Woodworking 101: The Basics
An Industry Review:
By Dr. Eugene Wengert
At least 75% of the wood manufacturing problems I review in the University's Forestry Department are moisture related. That's why eliminating wood problems begins and ends with monitoring and controlling the moisture content (MC) in wood.
As the moisture content in wood changes, wood expands or contracts, and this in turn causes a variety of problems. The moisture content of wood is measured as a ratio between the weight of the water in the wood and the weight of the wood itself. This ratio is stated as a percentage.
Freshly forest-cut "green" wood may have a moisture content of 30% to more than 200%, depending on the species. Before using any wood, it needs to be dried to reduce its moisture content. The "ideal" moisture content depends upon the use of the wood and the annual average relative humidity at the place where the wood is to be used. It is critical that the wood you work with be dried down to an MC within 2 percentage points of the equilibrium moisture content (EMC) of the in-use location. The EMC of air is numerically equal to the MC that will eventually be attained by any piece of wood when stored indefinitely at a particular humidity. Temperature has no direct effect on MC or EMC.
The relative humidity (RH) in most homes and offices in the U.S. (except in coastal areas and exceptionally dry areas like the desert Southwest) averages 30% to 40% RH. This is 6% to 7% EMC (see the tabulation), which means that wood in interior locations will average 6% to 7% MC. Therefore, lumber intended for interior use should be dried to 6% to 7% MC and should be kept at this MC prior to and during manufacturing. Two special notes: (1) Softwoods machine better at a little higher MC and shrink and swell less than hardwoods when the MC changes. Therefore, often the target MC for softwoods is 8% MC. (2) The term kiln dried has no special indication of MC for furniture, cabinet, or millwork manufacturers; don't specify kiln-dried lumber without also adding the actual MC that you want.
Wood is a cellulosic material, so it is constantly losing or gaining water to or from the environment. Therefore, it is important to remember that even after wood has been dried to the proper MC, MC can change during storage, manufacturing, or use.
One of the key points in working with wood is that, if the shape or size of wood changes, it is because of a change in MC. That is why metering of wood at all phases of manufacturing is critical.
Now let's review several ways of eliminating wood problems that may occur in the mill or in the plant environment.
1. Sunken Joints in Panel Manufacturing.
The most common cause of a sunken joint is that wood around the joint swells when it absorbs water from the glue itself. If the wood is planed or sanded flat before the swelling goes down (that is, before the MC equilibrates), the wood at the joint will shrink later, creating a depression at the joint.
The remedy for a sunken joint is to wait for the moisture around the joint to dissipate before planing or sanding. Generally, seven days at warm temperatures is sufficient.
In a few cases, a very dry wood panel machined flat in the dry condition and then permitted to regain moisture during storage will have very little swelling in thickness at the joint area (the glue is rigid), while the wood away from the joint will swell in thickness normally, creating a sunken joint appearance. The cure in this case is to make sure that the MC of the lumber at the time of manufacturing is within 2% of the EMC of the air, even in the humid summer months.
2. Uneven Surfaces in Panel Manufacturing.
Assuming that the panel was even when it was sanded or planed, the wood could change shape only if its moisture content changed. Therefore, planking is caused because individual strips or pieces were at varying moisture contents when the panel was glued up, and these MCs were not equalized before sanding or planing. For example, a 1" thick panel could experience a noticeable thickness change of up to 0.01" with only a 2% to 4% MC difference between individual strips. (That is, planking can be obvious when one strip is 6% MC at the time of gluing and the adjacent one is 8% MC.)
3. Panel End Splits.
Remember that the end grain of wood dries up to 50 times faster than the edge or the face. The ends therefore dry rapidly and shrink, while the center of the panel dries and shrinks more slowly. This difference leads to stress build-up and, potentially, end splits.
To correct this problem, keep the moisture content of the lumber within 2% of the EMC of the surrounding air. Because the EMC in the plant and in the home or office is lower in the winter, lumber should be dried to a lower MC in the winter (typically 5% to 7.0% MC), and/or the plant must be humidified slightly.
4. Cupped or Curved Panels.
A. Because the face of the lumber closest to the bark shrinks more than the other face, a glued-up panel with most or all of the pieces with same grain orientation will shrink more on the bark side as it dries, thus causing cupping. Most important, lumber should be dried to the correct MC to minimize all shrinkage. As an additional safety factor, individual pieces should be varied in their grain orientation-bark side up, then pith side, and so on. (Note: The reverse situation occurs if the lumber is too dry for the EMC.)
B. If the finish on both sides of the panel isn't identical, the two faces can swell at varying rates, resulting in warp. Therefore, the finish must be applied equally to both sides, especially water-based finishes. Again, all wood should have similar initial MC.
C. If panels are designed with a cleat or rail which is securely fastened to the bottom of the panel and if the panel changes MC (resulting in shrinking or swelling), the top face is free to move but the bottom face can't. Warping will result. Therefore, the cleat must be free to move slightly as the panel shrinks or swells. I recommend using slotted screw holes and not gluing the cleat to the panel. But the real cure is using lumber at the correct initial MC.
5. Splitting and Chipped Grain When Machining.
6. Fuzzy Grain When Machining.
7. Gluing, Laminating and Veneering Failure.
The adhesive, or middle link (#1), is determined by the strength of the adhesive itself. If the proper adhesive has been chosen and handled correctly, this link shouldn't be a problem. The next links (#2 and #3) represent the bond between the wood and the glue. In practice, weakness of this bond is the most common cause of glue-line failure. Weakness results because the adhesive cannot attach itself to the wood. Two common reasons are: (1) the surfaces to be glued are several hours old and have become less active because the glue-bonding sites are occupied with dust or with moisture from the air; and (2) the wood MC and the EMC aren't close enough, so the wood changed MC, swelled (or shrank), and now the surfaces are no longer perfectly flat (0.002" to 0.006" is the preferred gap between surfaces-no more or no less for maximum joint strength). Links #4 and #5 are the wood surfaces themselves.
8. Glue-Line Failure.
A final note on gluing. There are a considerable number of factors in the gluing operation that affect the strength, durability, and appearance of a glue joint: (1) factors relating to the adhesive itself; (2) factors relating to the handling of the adhesive from its arrival in the glue room until it is applied to wood, pressed, and heated; and (3) factors relating to the wood itself.
9. Cracked or Checked Finishes.
10. An Ounce of Prevention. . .
Remember: Anytime wood or wood products are stored, a change in MC can occur if the MC isn't equal to the EMC.
Humidity can be controlled by varying or controlling the storage temperature. How does this work? This concept is based on two facts: First, when air is heated, its RH goes down. Second, every morning, the outside RH is close to 100% RH in most of the U.S., summer and winter. Therefore, it is a simple matter to achieve the desired RH (and therefore EMC) by heating a storage facility a certain amount above the morning low temperature. (For example, using the tabulation shown, heating air 22 degrees F above the morning low results in 7% EMC.)
Practical Moisture Measurement
A. Oven-drying (requires scales and an oven at 212 degrees F; test is destructive).
B. Hand-held moisture meters (ideal for spot-checking; small pocket-sized units are ideal for checking work in process in most plants; the simpler they are, the more they will be used; the faster the readings are taken, the more likely these meters will be used.)
C. For rough lumber, an in-line system can measure the MC of each piece of lumber. (Ideal for the cut-up or planing operation; gets wet wood out of the plant before it costs you money.)
Final Practical Advice to Manufacturers
For a copy of this report contact Wagner Electronics via e-mail at: firstname.lastname@example.org
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison
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