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Processing - From Logs to Lumber


The design of wood sawing systems over time has evolved with changes in technology, resource and demand. The design of most sawmill is most influenced by four factors;

The hardwood sawmilling industry has been affected by changes in these factors and has some characteristic traits. The industry is typified by a significant number of small operators and a few larger ones. Timber quotas for mills may range from 1000 CU.M (cubic metres/yr) to 10,000 CU.M plus/yr.

The large proportion of small operations reflect a desire to maintain a simple, small, low capital operation either as a conscious decision to avoid risk, limits on the quantity of timber available, because it best suits their style or working or because further investment of resources for the production of typically low value material would be uneconomic.

The Wood Resource


In the case of hardwood sawmills, operations were traditionally based on the use of Old-Growth stands of large dimension. While quotas on the volume allowed to be sawn may have varied, mills would have been able to operate with some guarantee of timber volumes. Sawmills are now being required to cut progressively smaller material as Old Growth stands are exhausted or reserved for conservation purposes.

Markets


Due to the absence of better saw technology and/or drying technology, markets for hardwood timber have traditionally been in low value, undried timber (Green Scantling) mainly for structural uses. There have been little limited new markets for green timber and hence little product diversification.

Mill location


Many hardwood mills have traditionally been located in or near the forests from which timber is sourced, or in adjacent areas near markets. This scenario has changed little over time with the exception of some new mills which have been able to establish further from the resource with improved transport capability.

Available Capital


The majority of hardwood mills are small, often family-managed operations without substantial amounts of capital available to invest in either mill equipment or infrastructure. Larger mills have been established and managed by companies and the increased capital available is reflected in the size, output and efficiency of the sawmilling operation. Where any improvements in operation occur, they tend to be incremental.

While a variety of methods and sawing patterns are available, the above factors have influenced hardwood sawmills in Australia to the extent that most operations while differing in the size and productivity, consist of two main sawing processes; the Head Rig or Break Down Saw and one or more secondary breast bench saws. The saws have different roles but essentially reduce rounded pieces of wood to manageable and marketable pieces of timber.

The process involved in the conversion of logs to green sawn timber is best described by analysing these components in the chain of mill operation.

The Head Rig/Break Down Saw

The Head Rig breaks whole logs down to manageable slabs or flitches. Its function is to produce flitches that can be easily processed at the Breast bench using preparatory log cuts. Logs are loaded onto the log carriage by a small crane or bulldozer and turned with log 'kickers' by the sawyer to align the log ready for the first saw cut.

The log is passed through either a large single or pair of vertically opposed circular saws slightly offset, or a bandmill (source unknown) which slabs material from the log.

The slabbing process produces one or two flat sided flitches which are fed on rollers or transfer belt to be further processed on the breast bench where the secondary cuts are made.

Log carriages (source unknown) are used in all but the smallest mill operations in Australia to pass logs through either a circular saw or bandsaw (Woodmizer promotional brochure)

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Circular Saws

The advantages of circular saws compared to bandsaws for cutting hardwood logs are as follows;

  1. Low cost of the equipment
  2. Low cost of the saws and the low costs of maintaining them
  3. Simplicity in adjustment and operation
  4. Circular saws are relatively robust
The disadvantages are;

  1. Relatively wide kerf resulting in greater waste during cutting
  2. Power requirements are greater than other types of saws owing to the wider saw kerf and the angle at which the teeth strike the wood
  3. Circular saws have a limited depth of cut and are unsuitable for cutting wide boards. In some mills a vertical frame saw consisting of a reciprocating vertical frame in which are mounted one or two saws may be employed at the circular saw headrig to cut oversized logs.

Bandsaws

Bandsaws consist of a continuous band of steel with teeth on one or both edges mounted on two wheels, one of which is above and the other below the cutting field. These wheels can be 1.5 to 3m in diameter with the size of the saw usually designated in terms of the diameter of the wheels. The size of the wheels determines the gauge of the saws which can be used and to a large extent the size of the log. Power is applied to the lower and heavier wheel which acts as a flywheel driving the saw downward through the log as it is fed through the saw on the carriage. Often seen in softwood sawmills, bandsaws are also seen in hardwood mills where accurate sizing is required, as in the cutting of regrowth Ash type eucalypts. The advantages of having a bandsaw at the headrig are;

  1. The saw kerf can be reduced to almost half that of a circular saw
  2. Higher output than other headrigs due to higher feed speeds (typically 1-1.25m/s)
  3. Deeper cutting and hence large logs can be sawn. Less power is required due to the smaller saw kerf.

The disadvantages of band saws are;

  1. High initial cost of the equipment
  2. Relatively high cost of the saws
  3. Greater skill required to operate and maintain the saw
  4. Delicacy-bandsaws are quite delicate. If a log is forced into a bandsaw too rapidly it may rend the saw from its wheels. Band saws are also much more susceptible to damage from rock nails and other hard materials that may be embedded in logs.

The Breast bench

Flitches of timber from the Head Rig are conveyed to the Benchman at one end of the breast bench by a conveyor belt. The Benchman runs the slab through the saw. The dimensions of the material cut are decided by the Leverman, working with the benchman, who controls a gauge regulating what width of the slab is passed through the saw at any time.

The slab is pushed through the saw to produce smaller flitches returned by two people 'the "Tailor outs" as many times as required to get the timber dimensions necessary. This can be seen as two stages; cutting the flitches to the required size (sizing up) and then 'ripping off' - passing the flitches over the saw again and again until the flitch is completely sawn up.

The operator of the headrig is often the most skilled of sawmill production personnel followed by the Leverman and the Benchman. In some sawmills producing backsawn boards only and having a sawmill carriage capable of accurately turning and sizing the log, both primary and secondary cuts may be made at the head rig. Timber of the required final dimension is thrown onto the 'Green Chain', a line feeding sawn material to be stacked ready for dispatch.

To ensure that the saws remain in good condition, most employ a "Saw doctor". It is their job to ensure that the angles of the saw blades are maintained, the saws kept shard and inefficiencies in the sawing process removed.

Two types of sawing are employed in sawmills depending mainly on end-product requirements, log species and size namely back-sawing and quarter sawing. 'Sawing around' involving backsawing or quarter sawing involves turning a log as sawing proceeds so that the cuts are made in different planes. Back sawing aims to produce boards with the face tangential to the growth rings of the tree and quarter sawn products are cut with the face parallel to the rays and at 90 degrees to the growth rings. Wallis,N,K 1970)

Live sawing another technique involves using parallel saws to cut the log. The log is ripped into boards by means of a parallel series of saw cuts without any turning of the log.

Backsawing

Backsawing is the most widely practised method of sawing in Australia. The method is highly flexible and well suited to securing high grade timber from faulty and varied logs. Backsawing offers the following advantages over quarter sawing;

- Higher recovery and output

- Better use of a log to avoid defects

- A greater proportion of boards may be obtained

- Boards season more rapidly with less shrinkage in thickness

- Timber may be more readily nailed without splitting

Quarter Sawing

Quarter sawing is important for eucalypt species where losses during seasoning are high. In this area, the saving in avoidance of seasoning losses is frequently more than sufficient to offset the disadvantages of slightly lower recovery, and slower production obtained when quarter sawing as compared to back sawing.

Quarter sawing may be practised to;

- Reveal ray textures in decorative timbers

- Produce edge-grain material as for flooring and joinery

- Reduce the feature of gum veins in eucalypt logs

- Reduce the amount of defect in the timber sawn. Quarter sawn timber is less prone to cupping, warping and checking and shrinks less in with than backsawn material.

An Efficient Process?

The process involving the conversion of logs to sawn timber is not a particularly efficient one. Walker (1995) makes some important observations on the efficiency and effectiveness of mill operation. Walker suggests that a mill should be judged on its operational efficiency and profitability - a result as much of good management as mill design. Sawmills are characterised by the timber resource they cut, by their size, by the type of machinery employed to break down the logs and by the degree of automation. In Australia mills produce an assortment of sizes, lengths and grades which is too varied for production to be quite rational. There is no 'standard' design with each mill unique in its operation. Good mill design however can be recognised through the smooth flow of wood through the mill with no bottlenecks and with no machine waiting for material to cut. Mill design should involve repeated simulation of various design options, varying the resource characteristics, the saws and mill layout, and the market demand for different sawn products.

Throughput, volume conversion and grade recovery determine the profitability of a mill. The conversion is defined as the ration of the volume of green sawn timber that can be cut from a given volume of debarked logs. Usually the conversion is based on the nominal sizes being cut rather than the actual green dimensions which allow for sawing variation, shrinkage and planing loss, or it is based on the dry dressed sizes. In modern mills green conversion is typically about 55-60% and the sawdust generated not exceeding 10%. In old inefficient mills sawdust can amount to 25%. Grade recovery is concerned with maximising profits by cutting the more valuable grades or sizes rather than trying to achieve profitability by maximising throughput or volume conversion. It involves turning and careful examination of each log as it is being sawn to determine the best sawing strategy. A log carriage being needed to perform this.

The Costs

Most of the costs incurred by the sawmiller are in the handling and transport of timber to the mill. Specifically, costs are incurred in the following areas;

These are notional prices and differ between timber regions. In the mill, making the most of the timber available reduces costs. This is achieved by ensuring the primary cuts on the head rig are made to obtain the highest recovery of timber possible and reduce the amount of waste as woodchip. It also means committing resources to ensure that staff are well trained and in what they do.

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