Gathering Raw Materials
Lumber mills obtain trees from company, government and private properties. When a tree is of a mature size, it is cut down using a gas-powered chainsaw. Branches are trimmed off with chainsaws as well. Large off-road tractors, called skidders, are used to drag the tree lengths out of the forest. When terrain is too steep for skidders to maneuver, a machine called a yarder is used. The yarder pulls logs up steep slopes or across uneven land using sturdy cables. These skidders and yarders pull the logs to clearings where they are loaded onto tractor-trailer trucks and transported to the lumber mill.
Before the logs are cut they are inspected, debarked and bucked. Defects in the wood, such as knots, decrease the value of lumber. Therefore, each length is inspected for these defects and any unwanted metal pieces. Next, chain conveyors transport the logs into the mill. Grinding wheels or high-pressure water jets are then used to remove bark from the logs. The bark is sometimes used for decorative mulch, but in most cases it is discarded. Buck saws cut the logs to lengths predetermined by the mill. This process is called bucking. Preparation is done while the wood is still wet, and water may be sprayed over the lumber to keep it from drying out and shrinking.
The lengths of wood are measured and scanned for defects by optical sensors. Then, a computer maps out a series of cuts that would provide the most solid boards. The computer operator uses his judgment and experience to make a final decision on how the wood will be cut. A headrig saw cuts the boards lengthwise according to the operator's instructions. First, the curved pieces, called slabs, are cut from each piece of wood. These pieces are usually recycled as mulch or regarded as scrap. Then, the saw cuts the rest of the log into usable pieces.
Boards are not ready to be used until they are seasoned, planed and graded. Seasoning prevents rotting as the boards age. The wood will either be placed in a holding area over time or heated in a kiln, which draws more moisture out of the wood in a shorter period of time. Next, wood is sent through a planer to perfect the dimensions, smooth the surfaces and round the corners. Finally, machines and workers inspect the lumber and rate it based on the amount of defects present. Lumber of similar type and quality are packaged together and shipped to lumber yards for public sale.