Whilst trees are round and we live in a mainly square or rectangular world, trees must therefore be processed into other forms. This processing can be summarised as:
Primary processing – sawing logs into kants, and then to flat and square edged boards.
Secondary processing – precision sawing and milling (including planing). This is where the term added value is derived.
In contrast to metals, the task of processing wood is complicated by the anisotropic nature of wood. Even this anisotropy is not constant as the direction of grain can vary within a piece.
The conversion of logs into boards has become very technically advanced and computer led. After removal of the bark, logs are scanned in order to determine shape and dimensions, and from this the best way of cutting the log can be calculated. The saws are automatically set and the log turned to achieve the greatest yield from the log. Whole log X-ray tomography has been developed so that the internal structure of the log can be seen and the log then cut for best quality. In the case of large logs, sawing patterns can be determined by the operator (often band saw technology), and/or by computer. Mobile sawmills have been developed so that individual trees can be processed on site. These machines are usually of simpler design and easily maintained by the operator.
Above is a computer led Linck line operation, with one operator controlling the entire primary sawline.
The process of severing the wood fibres in order to cut wood down to other dimensions.
Primary bandsaws can be from 100mm to 300mm wide. They can also achieve a greater depth of cut which is essential for processing large logs, particularly tropical logs.
Sawing can be further divided into rip sawing and cross-cutting.
Rip sawing is the process of cutting wood along the grain or axis of the tree. Because all the fibres must be cut, it is essentially a chiselling process and requires a lot of energy. Much research and development has gone into reducing kerf (width of saw cut) and improving accuracy, which together reduce waste.
Cross-cutting is the process of cutting at right angles to the grain across all the fibres. The saw teeth scribe through the fibres like knives on either side of the cut. Many saws are required to do both processes with an inevitable loss of efficiency. There are two principle types of saw, circular saws and bandsaws although reciprocating saws are sometimes found for special applications.
Circular saws. Metal disks carrying teeth which can be shaped to ripping, cross-cutting or both. Few saws are plain steel nowadays. Most are tipped with hard metals such as cemented tungsten carbide TCT, Stellite © or polycrystalline diamond PCD. They have a limited depth of cut and remove a wider kerf than other types of saw, but they are robust and easily serviced. In primary sawmilling greater depth of cut can be achieved by using aligned pairs of saws. Because the angle at which the teeth meet the wood fibres changes with the curve of the saw, the efficiency is not as high as bandsaws. Circular saws can achieve very high accuracy, good finish and durability.
Bandsaws. These saws comprise a thin endless band of steel running over two, occasionally three, pulleys. They have the narrowest kerf, lowest power consumption and highest production rates possible because the saw teeth always meet the fibres at the same angle so that everything can be optimised. They can be further divided into narrow bandsaws and wide bandsaws. Narrow bandsaws are designed for small workshops and for cutting curves. The kerf is small and power consumption low. The blades are often disposable and not resharpened. They may be from 6mm to 50mm wide. The blades run over rubber faced pulleys and rely on precisely set guides to control them. Wide bandsaws by contrast may be from 60mm to 300mm wide and be more than 12 metres long. They run over iron pulleys with slightly curved faces. The blades are prestressed (tensioned) by rolling so that the blades hug the pulleys. The blades are steadied by guides to limit vibration although it is the prestressing that is mainly responsible for taking the cutting forces. The speed of cutting can be remarkable. Blades are often hard metal tipped with Stellite © such that very long life is possible. The flexing of the blades requires special steels and maintenance in order to avoid fatigue cracks. In primary sawmilling, bandsaws are often arranged in gangs so that a log can be converted in one pass.
Frame saws. In the distant past, logs were sawn with a single man-powered reciprocating saw – the pit saw*. It was a logical development to mechanise this and then to mount several saw blades together. Whilst the speed of cutting is slow, this is compensated for by producing many boards at once. These machines are known as frame saws and are now almost obsolete since a lot of wood is lost in sawdust (kerf) and it is a slow process to adjust and change blades
Milling includes such processes as planing, routing and chipping. Abrasive sanding can be viewed as milling with a very large number of cutting points.
The process of producing an accurate smooth surface or of shaping wood to profiles and other shapes is properly called rotary milling. A planed surface is not truly flat but is comprised of a series of shallow cuts that produce a visible wave field. Metal and woodworking machines have now converged and are very similar: it being only the tooling that is different.
Planing and moulding is typically carried out on machines that can shape all four faces at one pass (hence four cutter, six cutter etc). With multi-knife cuter blocks, very high accuracy and rate of feed is possible – 300 metres per minute and more. In the joinery industry, very complex sections are machined.
Routing utilises a single rotating cutter, often running at very high speed. They are often computer numerically controlled and cut in 5 axes so that very complex 3D shapes can be machined. In the small workshop the hand router is an invaluable and very versatile tool.
Abrasive. Because milling leaves a wave field, if the surface is to be painted or polished, it must be sanded smooth. Similarly surfaces produced in other ways such as veneering or after gluing, require sanding to a fine finish. In the manufacture of panels, abrasive sanding is used to calibrate panels to a precise thickness. Drum sanders are occasionally used for fine handwork, but for serious production wide belt sanders are used. These are capable of remarkable accuracy and very fine finish. Power consumption is high.
* Interestingly this is where the terms "top dog" and "under dog" originated - the "under dog" being the man in the pit under the log, pulling down the saw, and getting covered in sawdust/etc, in fact the worst of the two jobs! The "top dog" would be standing on the log, pulling the saw up.