Q: We are having a problem with our veneer. The surface is quite rough with lots of what I would describe as numerous long hills and valleys running with the grain of the wood. It appears as though the wood has been torn from the surface. As a result, we have to do a large amount of sanding to eliminate the hills and get a smooth surface. It is pine veneer. Can you help us figure what might be happening? Incidentally, we also are seeing some feathering of the grain after sanding. By feathering, I mean small, very thin areas at the end of a growth ring that seem to be separated from the wood below. Anything here that would help too?
A: I have examined the samples you sent. Both defects, the roughness and the feathering, are the result of mechanical damage to the wood's surface. It is not a growth defect.
Consider the roughness first. When cutting veneer, it would be normal or common for the actual cut or separation, if you watched it under magnification, to go in front of the knife and "follow the grain." The knife itself is quite large and thick and therefore breaks or splits the wood; it is behaving in a similar manner as driving a wedge into the wood. We might say that the wood is actually splitting ahead of the sharp razor edge of the knife. The magnitude or extent of this splitting would be much greater when using a dull knife or when veneering too fast.
To minimize this natural "splitting ahead of the knife" tendency, a veneer lathe will have a pressure bar (or nose bar) located right in front of the knife to compress the wood before it is cut. This pressure is a very effective technique to keep the split from going very far in front of the knife. However, as we sharpen the knife and it gets shorter, we need to continually adjust the location of the pressure bar to keep it just a fraction in front of the knife. Perhaps with your veneer the bar is not properly adjusted.
As the knife edge dulls, the knife tends to split the wood even more. In this case, we will note that the veneered surface will be quite rough, with lots of small "hills and valleys" where the knife pushed the wood out of the way and then finally tore out the wood fibers rather than cut them cleanly. This will be most severe where the veneer has softer areas, such as in the earlywood portion of the growth ring of pine.
In response to a dull knife, sometimes the pressure bar will be tightened, instead of re-sharpening the knife. Of course, tightening the bar will lead to potential pressure failures, require more energy to veneer and result in a poor surface too.
If the logs are not properly heated beforehand, this roughness defect is also more common. A little detective work at the veneer mill should correct this problem quickly.
Now, let's consider the feathering defect. Let me explain what is happening. The annual growth ring in pine is made up of two types of wood: a) the soft, light-colored wood, called earlywood or springwood, and b) the dense, dark-colored wood called latewood or summerwood. The color contrast makes it easy to observe the annual growth rings and these two types of wood within a ring.
When machining or sanding flat grain lumber or veneer, the hard, dense latewood, which is hard to cut or sand off, is often easily pushed into the softer earlywood, especially if the machining pressure is very high. High pressure can result from too much knife heel, excessive pressure from the feed rollers, or dull sandpaper. This pressure results in rupture or failure within the wood at the junction of the earlywood and latewood. The failure is quite small, but will be obvious after machining if the wood picks up moisture from higher RHs or water-based finishes.
Described another way, when cutting the harder, denser part of the annual ring, it is sometimes easier to push this hard wood out of the way by pushing it into the softer part of the ring rather than to cut it off. When moisture is added back to the wood, the squished cells pop back to their original size, creating raised grain and highlighting the separation.
The most likely sources of the excessive pressure are excessive stock removal (too much, too fast) or dull sandpaper. You mention that with the roughness, you have to remove substantial veneer to get a smooth surface did you change feed rates as the removal increased? In addition, the defect is more likely to occur with very dry wood (under 8 percent MC with pine), as then the forces required for sanding are higher and the dense latewood is even harder to cut and easier to push. My experience says that you can change sandpaper in the sander, and perhaps slow the feed slightly, and the defect will disappear.
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