Wood properties and defects
Properties. Density (mass for a given volume) tells how heavy and hard the wood is; oak is dense, pine is light. Moisture content is the water in wood compared with its dry mass: MC = (wet mass − dry mass) ÷ dry mass × 100. Grain is the direction of the fibres: wood is strong along the grain and weak across it. Wood also shrinks and swells with moisture, more across the grain than along it.
Defects. A knot is a dark round spot where a branch grew. Cracks and checks are splits, often from drying too fast. Warp means the board bends (bow, twist, cup). Decay and insect holes come from damp or pests. Sloping grain weakens a board. Graders look for these before they choose the use.
Lumber, veneer, plywood and particle boards
Lumber is sawn solid wood: boards, planks, beams. Veneer is a very thin sheet (about 0.5 to 3 mm) peeled or sliced from a log. Plywood is glued veneers with the grain crossed at 90° (always an odd number of layers: 3, 5, 7). Laminated wood (glulam) glues many boards along their length so you can make long, strong beams. Particle board is wood chips mixed with glue and pressed flat; fibreboard (MDF) uses fine fibres. Engineered boards use waste and small logs, and they stay flatter than solid wood.
Adhesives, abrasives, finishes, upholstery
Adhesives: PVA (white wood glue) for indoor joints; urea and phenol resin glues for boards (phenol is waterproof, so it is used for outdoor plywood); hot-melt for edges. Abrasives are papers and belts with grit: coarse grit (60) removes wood fast, fine grit (240) makes it smooth. Finishes: stain changes colour, varnish and lacquer make a hard film, oil soaks in, paint hides the grain. Upholstery materials: foam, springs, webbing, fabric and leather for seats.
Fittings, decorative and other materials, metals
Fittings join or move parts: hinges, drawer slides, handles, locks, screws, nails and dowels. Decorative materials: edge strips, laminates, glass and mirrors. Non-traditional materials include bamboo, cork, plastic laminates and recycled boards. Metals (steel, brass, aluminium) make hardware and the frames of chairs and tables.
Artificial (kiln) drying
Fresh (green) wood has 50 to 100% moisture. Air-drying is slow (months). A kiln is a heated chamber with fans and steam: temperature, humidity and air flow are controlled. Drying is done in stages: gentle at first, hotter later. Too fast makes checks and warp. Final moisture is about 8 to 12% for indoor furniture, higher for outdoor use. A moisture meter checks it. Boards are stacked with small sticks (stickers) between layers so the air can pass.
Cutting operations, machines and economy
Operations: crosscut (across grain), rip cut (along grain), and resawing (cutting thick boards thinner). Machines: band saw (long blade loop; logs and curves), circular saw (round blade; straight cuts), frame saw (many blades; sawmill), planer and jointer (make flat, even faces), and CNC routers. Economy: the yield (cutting efficiency) = useful volume ÷ log volume × 100. Typical sawmill yield is 50 to 70%. Kerf (the width of wood the blade turns to dust) and trim waste lower it. Better cutting plans raise the yield and the profit.
Health and safety
Wood dust can cause breathing problems and some dusts are harmful over years: use dust extraction and a mask. Noise is loud: wear ear defenders. Wear safety glasses. Tie back long hair and loose clothes. Keep guards on machines, use a push stick, never reach over a moving blade, and switch off and wait for the blade to stop before clearing it. Keep floors clear of off-cuts and keep fire extinguishers close, since dust and shavings burn easily.
Try it: weigh the water
At home, weigh a small damp piece of wood (a clean twig). Let it dry for a few days in a warm room and weigh again. The mass lost is water. Work out MC = (wet − dry) ÷ dry × 100. In the 3D, last step, move the slider and see how the board shrinks as the moisture falls.
Key formulas and definitions
- MC (%) = (wet mass − dry mass) ÷ dry mass × 100
- Yield (%) = useful volume ÷ log volume × 100
- Volume of a board = length × width × thickness
- Waste (%) = 100 − yield
Worked examples
1. A wet wood sample has mass 150 g. After drying it has mass 100 g. Find the moisture content.
MC = (150 − 100) ÷ 100 × 100 = 50%.
2. A log of 1.0 m³ gives 0.6 m³ of boards. What is the yield and the waste?
Yield = 0.6 ÷ 1.0 × 100 = 60%. Waste = 100 − 60 = 40%.
3. How many boards of 4 m × 0.25 m × 0.05 m make 2 m³?
One board = 4 × 0.25 × 0.05 = 0.05 m³. Number = 2 ÷ 0.05 = 40 boards.
4. Why does plywood use an odd number of layers?
With an odd number the two outer layers have the grain in the same direction and the sheet stays balanced and flat. An even number would pull the sheet to one side.
Common mistakes
- Dividing by the wet mass when working out moisture content. Divide by the dry mass.
- Cutting wood before it is dry. It will shrink and warp afterwards.
- Thinking plywood is weak. Crossed layers make it strong in both directions.
- Removing the machine guard to "see better". The guard protects your hands.