Timber recovery and conversion

4.3 Conversion of timber worksheet

1.1.6.1 Conversion of timber.pdf

Highly computerised and mechanised method of recovering plantation softwoods

Harvesting of timber

After the tree has been selected and marked by the spotter, and undergrowth has been cleared, if necessary, the logger fells the tree. Two cuts are generally made using a chainsaw, a mobile circular saw, a two-man crosscut saw or an axe. The direction of fall is controlled by the undercut.

After felling, the limbs and crown are cut off and the trunk is cut into manageable logs, which are loaded onto timber jinkers for transport to the sawmill.

In softwood pine plantations, where trees are of a uniform size and evenly spaced, at harvesting time (approximately 25 to 35 years) a one-man operated tree harvester grips and cuts the tree off near ground level, turns the trunk 90° to horizontal, strips all the branches, docks the log to a computer-set optimum length and stacks the log on a truck ready for transportation.

Recovery and conversion of plantation softwoods

Methods of conversion

The conversion of timber involves cutting the logs into marketable or commercial sizes. In Aust ralian hardwood mills it is usual to 'break down' the logs with twin circular saws or a log bandsaw and then resaw the flitches that are obtained to marketable sizes on a smaller circular saw. The flitches are also docked to set lengths while waney sections and splits are cut out.

Live sawing

The live sawing method results in the central planks being radial cut (quarter-sawn) and the two outer areas of the converted log being tangential cut (back-sawn). This gives an increased choice of grain pattern.

A problem with live sawing is the difficulty with cutting out faults like piping (hollowed-out pith area), internal gum pockets, and checks or shakes.

Back sawing

Back sawing (tangential cut) produces boards whose faces are, in general, tangential to the growth rings (see Fig. 2.6b). This is a highly flexible method well suited to securing high-grade timber from varied and faulty logs, as it allows for sawing around faulty parts and there is very little waste. Backsawing is more commonly used in Australia for the conversion of old growth hardwoods.

Timbers such as red cedar, Oregon, radiata pine and alpine ash, which possess distinct growth rings, are usually back sawn, as this exposes the most decorative figure. Timber for ceiling and floor joists, bearers, and axe and hammer handles is generally back sawn, as there is more strength in the direction of the growth rings. Back-sawn boards shrink mostly in width and tend to 'cup' or warp away from the heart of the tree.

Quarter sawing

Quarter sawing (radial cut) produces boards whose faces are generally parallel to the medullary rays and at right angles to the growth rings. The log requires even more frequent turning than in back sawing and more timber is lost through waste, but the recovery rate is still good, as quarter-sawn timber seasons very evenly.

Timbers with distinct ray figure, such as silky oak, sycamore, and, to a lesser degree, maple, reveal a very decorative figure called 'silver grain' when they are quarter cut. Timbers with interlocked grain, such as Queensland maple and walnut, show a valuable decorative 'stripe' or 'ribbon' figure. For this reason most valuable cabinet timbers that have prominent rays and/or interlocked grain are fully quarter sawn (that is, sawn within 10° of the rays). The seasoning process is slower for quarter-sawn timber than for back-sawn timber, but quarter sawn boards retain their shape and do not cup or check to any extent, and the edge grain that is revealed produces an even-wearing surface. This is why timber for such applications as architraves, mouldings and flooring boards is usually quarter sawn.

Why would a cabinet-maker use quarter-sawn timber when making fine quality furniture?

Quarter sawn timber while slower to convert provides several advantages to the designer/manufacturer. Quarter sawn timber has growth rings parallel to the short face, which typically shows the best grain on the face, provides better resistance to collapse and has better wear. It has lower shrinkage and less cupping and warping than other cuts thus providing stable, attractive material.

(a) Live sawn (b) Back sawn (c) Quarter sawn

Typical uses for quarter-sawn and back-sawn timber: (a) Quarter sawing for pieces to retain shape- 1 flooring boards for wear and less shrinkage, 2 mouldings and architraves, 3 window sashes, 4 door frames; (b) Back sawing for strength and growth ring figure-5 beams, girders, joists, etc., 6 hammer and tool handles, 7 shelving and table tops, heartside up.

Seasoning timber

Green or unseasoned timber contains a large amount of water, mainly within the many tubelike cells of which it is composed. Seasoning is simply the drying out of most of the water from the cell cavities and cell walls. The purpose of this drying process is to:

• reduce shrinkage, warping and checking to a minimum

• reduce weight to a minimum, with a consequent reduction in freight charges

• reduce liability of attack by wood-boring insects

• prevent blue stain and attack by other moulds, such as decay fungi

• increase the strength of the wood

• improve bonding properties so that adhesives, paints and finishes will adhere better and last longer

• make the timber lighter and easier to work.

The aim in seasoning is not to dry the water completely from the wood, which, under normal conditions of use, always contains some moisture. For example, under Sydney conditions a seasoned hardwood floor for a room 5 m x 5 m will contain about 55 L of water. The weight of water present is expressed as a percentage of the weight of the oven-dried wood, and this percentage is called the 'moisture content'.

The moisture content of freshly sawn timber often exceeds the weight of the wood itself. In green radiata pine, for example, moisture content is frequently greater than 200%. The ideal moisture content for seasoned timber varies with the relative humidity and temperature of the air; and to keep shrinkage and swelling (movement) to a minimum, timber should be seasoned to the EMC (equilibrium moisture content) of the locality where it is to be used. In Sydney, the average moisture content of the air is 12%.

Shrinkage takes place during the process of seasoning.

There are two stages in the seasoning of green timber , as the sap exists in two forms: as 'free moisture' contained in the cell cavities and as 'combined moisture' absorbed within the cell walls. In the first stage, the wood cells give up their free moisture, the combined moisture in the cell walls remaining until the cell cavity is empty. This condition is referred to as the 'fibre saturation point', at which the moisture content is usually about 30%. Shrinkage commences only when the cell walls start to give up their combined moisture, and continues until seasoning is completed.

When seasoning is complete it is known as the equilibrium moisture content (EMC) stage and is the stage at which the moisture content of the wood is equal to the relative humidity of the air surrounding it. As the air humidity changes (depending on the dryness and moisture of the air-as in periods of rain or drought, heat and wind), so too does the moisture content of the wood. This changes in sympathy with the air surrounding it, unless it is totally sealed from the atmosphere, which is just one of the reasons for lacquering/waxing/oiling a project.

Very little shrinkage takes place along the grain, as the cells do not shrink to any extent in their length. Similarly, the rays, which are long, thin cells running at right angles to the growth rings, restrict radial shrinkage. Most shrinkage, therefore, takes place in the direction of the growth rings.

This is called tangential shrinkage, and is usually about double that of radial shrinkage and about 100 times greater than longitudinal shrinkage.

Seasoning methods

Air seasoning

Air seasoning or the natural drying method requires the timber to be stripped out in stacks in the open, so that it dries slowly, in natural prevailing weather conditions. The secret to efficient air drying is good air circulation. To achieve this the stack is laid on bearers to keep it clear of th e ground, and strips of dry wood, usually about 20 mm thick and 25 to 40 mm wide, are placed exactly one above the other between the layers of boards. Any strips not in vertical alignment will cause distortion in the boards.

The strips in each layer should be spaced at 500 to 1000 mm intervals, immediately above the cross-bearers of the foundations. The boards should be placed along the stack so that there is a space of 25 to 50 mm between their edges to allow the vertical passage of air. Stacks should be protected from the effects of sun and weather by a simple roof or cover. The ends of the boards are usually sealed with a thick coat of paint to stop the end grain from drying out too quickly. As a general rule, air seasoning takes approximately six months for every 25 mm of timber thickness, but the time varies according to several factors, such as species of timber, size of boards, season and climatic conditions, site on which stack is built, and method of stacking.

Air seasoning

Explain why the growth rings on the widening jointed boards shown are alternated.

Timber is prone to movement (cupping or distortion). When creating a widening jointed board such as a table top, failure to alternate the boards’ grain direction can exacerbate cupping or distortion of the timbers. In the case of a table top this could create effective cupping across the entire width of the jointed boards. By alternating the grain the possibility of cupping is balanced out by flipping the direction of cupping between each board and thus providing a flatter top, which can be more securely fastened.

Kiln seasoning

Kiln seasoning, or the artificial drying method, requires that the timber be stacked in a similar manner to that for air seasoning, placed on special trucks and wheeled into oven-like rooms called 'kilns' where heat and humidity can be carefully controlled. The timber is dried using air circulated by fans over steam-heated coils. Steam inlets supply moisture to control the humidity (see image below).

Care must be exercised in the early stages to ensure that the free moisture in the cell cavities is not drawn off too rapidly, resulting in serious degrade in the form of 'collapse' (see image below). This is a flattening of softwood cell structure, causing distortion, severe shrinkage and great loss in overall strength.

A compartment kiln

Collapse during drying

Kiln drying is particularly suited to softwoods, as they are not prone to collapse and the entire seasoning process may be completed in the kilns. However, with hardwoods it is more economical to air dry to fibre saturation point (about 30% moisture content) and then kiln dry to equilibrium moisture content (approximately 12%), as degrade is then kept to a minimum. Depending on the species and the other factors mentioned above, 25 mm thick green timber takes from one to three weeks to kiln dry, compared with three to nine months for air drying.

There are two types of kilns in general use: compartment kilns and progressive kilns.

The majority of kilns used in Australia are the compartment type, which lends itself to an economical arrangement for combined air and kiln seasoning. These kilns offer greater control over the drying process for individual stacks, and this is important where a number of species is to be seasoned at the one time. Fans located in the ceiling induce air from the outside and drive it over heating pipes, down the side and through the stack, where it is returned to the fans over a second set of heat ing coils. A steam spray line and exhaust and inlet air vents located in the ceiling control humidity.

A progressive kiln is similar to an assembly line. Truckloads of timber ranged one behind the other are moved progressively along the kiln so that the green stock added at the cool moist end is gradually dried as it slowly moves along, until it emerges at the hot dry end ready for use. Progressive kilns lack the flexibility of compartment kilns and are suitable only when a continuous supply of timber of the same kind and thickness is available. Some progressive kilns combine the advantages of both types by dividing the tunnel into several compartments and controlling heat, moisture and air circulation in each. However, they generally are not popular, because many technical problems are involved in their control, especially in Australia where so many species of timber exist.

The advantages of kiln seasoning are a considerable reduction in the time required for seasoning, destruction of insects and fungi, reduction of moisture content to any desired level, and a measure of control to minimise the causes of seasoning defects.

Combined seasoning

As one might expect, combined seasoning combines the advantages of both air seasoning and kiln seasoning. It helps to overcome the main disadvantages of kiln seasoning; namely, distortion and checking due to too rapid removal of the free moisture from the outer wood cell cavities. The timber is first air seasoned for about three months until the free moisture in the cell cavities has dried out and is then kiln dried for three to seven days to remove the combined moisture from the cell walls. The final moisture content can be regulated so that the timber can be used either indoors or outside.

Storage of seasoned timber

In retail and domestic situations, the storage of timber should be undertaken so that scantlings, boards, battens and strips of timber remain straight and are not subjected to insect attack and moulds/fungi.

Storage needs to be in an airy environment that is not too dry or damp. Timber should be stacked horizontally, as vertical storage will allow the timber to bend under its own weight. Support should be provided for the timber along its length, as frequently as possible, and preferably on a flat surface. Avoid storing timber on the floor because of damp, which leads to mould, fungi and insect attack.

Understanding Timber Movement

This video covers reasons for timber movement and then discusses methods of securing table tops and joinery which consider the movement of timber.

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