Planing defects such as snipe, ripples and raised, chipped or fuzzy grain can ruin a project. Here are simple solutions.
Have you ever planed a piece of wood and then found that the first 6 inches or the last 6 inches of the lumber is a bit thinner than the rest of the piece? This machining defect is called snipe.
It happens when the lumber is not held tightly against the bed plate of the planer, but lifts off the plate and is pushed into the planer knives. The reason the lumber is not held tightly against the bed plate for a planer that has the planer head on the top of the lumber is that gravity is pulling the other end of the lumber down. The chip breaker or pressure bar just cannot hold the piece against this force of gravity. For a bottom-head machine, it is likely that the infeed table or outfeed table is too high.
The solution is simple. For a top-head machine, the infeed and outfeed tables where they connect to the machine must be exactly level with the bed plate. Then, further from the machine, the tables must go up a few thousandths of an inch higher than the bed plate of the machine for every foot of length. The table must also be long enough to hold the full length of the lumber. For a bottom-head machine, the slope would go the other direction.
Oftentimes, adding slightly to the problem is that the pressure bar has been worn from abrasion with the lumber. Although its original setting was correct, with wear it becomes too loose.
Have you ever planed lumber, only to find that after planing you need to sand it a great deal to get rid of the planer ripples and get it smooth? Why does this happen? There are three basic causes.
The first is that feed speed is too fast. Understand that a planer knife is actually spinning in a circle, so its cut on the lumber is not flat but is a small groove or dished area, a small arc of the circle. If you feed slowly, around 15 to 20 knife marks per inch, the difference between the highest point or lowest point in this dish or ripple is very small and will require little sanding to achieve a really smooth surface. If you plane fast with fewer than 15 marks per inch, the arcs becoming wider and deeper with more sanding needed.
The second cause of such ripples is that the bed plate of the planer is no longer flat but has been worn after rubbing against many board feet of lumber. In this case, especially with narrower pieces, the lumber will not be held tightly but will chatter up and down, creating some large ripples that require a lot of sanding.
A third potential cause will happen when the knives are fairly dull. The knife will often push some of the wood down into the lumber face rather than plane it off. Coming out of the planer, and even after sanding, the piece will look smooth, but as soon as a little moisture hits the surface – from high relative humidity or water-based finishes – the crushed areas will spring back and give the lumber a rippled appearance.
Have you ever had a fairly smooth surface, but then a few days later the surface was uneven with the unevenness being aligned with the grain of the wood? This is a common event when dull knives are used, or high stock removal occurs; each knife is removing a lot of material.
It will only occur with woods that have a large density difference within an individual annual growth ring; that is, the early-formed wood, sometimes called spring wood, is lower density than the later-grown wood, sometimes called summer wood. It results when the knife pushes the dense latewood into the soft earlywood, instead of cutting the wood fibers.
Dull knives and low moisture content accentuate this effect. Then, when the surface is exposed to moisture, the crushed fibers spring back, raising the dense latewood above the surface. This defect will occur only on one side of the lumber if the lumber is planed in the same direction on each side.
The solution is to take smaller cuts and use sharper knives. However, with a wood like southern pine, which has such a large contrast within the growth ring, raised grain is hard to avoid.
If the damage done is so severe that the grain actually fails and tears, then the defect might be called torn grain. With torn grain, you can take your fingernail and flick up the loose edges of the grain.
Have you ever seen an area of planed lumber that has a multitude of small areas where the wood has been torn out and left small depressions that need to be filled with putty to get the area smooth? This chipped grain is common around knots. Old-timers will sometimes tell you that you were planing against the grain, so you should flip the piece around, end for end. That may work sometimes, but it is hardly a productive way to handle this defect.
The defect is caused when a knife that is a bit too slender, or the rake angle is too large, enters the wood and figures out that it takes less energy to split the wood ahead of the knife instead of cutting the fibers. Dull knives, high feed speeds, deep depth of cuts, low moisture content and high temperature dried wood are more likely to incur this splitting.
If the grain of the piece comes out of the face – planing with the grain – then the split, which will follow the grain, will go ahead of the knife but into the waste wood that will be removed in planing. In this case, no defect. But if the grain is going into the piece – planing against the grain – then the split will travel into the good part of the lumber. Eventually, the knife will tear the wood out, but the end result is a small, deep hole.
In addition to sharpening knives, removing less material per knife cut, assuring that the wood is not over-dried (under 6.0 percent MC for hardwoods and under 10.0 percent MC for softwoods), and is dried under 160F, the knife should be made a few degrees more stout.
The knife is still sharp, but it has more metal. It will act more like a plow when cutting wood rather than a wedge. The bad news of doing this is that the knife will work well for dense woods, but will likely fuzz the lighter-weight woods. Many shops will have two different heads one for denser woods and one for lighter-weight woods.
A comment about planing against the grain: Some woods and some pieces of lumber will have swirly grain, so even on one face of the lumber, you will be planing against the grain in some areas. Two examples: maple has small pockets of swirly grain; mahogany has long stripes of "against the grain" wood.
Have you ever noticed that, after planing, the lumber will seem to have fine fuzziness? It may be all over, or it may be in certain areas. It is hard to detect unless there is very bright lighting on the exit end of the planer. Fuzz does cause blotchiness when staining and a roughness of the finish.
Fuzziness results because the wood fibers are too weak to be cut cleanly, so they fold over. With the addition of moisture, they pop back up and really look rough. Oftentimes the weak fiber results because of a growth defect called tension wood, which is found only in hardwoods. Weakness also results when the MC is high. Also, low-density species have weaker fibers, especially aspen and cottonwood.
Slender, sharp knives (large rake angles) will help the situation. Often, in practice, the cure is to use a sanding sealer or wash coat on the lumber after planing but before sanding. This treatment stands up the fibers and then holds them up so they can be sanded cleanly.
Source: 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 and can be reached at [email protected]. You can read past Wood Doctor columns at WoodworkingNetwork.com.
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