Radial Milling Technique
Uses for Radially Sawn Timber
When white people first arrived in Australia, they came to a land which displayed what was a seemingly inexhaustible smorgasbord of large diameter trees. They had the choice of a wide range of eucalypt species, Cedar from northern Australia, or Huon Pine from the south.
Australia's forests were first exploited at a time of innovation and industrial change. With the shift to steel manufacture, the mass consumption of timber for boat-building was coming to an end. As a result, many of the qualities of Australian timbers went undiscovered or unexploited. The time of change brought demands for massive amounts of timber for railway sleepers and blocking for roads. Many of Australia's finest forests were exported for this purpose.
Trees of a large diameter were easily converted to produce quality products. There seemed so much timber, that massive areas of forests were cleared and burnt or left to rot.
The mass exploitation of Australia's forests continues, with the bulk of production of timber being for house-framing materials. Generally, this material is cut from the larger of the trees, the smaller and degraded logs being smashed up into chips, mainly for paper production.
As Australia works its way through its old growth forests, it is faced with new problems - a new resource of smaller diameter regrowth trees, which is vastly different from the old.
When a tree grows, stresses are produced. Cells at the heart of the tree distort slightly from their original shape as they "die", and this produces stress. As the outer layers of the tree grow, they compress the core. Such compression of the core compounds the stresses and will cause a piece of wood cut from the tree to curve away from the central axis of the tree. This produces 'spring' in a quarter-sawn piece of timber or 'bow' in back-sawn timber.
This is a major problem with the small diameter regrowth eucalypt. As the tree matures, the compression of the central core may cause it to break-down and the core stresses to dissipate. The central core generally becomes unusable for sawn timber.
Apart from these stresses, which are essentially radial, other stresses arise during the conversion and seasoning of timber. During drying, timber generally shrinks twice as much parallel to the growth rings as it does radially or across the growth rings.
Consequently, original shapes are distorted, with rounds becoming oval-shaped, squares becoming diamond-shaped or rectangular, etc.
Timber is generally back-sawn (this is with growth rings generally parallel to the broad face) or quarter-sawn (with growth rings generally at right-angles to the broad face). Backsawn boards tend to cup away from the heart due to the differing shrinkage rates, while a quarter-sawn board will tend to lose more in thickness than in width while still retaining a basic rectangular shape.
Small diameter eucalypts, with their unusable central core, high growth-stresses and small radiused growth-rings (which can cause extra distortion in drying) are particularly difficult to convert by conventional means.
Recovery rates (percentage of log volume converted into usable timber) by conventional milling means are generally around 50 percent for larger trees and may be around 25 percent for smaller diameter trees. The quality of timber produced from these small logs is often low and sometimes a log will be put over a bench and just an 'easy minimum' recovered, the remainder of the log being chipped. Ready money is available (around $60 per cubic metre) for woodchips for local paper mills and woodchip exporters.
With less attractive royalty rates payable for lower graded logs, there is little incentive for sawmills to maximise recovery. For the same reason, there is little incentive to grow hardwood timber, resulting in the bulk of Australian plantations growing introduced softwood species. Using conventional methods, these softwoods do offer earlier utilization, as they have less stress and their core is generally usable.
Radial sawing of timber offers a solution to the previously stated problems...
The radial technique involves repeated cutting of a log from the outside to the centre, along its longitudinal axis. After each cut, the log is rotated the desired number of degrees and cut again to give an elongated, wedge-shaped element with the desired thickness for the particular application.
All classes of logs can be sawn efficiently and economically. Stresses are relieved evenly in the log, leaving a large number of timber elements with similar shape and stress characteristics. Wedges cut from small diameter, high growth-stress trees will each spring away from the centre of the tree to a similar degree. The resultant wedge-shaped elements are highly stable, retaining their basic wedge shape when they dry and do not suffer from distortion as is generally the case with conventionally sawn timber.
Initial recovery-rates are high, with 70 to 80 percent of the log being turned into sawn timber. Some would argue that the high recovery-rate is all very well, but it produces unusable triangular sections...
Radially sawn timber has a multitude of uses; here are some of them:
In their basic shape, the radial wedges can be used as tapered weatherboards. Weatherboards cut in this manner can be produced efficiently and with a minimum of waste. The outside of the log can be removed or retained for a rustic appearance. The appearance of the rustic weatherboards can be most impressive as the story of the tree can unfold in a wave across the wall. Conventionally tapered weatherboards are resawn from rectangular pieces of timber cut fom high-quality logs. The rate of recovery of quality weatherboards is quite low.
Two wedges inverted and joined together can make a single member with two sets of parallel faces. Since the spring is reasonably even, two such elements can be connected so as to balance each other and make a reasonably straight member.
Laminated bench tops are usually made from very narrow strips of timber to increase the stability, so that in many cases, a laminated bench or table top made from radially sawn timber would have similar or less glue lines than a conventional product.
The consistency of production and the ability to group a large number of similarly-shaped elements from one section of log, will generally lead to a product with a high quality appearance.
Radially sawn wedges can be substituted directly, as individual units, for a variety of uses. A wedge, flattened at the apex of the two radial faces and the outer tangential face, can be used as a joist-bearer, rafter or simple beam.
The 'spring' of smaller trees can straightened-out over stumps or bearers while the lesser 'spring' of larger trees and sections can be cut-out if necessary.
Many mouldings, such as skirtings, architraves, picture frames, wind-moulds, etc, are often basically wedge-shaped anyway and can be produced with very little wastage by the radial sawing method.
Drop-seal wall panels can be produced by cutting a rebate in the outer tangential face of a segment to accept the smaller edge of another segment. These fit together to form a strong panel with very little movement.
Panels constructed in this way have a direct application as in-fill walls in pole-frame houses or in combination with earth walls in post and beam, non-loadbearing mudbrick housing. Other applications could be in the area of external fencing.
1. Andrew Knorr, through his company, Radcon, still has the only commercial radial milling operation of which the Good Wood Guide is aware, although the Eco Timber Co. of Murwillumbah is setting up as a distributor of the mills. Hopefully he will find the time to give more presentations on his technique and continue to 'spread the word'. The Guide strongly hopes that we will eventually see many resource-efficient radial mills operating throughout regional NSW and Queensland. (See the Alternative Directory listing in the Timber section, under Radial Sawmillers.)