Fast-growing timber also needs a high yield, this is how to saw eucalyptus!

Eucalyptus is a fast-growing timber with a short growth cycle, strong adaptability and high yield. It is one of the three fastest-growing tree species in the world recommended by the Food and Agriculture Organization of the United Nations, along with fast-growing forest tree species such as pine and poplar.

Eucalyptus is not a type of tree, but a general term for all types of eucalyptus. There are about 808 species and 137 subspecies or varieties, totaling 945 species. Among them, there are more than 100 species with important economic uses. The vast majority of eucalyptus trees are distributed in the Australian mainland and its neighboring islands.

About a hundred years ago, eucalyptus was introduced to China as a garden tree species, shelterbelt, and roadside greening tree species. After the founding of the People's Republic of China, large-scale commercial planting began in the Leizhou Peninsula of Guangdong and Guangxi.

Since the country introduced eucalyptus, eucalyptus forests have developed rapidly, and eucalyptus timber has accounted for more than 30% of the country's timber, which has timely filled the timber gap and greatly maintained the country's timber security.

▲Eucalyptus veneer

▲Eucalyptus tree cubes

Eucalyptus wood has become an important source of plywood and solid wood in the timber market, but due to the growth stress of eucalyptus itself and the limitations of processing technology, the yield of eucalyptus wood is not high.

During the sawing process, the wood is prone to defects such as end cracks, splitting, and warping due to the release of its growth stress, which greatly limits the use of wood in solid wood processing.

Problems encountered in eucalyptus sawing

When eucalyptus wood is sawn horizontally, the longitudinal growth stress at the end of the new wood is released and converted into tangential and radial tensile stress. The tensile stress exceeds the tensile stress that the wood itself can withstand, and the end of the wood cracks and deforms.

During the eucalyptus timbering process, the tangentially cut boards from the outer side of the log will bend outwards due to the release of longitudinal tensile stress.

When sawing the tangentially cut board close to the pith, the longitudinal compressive stress is released and the board bends toward the pith.

When sawing center-sawn lumber, opposite tensile stresses are generated on both sides of the pith at the ends of the lumber, causing cracks along both sides of the pith at both ends of the lumber.

Eucalyptus wood can crack and deform during sawing, reducing its quality and even rendering it scrapped. This significantly reduces the yield and utilization of the wood. The sawing quality of the wood also affects subsequent processing and drying.

Therefore, wood processing companies and scientific researchers have been working hard to explore sawing equipment and methods suitable for eucalyptus to reduce cracking and deformation of the boards.

Development of eucalyptus sawing equipment

In the traditional sawing and timber processing stage, the traditional sawing equipment used for eucalyptus timber is mainly divided into band saws, circular saws and frame saws.

With the advancement of science and technology, eucalyptus sawing technology has been continuously innovated and improved.

The lumber industry has integrated high-tech technologies such as numerical control and intelligentization into sawing equipment. The application of advanced technologies such as chipping-lumbering combined systems, scanning technology, saw blade monitoring systems and control systems has not only greatly improved the yield of eucalyptus, but also effectively suppressed the warping of sawn timber and reduced thickness tolerances.

band saw

Band saw is a traditional wood-cutting equipment. Commonly used band saws include sports car band saw and double band saw.

Sports car band saw generally refers to a single saw blade band saw, this method can be used to saw according to the material.

The twin band saw uses a dual blade sawing mechanism, sawing both sides of the log simultaneously, evenly releasing growth stress during the sawing process. The twin band saw cannot be sawed while looking at the log, but it is more productive than a racing band saw.

▲Sports car band saw

Band saws are more suitable for sawing large-diameter eucalyptus wood. They have high sawing quality and narrow sawing paths, and the log yield is high. However, the thickness of the sawn wood varies greatly, and the secondary processing loss of standard wood is large.

circular saw

Circular saws have been widely used in European countries since the early days of wood processing. Currently, the most widely used ones are double-axis multi-blade circular saws.

The double-axis multi-blade circular saw is more suitable for sawing large-diameter eucalyptus wood. It can produce boards of required specifications in one sawing. The process is simple, the production efficiency is high, and the processing precision is high, but the saw path loss is large and the yield rate is low.

frame saw

Frame saws are suitable for sawing eucalyptus wood with smaller diameters.

Compared with band saws and circular saws, it has the advantages of being able to saw multiple pieces of timber of different thicknesses at one time, high production efficiency, low cost, simple production process, etc., and is easy to be used in conjunction with a single re-saw system.

Chipping-lumbering combined system

The chipping-lumbering combined system uses a squarer to chip the veneer and edge strips of logs to make standard lumber or square lumber, which is then cut into boards using a multi-blade saw. It integrates chipping, longitudinal splitting, and edge trimming equipment in a compact structure.

Scanning technology

Scanning technology is mainly used for log positioning in sawing systems. By scanning the shape of the end or cross-section of the log, analyzing the relevant data, and providing the precise parameters of the analysis to the computer control system, the optimal sawing position is finally determined.

In the application of sawing logs, the yield rate can be increased by 3% to 5%.

Saw blade monitoring system

The saw blade monitoring system mainly consists of three parts: a display, an oscilloscope and a non-contact position sensor. It is mostly used on multi-band saw machines and sports car band saw machines to ensure the feeding speed of the sawing system.

The use of saw blade detection system can increase lumber production by 5% to 7%.

control system

The control system coordinates all functions through a central microcomputer. The central microcomputer calculates and identifies the size, shape and quality of the log by scanning the system data, then selects the best sawing section and adjusts the position of the log on the machine tool.

Development of eucalyptus sawing technology

The sawing method has a direct impact on the log yield and sawn timber quality. The choice of sawing method also needs to be based on the tree species, wood diameter and purpose.

At present, the sawing techniques used for eucalyptus mainly include tangential balance sawing, flip sawing, three-side sawing, four-side sawing, radial sawing, wedge sawing and curve sawing.

Tangential balance sawing method

The tangential balanced sawing method is to saw the log symmetrically from both sides along the pith direction. Since the sawing direction of the board is parallel and symmetrically distributed on the cross-section of the log, the growth stress can be evenly released during the sawing process, which can alleviate the warping of the board.

The tangential balanced sawing method is mainly used for sawing eucalyptus wood of small and medium diameter (diameter less than 400mm) to obtain the widest possible sawn timber.

Flip down sawing method

The flip sawing method first saws down the log in a vertical direction from one side, then flips it 90° three times in a row and saws down. The number of boards sawed from the first three sides depends on the diameter of the log.

This method has a high yield of sawn boards. Since the growth stress of eucalyptus radially sawn boards is more balanced than that of tangentially sawn boards, the sawn boards have less end cracks and warping than those of tangentially sawn boards, thereby improving the overall quality of eucalyptus boards.

Three-side sawing method

The three-side down sawing method first saws off one side of the board from the edge of the log, then flips it outward 90 degrees, places the sawn surface on a bench, and then saws it into boards in parallel.

For medium-diameter eucalyptus (diameter range of about 300-500mm), the three-side down sawing method can be used for sawing, which not only reduces the workload of some trimming, but also obtains wider sawn timber.

The principle of the three-side sawing method is similar to the chord-balanced sawing method, except that the three-side sawing method has one more cutting surface. This cutting surface is conducive to stabilizing the eucalyptus wood on the machine tool, improving the sawing stability and the accuracy of the board specifications. At the same time, this method is simple to operate and highly efficient, so it is widely used by eucalyptus wood processing companies.

Four-sided symmetrical chordal surface sawing method

The four-sided symmetrical tangential sawing method is also called the rough square sawing method. The log is flipped 180° once, and the skin on both sides of the log is sawed symmetrically to obtain two rough square sides. Then it is flipped outward 90° and the required specifications of boards are sawed in turn.

This method is invented based on the wood structure and growth stress characteristics of eucalyptus wood. Symmetrical sawing of the two sides of the eucalyptus wood can evenly release a portion of the growth stress and reduce residual stress, thereby effectively reducing the cracking and bending caused by the release of growth stress during the subsequent sawing process.

This method is suitable for sawing large-diameter eucalyptus (diameter greater than about 400mm). It can reduce the cutting height and trimming workload of the rough square, and the specifications of the resulting sawn timber are more uniform.

Radial sawing

The radial sawing method is to first saw the log into quarter lumber, and then saw each quarter lumber into quarter-sawn boards of the required thickness.

This method is relatively complicated to operate and increases labor and time costs, so few companies use this method to process eucalyptus.

Wedge sawing method

Wedge sawing, also known as radial sawing, is an innovative method for efficiently sawing small-diameter logs in plantations. Using wedge sawing for small-diameter logs with a diameter of less than 400 mm can effectively increase output value.

The wedge-shaped sawing method saws the eucalyptus wood in the order of its growth rings. The width direction of the board is consistent with the direction of the growth rings, which can reduce the residual stress of the board and evenly distribute the growth stress. In the subsequent drying process, the drying stress is also more evenly distributed on the board, which improves the overall dimensional stability of the board and effectively reduces defects such as cracking, warping, and horizontal bending of the board.

However, since this sawing method is relatively complicated to operate and has high processing costs, it is generally only used for processing plates for special purposes.

Curve sawing method

The curved sawing method is to saw the logs according to their structural shape. It can not only cut along a straight line, but also perform curved sawing within the range of limited parameters, which can significantly improve the yield and utilization rate of logs.

The curved sawn timber produced by the under-curve sawing method can be straightened after drying and other processing, thereby improving the properties of the wood.

Summarize

The harvesting and utilization of fast-growing eucalyptus trees in plantations are mainly small and medium diameter timber, and the cracking and deformation of small and medium diameter logs are more obvious.

At present, most of the small and medium-sized enterprises in China basically use traditional sports car band saws, double-axis multi-blade circular saws and simple three-sided sawing methods. They are mainly traditional sawing equipment with low cost and easy operation.

The three-side down sawing method has a simple process, high yield rate and processing efficiency, but defects such as cracking and deformation of the sawn timber cannot be effectively suppressed. Therefore, the resulting sawn timber is difficult to be directly used for the processing and utilization of wood products such as furniture and handicrafts.

Combining the current intelligent sawing technology, developing a fully automatic sawing system suitable for eucalyptus processing and improving the precision and quality of products through the use of automation technology is the development direction of eucalyptus sawing.