Wood Production
Trees are harvested in fall or winter, because water content and environmental concerns (with regards to fires) are ideal then. Wood is harvested from forests as logs, and transported to saw mills to be cut into dimensional shapes.
Dimension Lumber: Wood from 2 inches to 5 inches. Sawn on all four sides. Common shapes are 2x4, 2x6, 2x8, 2x10, 2x12, and 4x4. Surfacing generally removes ¼ to 3/8 in. per side. Used for studs, sill and top plates, joints, beams, rafters, trusses and decking.
Heavy Timber: Wood larger than 6 inches in one dimension. Sawn on all four sides. Common shapes are 4x6, 6x6, 8x8 and larger. Surfacing generally removes 3/8 in. per side. Used for heavy frame construction, landscaping, railroad ties, and marine construction.
Round Stock: Posts and poles used for building poles, marine piling, and utility poles.
Engineered Wood: Products manufactured by bonding together wood strands, veneers, lumber, and other forms of wood fiber to produce a larger and integral composite unit. Types include structural panels (plywood, oriented strand board, composite panels), glued laminated timber (Glulam), structural composite lumber, and composite structural members.
Specialty Items: Milled and fabricated products that reduce on-site construction time, including lattice, handrails, spindles, radius edge decking, turned posts, etc.
Sawn Wood Production: (1) Sawing into desired shape, (2) seasoning, (3) surfacing, (4) grading, (5) preservative treatment (optional).
Surfacing (planing) of wood products can be done before or after drying. Post drying surfacing is superior, because it removes small defects developed during the drying process. When surfaced before seasoning, dimensions are slightly increased to compensate for shrinkage during seasoning.
Cutting: Harvested wood is cut into lumber and timber at saw mills using circular saws, band saws, or frame saws. The most common patterns for sawing a log are plain (slash), quarter, and combination sawing.
Categories of Sawing:
1. Flat-sawn, 45o or less
2. Rift-sawn, 45 o to 80 o
3. Vertical- or edge-sawn, 80 o to 90 o
Flat-sawn boards have desirable exposure of grain for decorative applications. Flat-sawn boards tend to distort more than vertical-sawn boards in response to moisture fluctuations. Therefore, vertical-sawn boards are used more for structural applications.
Plain-sawing is rapid and economic straight-cutting of lumber whereas quarter-sawing maximizes the amount of vertical-sawn cuts.
Seasoning: Green wood contains 30% to 200% moisture by the oven-dry weight. Seasoning removes excess moisture from wood. For structural wood, recommended moisture is 7% in the dry southwestern states to 14% in the damp coastal regions. Framing lumber has an average moisture content of 15%.
Wood is seasoned by air and kiln drying. Air-drying is inexpensive, but slow. The green lumber is stacked in covered piles to dry. These piles of lumber are made of successive layers of board separated by 1-inch strips so that air can flow between layers.
Typically, it takes three to four months to dry wood.
After air-drying, the lumber may be kiln dried. A kiln is a large oven where all variables can be closely monitored. Drying temperatures in a kiln range from 20 C to 50 C, typically requires 4 to 10 days.
Drying to quickly results in cracking and warping.
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