The technology used in a softwood sawmill varies greatly, depending on the types of logs available and the products being produced.
But most sawmills process logs as shown in the diagram on the right.
Debarking
Before sawing, the bark is removed from the log. This makes it easier to handle, reduces fouling of saws and transfer systems, reduces wear on saws, makes the logs easier to assess, leading to better conversion and grade recovery.
Logs are generally debarked by Cambio ring or Rosserhead type debarkers that chip the bark away using blunt knives, or teeth mounted on a rotating ring or wheel. High-pressure water may be used for debarking logs that are heavily fluted or of poor form.
The Headrig
This is the primary log breakdown saw - converting the log into boards or large dimension timber. It is the first saw the logs meet as they enter a mill. Bandsaws, circular saws and chipper canters are the three types of saws used as headrigs in New Zealand. Frame saws and chainsaws are also used but are not common. Bandsaws, frame saws, chainsaws and circular saws cut wood to produce lumber and sawdust.
Chipper canters chip the edges of logs, cants or flitches. This generates two parallel machined faces and woodchips which can be used in the production of other products like MDF and paper.
The circular saw has a circular blade rotating around a central axis (arbor). The blade, which is self-supporting, is thicker than some other saws and has a larger kerf, producing more waste (sawdust). They can distort in use through heat expansion and centrifugal force. For these saws to cut true, a region of the blade must be tensioned. This involves hammering or squeezing the blade in a zone about half to two-thirds of the way from the centre so that the metal is slightly thinner and pre-strained. When the saw is running, the pre-strained forces counteract the distortive forces, maintaining blade shape and cutting accuracy. Circular saws are generally too wasteful for use on large logs.
Bandsaws are used where larger logs are being broken down. Because they are not self-supporting they can have a narrower kerf and are more efficient. A bandsaw is an endless steel band that is mounted and strained between two large wheels. The lower wheel pulls the blade down through the log as it is feed into the saw. The log is fed to the saw on a log carriage that can position or rotate the log to present the desired face to saw. Like circular saws, bandsaws also require tensioning.
Cutting Patterns
A cutting pattern determines how or in what order cuts will be made on a log. Different patterns have been developed to suit different logs and optimise utilisation and value recovery. The four basic patterns are:
• Live-Sawing
• Grade Sawing
• Cant-Sawing
• Quarter-Sawing
Each has various advantages and disadvantages in processing speeds and costs, grade targeting and product quality.
Resaws
These are the secondary breakdown saws, processing the flitches and cants produced by the headrig. Resaws rip the wood to narrower widths using either circular saws or band saws. Many kinds of resaw exist, the main ones being:
• horizontal band saw
• vertical band saw
• moving edger
• rotary gangsaw
• frame gangsaw
• breast bench
Edgers remove the outer rounded part of the board where the bark was once attached, producing boards with four flat edges. Gangsaws have at least two saws next to each other, cutting in parallel. The saws are set at intervals along an arbor, which determines the width. The breast bench is a single circular saw that rips off one board at a time. The flitch or cant is recycled until no more recoverable wood remains.
Docking
Involves cutting the ends of boards to remove defects, improve presentation or to achieve a specified length. These saws are generally crosscut circular.
Grading
This process sorts the timber into different quality classes to determines what the end-use will be. Grading maximises the value of the timber - for the general New Zealand market, export (Australia, Japan and USA) or specific customer requirements.
Antisapstain
Radiata pine lumber is particularly susceptible to sapstain fungus. This can 'stain' the timber a blue or black colour within a few days, making it unattractive and consequently of less value. The fungi do not cause physical damage to the timber.
Many sawmills apply antisapstain treatment to freshly sawn timber by spraying or dipping it in a chemical solution which protects the timber until it can be further processed or accepted by the customer (usually 6 to 8 weeks).
Sort/Stack/Tally/Package
Timber is sorted in packets by grade, size and length at a 'sorting table'. In larger mills this is traditionally done on a 'green-chain' rather than a table. Sorting tables are circular rotating devices on to which timber is fed from the mill. Around the table are pull-off points where the timber is stacked into a packet. Green-chains have a conveyor-type system rather than a rotating table and allow more sorts than a table.
As timber is stacked, a tally is kept of the number and length of pieces. This and other details are used for inventory purposes. It can also be used to calculate total volume in the packet and helps the mill assess its efficiency.
Packets may be coded, tallied, wired (or strapped) and transported from the mill or stored for further processing.
Residues
Nearly all sawmills in New Zealand have mill residue and waste management streams. Residues include wood chips, slabs, sawdust, and bark. Mills that produce wood chips sell them to other processing facilities or export agents. Slab wood is also sold to other processing plants and is generally chipped. Many mills use sawdust and bark to power boilers that produce heat and electricity. The energy is used to run the mill and can offer significant savings.
Further Processing
Many mills further process the sawn timber - remanufacturing into fingerjointed products, panels or mouldings. Generally, mills dry the timber they produce, and some apply preservation treatment.
Drying
Timber is dried to improve its strength and to reduce the potential of damage by decay from fungi. Several options are available, from air drying which takes months, to high temperature kiln drying that can dry timber in less than a day. Dry timber is lighter and easier to handle, fastens and paints better, and is dimensionally more stable – it does not warp or split.
Preservation Treatment
Chemicals are used to treat the wood so it will resist damage-causing insects
or fungi. Timber can be treated to different levels depending on end use.
See Wood Preservation.