Tech - Materials

Materials

Deciduous trees- include hardwoods such as beech and oak. Deciduous trees nearly all have broad leaves. During the winter these leaves fall off.

Conifers- include softwoods such as pine and cedar. Soft wood trees have needles instead of broad leaves. Most softwood trees are evergreens which mean they keep their needles all year round.

From trees to planks

Felling

Trees are growing, living structures. They take in water and nutrients from the soil and transport it to the leaves, energy from sunlight converts water and carbon dioxide from the air into plant food. As this process happens during the summer months, trees are usually cut down in the non-growing season.

The tree is felled and the trunk cut into logs 4-6m long. The logs are then transported to the sawmill where the bark is removed and they are cut into planks.

Seasoning

The planks are left to dry before they are finally sawn into planks for sale to the public. The drying process is called seasoning and can take between 6 months and 2years if the timber is left outside. The process can be speeded up if the wet planks are placed in a hot building called a kiln. Kiln drying takes 1-3 weeks

Conversion

“sawing wood into planks”

There are two main methods of doing this:

1.      Plain sawing

2.      Quarter sawing

Plain sawing is quick and cheap to do but some planks will wrap more than others. Wet planks have a tendency to bend away from the heart as they dry out.

Quarter sawing

Is slow and expensive to do and is usually only used for expensive hardwoods such as mahogany. Planks cut in this way tend to wrap less then plain sawn ones because the growth rings are across the plank. The lower, radial, method is the most expensive and tends to be reserved for special hardwoods such as oak.

Grain

Grain is the term given to the pattern of the wood. These patterns are made by the growth rings of the tree.

Properties of wood

The structure of a tree as it grows allows nutrients to travel up and down the trunk to keep alive. This structure is a series of short bonded tubes called cells. If you looked at them under a microscope they would look like small drinking straws. It is this tubular structure that gives timber its strength.

The tubes or cells are larger towards the outside edges of the trunk, so during seasoning they shrink more than the cells in the middle. This is why timber tends to wrap.

Mechanical properties of timber

Because timber has cells arranged in bundles, it may help you to think of a plank as a bundle of straws, and imagine how it will react to different forces

Tension along the grain

Timber will be strong in tension along its length, that is, it can resist large pulling forces.

Compression down the grain

while timber is good at resisting compression forces applied to each end it is not as good in compression as it is in tension. A piece of timber could be up to 50% weaker in compression than in tension.

Compression and tension across the grain

Timber is weakest when a compression or tension load is applied across the grain.

Bending

Timber will bend when a load is applied to it. You can reduce the bending and increase the strength of your project by turning planks on their edge.

Man made boards

The term man board is given to wood that has been cut and glued to form a board or sheet.