Wood working02.03.2016

Peeling lathe turns logs into veneer ribbon

Series of articles on plywood manufacturing, part 1

Logs are bundled in the grapple of a wheel loader before soaking

Logs are bundled in the grapple of a wheel loader before soaking

On approaching Koskisen’s plywood mill in the bitter cold of winter, one’s attention is first drawn to the sight of steam rising from a basin where logs are soaked for peeling. The water temperature in the basin is kept at around 40 degrees Celsius throughout the year, warmed ecologically by the surplus heat of the mill’s dryers. A wheel loader arrives on the scene and lowers a bundle of logs into the soaking basin. Bundled logs are easier to move in the basin.

“The logs are soaked to achieve the right temperature through and through. Soaking also softens the wood, making it easier to peel,” says Miikka Lehtinen, Production Manager at Koskisen’s plywood mill.  

The mill uses roughly 1,500 m3 of roundwood a day, which represents some 30 lorry loads. The logs are soaked after sorting or weighing and measuring. The soaking process takes about 24 hours, after which the logs are transferred to the next work stage. 

Log crane at work.

Log crane at work.

From the soaking basin, a crane lifts the logs onto the line, where they are debarked and measured before being cross-cut. On the line, a metal detector rejects logs that contain metal, and the bark is guided to combustion. The work of the cross-cut sawline operator is facilitated by modern technology: the log scanner shows a 3D image of the log and provides a cutting suggestion.

“Production capacity has been raised, and last November we hired 50 new employees at the plywood mill. We also invested in a new log scanner,” recounts Lehtinen. 

Koskisen’s plywood mill are used to cut the logs into the standard dimensions used in the plywood industry: 50, 60 or 80 inches (1,300 mm, 1,600 mm, 1,970 mm). Cross-cut saw line operator Seppo Penttinen has worked at Koskisen for roughly 30 years. Cross-cutting at the mill is clearly handled with extensive skill.

 “Although a lot of information is provided in the monitor to help the cross-cutting, you still have to keep an eye on the saws while controlling them,” he stresses. 

Photo: Log centring is an important stage before peeling.

Log centring is an important stage before peeling.

The cross-cutting byproducts are also used as much as possible: off-cuts are chipped and the chips are used in pulp production. Mini chips, however, are used to make Koskisen chipboard.  

After cross-cutting, the logs make their way along the conveyor to the peeling lathe, where the warm scent of wood fills the air. Each of the standard lengths has its own lathe, i.e. a total of three lathes. Before peeling, the log is centred in order to achieve the optimal peeling outcome. Koskisen mainly uses birch, but also some spruce and aspen. 

Lathe operator Jussi Mattila keeps track of the peeling process.

Lathe operator Jussi Mattila keeps track of the peeling process.

Birch gives the best peeling result of these tree species. The peeling speed is 260 metres per minute, which means the log is not in the peeler for very long. A log with an average diameter of 24 centimetres will produce roughly 20 metres of veneer,” says lathe operator Jussi Mattila. 

With Mattila’s employment at Koskisen dating back to 1999, there’s no shortage of experience and skill also at the lathe.  

The veneer achieved from the surface of the log, i.e. face veneer, is the most valuable due to its appearance and lack of knots. This can be seen during the peeling process, despite the speed of the lathe. Initially, the beautiful wood grain shows on the veneer, but the further the peeling progresses, the knottier the veneer is. The core, or the centre part that remains of the wood, goes to the chipping plant.

“It takes around 3.5 cubic metres of logs to make one cubic metre of plywood, and more if further processing is involved,” explains Lehtinen.

Veneer ribbon peeled from a log.

Veneer ribbon peeled from a log.

Koskisen’s plywood mill runs in three shifts, seven days a week. The high quality of the plywood is monitored with continuous quality measurements and inspections during various stages of the process. In particular, the thickness, dimensions and tensile strength of the veneer are monitored during peeling. Strength properties are especially important in end products that require good load-bearing capacity, such as KoskiDeck or KoskiForm.

Peeling at the plywood mill in a nutshell

  • The plywood mill uses approx. 30 lorry loads of logs a day
  • The main tree species used are birch, some spruce and aspen
  • three shifts, seven days a week
  • 3.5 m3 of roundwood per 1 m3 of plywood
  • Koskisen’s specialities in plywood production:
  • 1,970 mm as the main dimension
  • peeling of aspen
  • steaming, which supports soaking