Below are some questions and answers pertaining to lumber and best practices for solid wood machining operations, as submitted to Dr. Gene Wengert, the Wood Doctor.
Q. We have been buying steamed walnut, but the latest batches seem to have more color variation and the sapwood is really white. What is the reason for this poorer color?
A. The process of steaming walnut is quite tricky. It must be steamed at about 200F to 205F. This temperature must be reached as soon as possible in the steamer. Cooler temperatures, including long heat-up times, can be fatal for developing the best color. Usually three days is required for steaming, but this can vary due to building construction.
In addition to heat, it is super critical that the humidity be 100 percent at all times, including heat-up. It is hard to maintain 100 percent RH above 205 F. Anything less than 100 percent RH means that the wood will dry a little bit and once the wood dries, developing the dark color is really tough.
Where we see humidity problems is when high 15 psi or higher pressure steam, that is 250F for example, will go into the steamer and this is too hot and the RH will not be 100 percent RH. What is required is that the steam must be boiled through water, usually in a trough filled with water in the steamer, which will cool the steam to 212F and also develop 100 percent RH.
The correct heat and humidity will not steam walnut (develop the desired color) if the wood has started to dry before going into the steamer. In warmer temperatures, the walnut may be indeed drying a bit before entry. Keep the piles covered with plastic before steaming to prevent drying. If wood is prewired before steaming, we also see a lot of long end checks when the ends are trimmed off.
In your case, I suspect, as it is most common, that the water trough is plugged or does not have enough water. Maybe the steam pipe has corroded so the small holes are too big now. Close inspection of the equipment and the process should quickly lead to a specific answer.
Q. When our crosscut or chop saws cut the end of oak strips, the end has many small splinters. Do these affect or cause the joint failure we see?
A. First, the chop saws need a plate at the back of the saw to hold the fibers against the wood piece as the saw completes the cut. It needs to be very close to the saw blade itself. Sometimes this is called a backer; in a planer, it is a chip breaker. When this is not done, the saw will break them off instead of cutting them, which is what you see.
Second, it is likely that it will affect gluing slightly with oak, but less so with softer woods. However, the effect is only at the very end. If the failure goes beyond the last 1/8 inch, then there is a bigger issue with the surface preparation, pressure or other factors. As you know, the joint should be 150 percent stronger than the wood itself. So, you need to look for why your joints are not full strength.
A common cause for a weak glue joint is the strips are cut one day and glued the next. Because the moisture in the air and wood are not in equilibrium, the wood changes moisture especially at the ends, which absorb moisture faster than the faces. A size change occurs, so the two pieces of wood being edge glued are not within 0.002 to 0.006 inch. You may need an outside person to come and examine the situation.
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. 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.
One trick is to find a piece of carbon paper and rub it gently on the wood surface after planing. This will highlight the surface marks. 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 objects 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. We have a machining problem. As we machine a piece, it is good until the last half inch. At this end, we all too often get a piece that chips out all the way to the end. Where should we look?
A. When a knife comes around and begins to take its cut, it always asks this question “Is it easier to cut the wood or to push it (or split it) out of the way?” In your case, it is easier to push it out of the way sometimes.
My answer begins with another question “What makes the wood so strong at the end that pushing or splitting is easier or, conversely, what is it that makes the end piece easier to split off?” From a practical standpoint, we have several answers that need to be checked out:
1. The knife could have too large of a rake angle, so it acts like a splitting chisel or wedge and not like a cutting knife. In this case, on a microscopic level, there is a small crack that runs ahead of the knife, with the crack following the grain. As we get near the end, this crack weakens the wood enough that splitting the piece off is easier. If this indeed is the cause, you would also notice some chipped grain here and there.
2. The knife could have a rake angle that is too small. In other words, the knife is plowing the wood off, and that continues to the end. In this case, we would also see more fuzzing in the rest of the piece…more than normal.
3. The energy required to cut a large amount of wood off in one cut is higher than a small cut. Plus higher feed speeds make it worse. Fast, deep cuts are tough so splitting is easier.
4. We know that drying lumber over 160F and drying to moistures under 5.5 percent MC make the wood more brittle. Brittleness means splitting is easier.
5. Dense wood is harder to cut, but splitting is still easy. Fast-grown hardwoods with large pores like oak and slower-grown softwoods are more dense usually.
Check these out. If these are not the cause, we will need more information.
Source: Gene Wengert, “The Wood Doctor,” has been training people in the efficient use of wood for more than 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison. You can read past columns at WoodworkingNetwork.com.
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