can be part of a small hand-lift truck or can be installed in a motorized large lift. The key is to know, for a given species, the typical weight per board foot.

Tables are published with this information, but it is probably better to develop your own conversion of weight to board feet, to accommodate regional weight variations for a species and to accommodate thickness variations. Naturally, moisture content will affect the values as well. One Internet source for free weight data for various species, thicknesses and moisture content is www.woodweb.com/knowledge_base/Calculating_the_weight_of_lumber.html

With both estimating techniques, you would compare the estimate to the actual sellers' tally. If the discrepancy is too large, then an individual tally is necessary.


Q: We just bought some planed, kiln-dried lumber as we needed it in a hurry to complete an order. Our lumber supplier indicates that for planed lumber, it is graded from the best face. We went with his explanation, but is this standard?


A: Yes, planed lumber is graded from the best face, while rough lumber (at any MC) is graded (with a few minor exceptions) from the poorer face.

Have something to say? Share your thoughts with us in the comments below.

Profile picture for user genewengert
About the author
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.