Unit 4 Lumber Manufacture Sawmill Operations • Sawing Methods • Seasoning Methods • Planing and Grading • Lumber Defects • Wood Protective Treatment

Trees are cut using a feller buncher that grasps a tree as it is sawn off at the base. Trees sustain less damage when felled in this manner. The manufacture of lumber begins in the forest where trees are cut. See Figure 4-1.

Log loaders stack logs on a truck for transport to a sawmill. Limbs and branches are removed, and the tree is cut into sections (logs) small enough to be transported by truck to a sawmill. See Figure 4‑2.

A log can be cut into many shapes and sizes of lumber. Logs are processed into the different sizes of lumber used in construction work. See Figure 4‑3.

Logs are conveyed into the sawmill from a log merchandising deck. At a sawmill, logs are stored in ponds or are stored on the ground and continually sprayed with water to prevent shrinkage caused by drying. The logs are then trans-ported to the sawmill from the merchandising deck as shown in Figure 4-4.

The logs pass through a debarker to remove bark and dirt, which may dull saw blades used in subsequent operations. Initial sawmill operations include debarking, which is a process of stripping the bark from a log. See Figure 4‑5. The debarked logs are cut into smaller sections, which are then cut into boards by a bandsaw.

Boards are fed into the trimmer by conveyor belts Boards are fed into the trimmer by conveyor belts. A trimmer consists of a series of saw blades that remove defects and cut the boards to standard lengths. Boards travel by conveyor belts into a trimmer, which cuts the boards to standard lengths and also cuts off pieces containing defects. See Figure 4‑6.

Lumber sawing methods include plainsawing, quartersawing, and rift sawing. Two primary methods are used to cut logs into lumber—plainsawing and quartersawing. See Figure 4‑7. Another sawing method, known as rift sawing, is used only for hardwood lumber. Each sawing method gives lumber a distinct grain pattern and also affects the performance characteristics of the lumber.

Lumber is stacked on tram cars and transported into a kiln for seasoning. Note the stickers placed between rows of lumber to allow for complete circulation of air. When kiln drying lumber, the newly produced lumber is placed in a temperature-controlled building called a kiln, which acts like a large oven. See Figure 4‑8. First, steam is introduced into the kiln to increase the humidity (amount of moisture in the air) in the kiln. As the temperature in the kiln is gradually increased, the humidity level is decreased. Kiln-dried lumber is stamped with a “KD.” Kiln drying is usually used for higher grades of hardwood lumber used for finish work. Kiln-dried lumber is more expensive than air‑dried lumber.

Lumber is graded and sorted according to thickness, width, and length as it moves along a conveyor belt. At the mill, the rough lumber passes through planers, which are machines with rotating knives that surface (smooth off) the sides and edges of the lumber. As the planed lumber moves along a conveyor belt, highly skilled workers called graders examine the boards for defects and mark each piece according to grade. The graded pieces are later sorted according to thickness, width, and length. See Figure 4‑9.

Knots are found in most grades of lumber Knots are found in most grades of lumber. A small number of sound knots will not significantly affect the lumber strength. Knots are found in most lumber. See Figure 4‑10. If knots are sound (remain firmly in place), a few of them will not significantly affect the lumber strength. Knots are classified by their diameter as follows: • pin knot—1/2″ or less • small knot—more than 1/2″ but less than 3/4″ • medium knot—more than 3/4″ but less than 1 1/2″ • large knot—more than 1 1/2″

Splits and wanes are among the lumber defects that occur from natural causes. Other natural defects occurring in lumber are as follows: • wane—absence of wood or the presence of bark on the edge or corner of a piece of lumber. See Figure 4-11. • shake—lengthwise separation of wood fibers between or through the annual rings • check—separation of wood fibers across the annual rings • split—separation of wood fibers across the annual rings (similar to a check) but extending entirely through a piece of lumber • pitch pocket—opening in the wood that contains solid or liquid pitch • pitch streak—section of wood fibers saturated with enough pitch to be visible

Warpage can occur during the evaporation of water from wood cells. Warping is a distortion that occurs during the evaporation of water from the wood cells. Uneven shrinkage in the wood produces warping and results in twisted and uneven shapes of lumber. Common warpage shapes are the crown, bow, twist, and cup. See Figure 4‑12. A crown (crook) is a deviation from a flat plane of the edge (narrow face) of a piece of lumber from end to end. A bow is a deviation from a flat plane of the wide face of a piece of lumber from end to end. A twist is a deviation from the flat planes of all four faces by a spiraling or torsional action, which is usually the result of improper seasoning. A cup is a deviation from a flat plane, edge to edge.

Dry rot is the breakdown of wood caused by a wood‑destroying fungus. Dry rot causes wood tissue to deteriorate, reducing the strength of a wood member. See Figure 4‑13. Dry rot is the most common type of damage caused by a fungus. The fungus that causes dry rot lives in the wood and can be seen only with a microscope. Since dry rot fungus must have water to live, it can survive only in wood with a moisture content of at least 20%. The term dry rot is misleading, since the deterioration begins under damp conditions. However, it is often not detected until after the wood has dried out.

Levels of wood deterioration vary depending on humidity levels and temperature. In some areas of the United States, moisture and oxygen levels and temperatures are more favorable to wood deterioration than in other areas. See Figure 4-14. Therefore, wood preservatives are used to eliminate wood fiber as a food source. Wood preservatives contain chemicals that protect the wood against fungus decay and insect attack. Preservatives are divided into three major types—waterborne, oil‑borne, and creosote.

Waterborne preservatives are used for residential, commercial, marine, agricultural, recreational, and industrial construction applications. Waterborne preservatives are used to treat lumber and panel products for residential, commercial, marine, agricultural, recreational, and industrial construction applications. Waterborne preservatives are the most common preservative specified for residential, com-mercial, and marine construction applications. Water-borne preservatives are clean, odorless, and easy to paint. Waterborne preservatives do not contain any arsenical or chromium compounds, but they provide excellent decay and termite resistance. Common waterborne preservatives are listed in the table in Figure 4-15.

Untreated lumber is placed in large cylinders and impregnated with preservative. Wood preservatives may be applied by pressure or non‑pressure processes. The pressure process is con-sidered to be the most effective since preservatives penetrate deeply into the wood. See Figure 4-16. Un-treated lumber is loaded onto tram cars and rolled into long steel cylinders, which are then sealed. The cylinders are filled with the preservative. Intense pres-sure builds up inside the tank and forces the preservative deeply into the wood.

Grade marks for pressure-treated lumber include information about the exposure conditions for proper use of the lumber, and also indicate the type of preservative used on the lumber. Proper use of pressure-treated lumber is determined by its exposure to the weather and exterior conditions. Most pressure-treated lumber is either rated for ground con-tact or above-ground exposure conditions. Ground contact exposure conditions include situations when pressure-treated lumber is in contact with the ground or freshwater. Above-ground exposure conditions include applications such as sill plates, beams, joists, and decks. A grade mark used for pressure-treated lumber is shown in Figure 4-17.