Timber BADI Year 1

Brainstorm: uses of timber Properties – why its good / bad

Timber Wood is an important engineering material Used for: Furniture Houses Bridges Piers jettys etc Telegraph poles, pit props, fence posts railway sleepers And many other applications

Description of Wood Properties (1) Weight:  (Density) Average ovendry weight. Rankings based on pounds per cubic foot: < 33 = low (light); 33 to 43 = medium (moderately heavy); > 43 = high (heavy). Hardness: Side hardness or ability to resist compression perpendicular to the grain.  Rankings based on fiber stress at proportional limit in pounds per square inch (psi): < 850 = low (dents easily); 850 to 1250 = medium (moderate hardness); > 1250 = high (hard).

Description of Wood Properties (2) Stiffness: Elasticity or ability to resist bending stress. Rankings based on modulus of elasticity in million psi: < 1.25 = low (bends easily); 1.25 to 1.65 = medium (moderate stiffness); > 1.65 = high (stiff). Bending Strength: Maximum bending stress before failure occurs. Rankings based on modulus of rupture in psi: < 11000 = low (weak); 11000 to 14000 = medium (moderately strong); > 14000 = high (strong).

Description of Wood Properties (3) Shock Resistance: Impact strength or toughness. Measured by dropping a 50 pound hammer on a board supported on both ends at successively increasing heights until complete rupture occurs.  Rankings based on max height of hammer in inches: < 30 = low; 30 to 55 = medium; > 55 = high. Decay Resistance: Ability to resist deterioration due to decay fungi. Rankings based on relative decay resistance: low (little resistance); medium (some resistance); high (very decay resistant). Heartwood is generally more decay resistant than sapwood.

Description of Wood Properties (4) Stability: Dimensional stability in service associated with changes in humidity levels. Rankings based on average seasonal movement of kiln dried wood: low (not stable); medium (fairly stable); high (very stable). Working ease: How easily the wood is worked. These rankings are somewhat more subjective than those for the other criteria but they take into account things such as blunting effects on cutting edges and how easily the wood splinters, chips, and burns. Rankings: low (works with difficulty); medium (works fairly easily); high (works easily). Type: Hardwood (angiosperm) or softwood (gymnosperm).  Softwoods are typically less dense than hardwoods, making them easier to cut and nail without pre-drilling. Pines, firs, hemlocks, cedars, and spruces are all softwoods.

Table of properties of common woods Wood Species Weight Hardness Stiffness Strength Shock Res Decay Res Stability Working Alder, Common med low high Beech, European Birch, European Boxwood Cherry, European Chestnut, European Douglas-fir Elm, European Hornbeam, European Horse Chestnut Larch, European Lime, European Magnolia Oak, red Pear Poplar Port-Orford cedar Sycamore plane Yew, European

Man made boards Man made boards often made use of waste materials. For example saw dust is use to make MDF and hardboard. The saw dust is held together with a glue. Man made boards are often used as cheap alternatives to real woods. Man made boards don't look as expensive as real woods. Man made boards are often covered with a thin layer of real wood to improve their appearance (Veneered). Because they don’t have grain they are often superior to timber because they do not warp shrink or swell

Chipboard Made from compressed wood chips. Strength varies with density of board. Bonded with resin. Often veneered to improve appearance. Cheap. USES: Cheap floor boards, self assembly furniture

MDF Medium Density Fibreboard Higher density when chipboard, therefore stronger. No grain, strong, though edges need treating before painting. Cheap. Doesn’t warp and dimensionally stable. Good for areas that will sometimes get wet, like windowsills. USES: Furniture, model making

Blockboard Blocks of cheap wood sandwiched between thin sheets of better quality wood. Very strong. USES: Furniture

Hardboard Made of sawdust, similar to MDF. Smooth on one side and rough on the other. Very cheap. USES: Construction material

Plywood Layers of wood bonded together with grains at right angles to each other, very strong. USES: Flooring, wall panels, furniture, boats (marine ply)