Lumber: You’re probably surrounded by it right now. You’ve probably bought it before. Perhaps you’ve even built something with it.
You know what it is but how is it made?
In the United States, most trees destined to be cut into lumber are grown in managed forests either owned by the lumber company or leased from the government. After the trees have reached an appropriate size, they are cut down and transported to a lumber mill where they are cut into various sizes of lumber.
This is where it gets interesting: the manufacturing part. It’s broken down into steps which we’ll outline below:
Felling
Selected trees in an area are visually inspected and marked as being ready to be cut down, or felled.
Most tree felling is done with chain saws. Two cuts are made near the base, one on each side, to control the direction the tree will fall. Once the tree is down, the limbs are trimmed off with chain saws, and the tree is cut into convenient lengths for transportation.
If the terrain is relatively level, diesel-powered tractors, called skidders, are used to drag the fallen tree sections to a cleared area for loading. If the terrain is steep, a self-propelled yarder is used. The yarder has a telescoping hydraulic tower that can be raised to a height of over 100 feet. Guy wires support the tower, and cables are run from the top of the tower down the steep slopes to retrieve the felled trees. The tree sections, or logs, are then loaded on trucks using wheeled log loaders.
The trucks make their way down the graded road and onto public highways on their way to the lumber mill. Once at the mill, giant mobile unloaders grab the entire truck load in one bite and stack it in long piles, known as log decks. The decks are periodically sprayed with water to prevent the wood from drying out and shrinking.
Debarking and bucking
Logs are picked up from the log deck with rubber-tired loaders and are placed on a chain conveyor that brings them into the mill. In some cases, the outer bark of the log is removed, either with sharp-toothed grinding wheels or with a jet of high-pressure water, while the log is slowly rotated about its long axis. The removed bark is pulverized and may be used as a fuel for the mill's furnaces or may be sold as a decorative garden mulch.
The logs are carried into the mill on the chain conveyor, where they stop momentarily as a huge circular saw cuts them into predetermined lengths. This process is called bucking, and the saw is called a bucking saw.
Headrig sawing (large logs)
If the log has a diameter larger than 2-3 ft, it is tipped off the conveyor and clamped onto a moveable carriage that slides lengthwise on a set of rails. The carriage can position the log transversely relative to the rails and can also rotate the log 90 or 180 degrees about its length. Optical sensors scan the log and determine its diameter at each end, its length, and any visible defects. Based on this information, a computer then calculates a suggested cutting pattern to maximize the number of pieces of lumber obtainable from the log.
The headrig sawyer sits in an enclosed booth next to a large vertical bandsaw called the headrig saw. He reviews the suggested cutting pattern displayed on a television monitor, but relies more on his experience to make the series of cuts. The log is fed lengthwise through the vertical bandsaw. The first cut is made along the side closest to the operator and removes a piece of wood called a slab. The outer surface of the slab has the curvature of the original tree trunk, and this piece is usually discarded and ground to chips for use in paper pulp.
The carriage is returned to its original position, and the log is shifted sideways or rotated to make subsequent cuts. The headrig sawyer must constantly review the log for internal defects and modify the cutting pattern accordingly as each successive cut opens the log further. In general, thinner pieces destined to be made into boards are cut from the outer portion of the log where there are fewer knots. Thicker pieces for dimension lumber are cut next, while the center of the log yields stock for heavy timber pieces.
Bandsawing small logs
Smaller diameter logs are fed through a series of bandsaws that cut them into nominal 1 inch, 2 inch, or 4 inch thick pieces in one pass.
Resawing
The large cut pieces from the headrig saw, called cants, are laid flat and moved by chain conveyor to multiple-blade bandsaws, where they are cut into the required widths and the outside edges are trimmed square. The pieces that were cut from smaller logs may also pass through multiple-blade bandsaws to cut them to width. If the pieces are small enough that they do not need further cutting, they may pass through a chipper, which grinds the uneven edges square.
Drying or seasoning
The cut and trimmed pieces of lumber are then moved to an area to be dried, or "seasoned." This is necessary to prevent decay and to permit the wood to shrink as it dries out. Timbers, because of their large dimensions, are difficult to thoroughly dry and are generally sold wet, or "green." Other lumber may be air dried or kiln dried, depending on the required moisture content of the finished piece. Air-dried lumber is stacked in a covered area with spacers between each piece to allow air to circulate. Air-dried woods generally contain about 20% moisture. Kiln-dried lumber is stacked in an enclosed area, while 110-180°F heated air is circulated through the stack. Kiln-dried woods generally contain less than 15% moisture and are often specified for interior floors, molding, and doors where minimal shrinkage is required.
Planing
The dried pieces of lumber are passed through planers, where rotating cutting heads trim the pieces to their final dimensions, smooth all four surfaces, and round the edges.
Grade stamping and banding
Each piece of lumber is visually or mechanically inspected and graded according to the amount of defects present. The grade is stamped on each piece, along with information about the moisture content, and a mill identification number. The lumber is then bundled according to the type of wood, grade, and moisture content, and the bundle is secured with steel bands. The bundle is loaded on a truck or train and shipped to a lumber yard for resale to customers.