Milling quality wood from our farm grown trees
Rather than provide a review of the various types of portable mills, this page looks at the science of sawmilling. My focus is on milling stable, furniture grade timber out of farm grown native hardwoods because that’s where I believe the best value lies for tree growers.
Hardwoods and Softwoods
Trees are classified into two groups: the hardwoods and the softwoods. The terms arose in Europe and refer to the heavy dense Oaks and the much lighter Pine. Strictly speaking, the distinction is botanical: the hardwoods include all the flowering trees (eucalypts, wattles and most rainforest cabinet species) whereas the softwoods are the cone-bearing trees like Pine, Cypress and Australian Kauri. Because the hardwoods have a very different wood structure to the softwoods it is not surprising that they differ in the way they perform when milled. In most cases the hardwoods are more difficult to manage and require a more sophisticated approach.
Sapwood, Heartwood and Pith
The sapwood is the band of creamy white or yellow wood on the outside of the log which provides the pipe through which water and dissolved minerals are transported up from the roots to the leaves. It also stores the food provided by photosynthesis including starch which makes it very susceptible to fungal and insect attack. The Powder Post Beetle (Lyctus borers) can turn dry(?) sapwood of the many susceptible hardwoods (including most eucalypts, wattles and oaks) into dust. Other than a small number of species that are recognized as non-susceptible it is illegal to sell timber containing untreated sapwood in most states. Treatment for internal use usually involved dipping in a boron mix.
The heartwood doesn't perform any function in the growth of the tree other than supporting the stem. The colour is the result of resins, minerals and other compounds being deposited in the cells as they are decommissioned from their role as sapwood. These deposits, and the lack of food, add durability, colour and strength to the timber. In many cases it is the colour and durability of many of our native timber species that makes them worth milling.
The Pith is the fine meandering track of corky cells in the centre of the log left behind by the growth of the tip of the tree. Because of the way the timber dries, a piece of wood with the pith included almost always cracks and splits. The first few growth rings around the pith usually contain what we call ‘juvenile wood’ which lacks the density, strength and stability of the wood in the outer growth rings. Old-time millers talk of ‘boxing out’ the heart to remove the pith and juvenile core.
Backsawn and Quartersawn
Depending on the species, product preferences and sawing methods, a miller may aim to cut boards that are either backsawn or quartersawn. Backsawn boards are cut so that the face of each board is roughly tangential to the annual growth rings. Backsawing allows for a higher recovery of boards from small diameter or pruned logs, although the timber may be more susceptible to drying defects (checking) and less stable in use (more prone to movement with changing humidity).
When the sawn boards are cut so that the face shows the vertical lines of the growth rings it is said to be quartersawn. Quartersawn timber has a number of advantages including lower shrinkage and less cupping and checking during drying. Because timber moves less in the radial than the tangential plane, quartersawn boards also move less in response to changes in humidity. This can be critical for applications such as solid timber flooring or table tops.
In species with prominent rays (such as Silky Oak and Casuarina) the general preference is for quartersawn boards to show off the unique rays that give these species their ‘oak’ characteristic. Similarly, if a log shows any sign of interlocking or wavy grain, the aim should be to maximize the proportion of perfectly quartersawn boards to show the ‘fiddleback’ grain.
Natural Growth Stresses
The formation of tension wood is common in many hardwood trees. As new growth rings are laid down around the tree, a longitudinal (up the tree) tensile stress is imposed as these cells mature. The result is that the surface of the log can be in tension (like an elastic band) and balanced by compression stresses (like a compressed spring) in the central core. The tension is greater in leaning trees. It is the formation of highly strung wood on the upper side of a branch that allows eucalypts and oaks to hold up huge horizontal branches. These stresses make it virtually impossible to mill a straight piece of timber out of a branch or severely leaning tree.
Growth stresses in straight logs can arise in tall skinny trees and are often responsible for end splitting in logs. It is the tension wood that allows the tree to flex in the wind without breaking. The release of the growth stresses during milling mean that both the sawn boards and the log from which it was cut tend to move to a new point of equilibrium. On most mills this results in the next board cut from the log being fatter in the middle than at each end. This is why experienced millers perform occasional straightening cuts during the milling process. Softwoods, such a Pine, form compression wood on the lower side of a leaning stem or branch which can create similar problems.
Research supports the observation that growth stresses in hardwood trees are of less concern in larger diameter logs cut from short straight trees. It has also been shown that soaking the logs in a dam or under a continuous water spray can relax the stresses over time but this may take several months.
The ideal log
If you plan to mill eucalypts, wattles or casuarinas the only practical option is to discard the outer sapwood and the central core (say 10cm square) with the aim of cutting straight clean heartwood boards. Unfortunately, the sapwood band is usually wider in healthy, fast growing trees. This means that many logs harvested from farm plantations don’t yield as much high quality heartwood timber as logs of the same size from slow growing native forest.
Log diameter is even more critical if you want to maximize the proportion of the sawn boards that are quartersawn or the logs show signs of severe growth stresses. Other factors, such as log taper, buttressing, the presence of knots, rot or other defects will all affect the recovery and quality of the timber.
The best strategy is to aim to mill logs that are straight, round, clear of defect and larger than 50cm in diameter. Of course this is not always possible but it is important to recognise that milling anything less than this will result in lower yields of quality timber. As a grower, I aim to produce quality logs that make on-farm milling a viable option. My view is that milling anything less than 40 cm in diameter is probably a waste of time.
There are many ways to mill a log. The simplest option is to simply cut a series of slabs or boards off one side of the logs (the through-and-through method). This is fine if the logs are very large and stable but, if there are growth stresses in the log the removal of a board from one side will result in the remaining log bending out of shape as it rebalances. If the log does bend as the boards are removed then it may be necessary to do a ‘straightening’ cut to level the logs before any more boards are cut.
The preferred sawing pattern for small hardwood logs is one that balances the growth stresses as the log is milled and maximises the proportion of quartersawn timber. The sawing pattern presented is a method that can be only used on a mill that allows you to cut through the full width of the log and rotate the log a number of times during the milling process. This is possible on most portable bandsaws. Some of the boards would need to be resawn to remove sapwood, the pith and any other defects but this would be best done after the timber is dried as other defects may become evident.
A possible quatersawing pattern for use on a horizontal bandsaw. Note that the log must be rotated as it is broken down. The final sections can be cut on a resaw bench or done individually on the bandsaw.
Types of farm scale mills
I have experience with three types of farm-scale sawmills each of which has its particular advantages and limitations.
The cheapest option is a chainsaw mill. These involve the use of jigs and bars that hold a chainsaw in place as it is pushed through the logs. A conventional chainsaw cuts a very rough kerf that is nearly twice that of a regular circular saw blade and much more than a band saw. For valuable logs the volume of sawdust could be a real issue. I use a finer ripping chain and a narrow bar that is available on order through Stihl distributors. For ripping the teeth on the chain are sharpened to a 10 degree angle (rather than 35 degrees). Using the special ripping chain reduces the kerf to about 5mm and makes a much smoother cut which not only saves wood but also makes cutting much easier.
The most commonly portable mill in Australia is the circular swing-blade portable sawmill. The Lucas Mill is one example. Once the log is positioned underneath the mill the operator can cut final dimension timber in a series of back and forth passes. During the cutting process it is possible to change the depth and width of each cut to maximise the proportion of quartersawn timber. Because the log sits on the ground it is difficult to rotate it so as to balance the growth stresses. This may mean that it is necessary to run a few straightening cuts to level the top of the log during the milling process. The operator can do this easily when it is lowered to the successive levels.
The Lucas mill is a robust and practical machine that can be used to mill a wide range of species. To mill small logs it would be worth set up a jig to allow the log to be rotated and to hold it steady. There is a limit to the width of board that the circular saw can cut although for very wide boards or slabs a chainsaw-type bar can be added.
Portable bandsaw mills usually have a lock deck which doubles as a trailer. The advantage of the deck is that the log can be rotated easily and held in place for each successive cut. The wider girth of the band saw also allows the operator to cut right through the centre of a relatively large log in order to follow a quartersawing pattern. Bandsaws have about a 3mm kerf which produces very little sawdust and if sharp and properly tensioned they can be very accurate and smooth. This can be a great advantage when milling specialty species. The trade off is that the bandsaw mills are generally more expensive and require greater operator knowledge and care.
Farm-scale milling is not a solution to every problem and in many cases I would advise growers to sell their logs to a professional rather than try and do it all themselves. One option is to rough saw the timber into large sections which are more efficient to transport long distances to a mill who can value-add more efficiently. The alternative is to spend more time and money setting up a timber drying and marketing business. Drying timber is a whole other article.
As a grower having the option of milling my own trees means that I can take advantage of the odd tree that blows over or dies. Having a mill also allows me to cut samples of my timber to show prospective buyers. I have used the chainsaw mill to cut logs in urban backyards that would have otherwise gone for firewood. My portable bandsaw mill is working well. There is a lot to learn but it’s fun seeing logs end up as clean boards, neatly stacked for air drying.
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