Wolfram Ulrich
Head of Global Branch Strategies
Bosch Rexroth
Lohr am Main, Germany

Curve saws can follow the outer contour of bowed logs, which yields more usable lumber than straight cuts. Feed speeds can exceed 500 fpm.

Highly automated sawmills are the norm in today's lumber industry. Modern lines debark, trim, cut, and saw up to 20 logs/min — depending on the type and size — with tolerances in the tenths of millimeters.

But speed is not the only path to improving production. Wood processors increasingly need to reduce waste and make the most efficient use of raw materials. One recent innovation is curve saws, which are not restricted to straight cuts. They can also follow a trunk's outer dimensions or specially calculated profiles to cut boards more efficiently.

The aim is getting the most usable lumber from each log. Yield is critical when turning tree trunks into planks and boards because raw materials make up about 80% of cost of the finished products. Even a 1% increase in yield can substantially boost sawmill profits.

Depending on the shape of the individual trunk, curved paths carve out up to 20% more finished wood. The curved boards and timbers are then straightened during drying. Cutting lengthwise along annual rings also lessens the degree of cross-grain cuts. This, in turn, reduces internal tensions. The result is more-homogenous lumber with greater load-bearing capacity and higher commercial value.

CALCULATING CUTS

Curve-sawing technology combines developments in sensors, microprocessors, and electrohydraulics. Because no two logs are exactly the same, multilaser scanners and cameras first measure trunks and pass the digital details to a master computer — typically an industrial PC. Special software takes the size and shape and generates a 3D model of the log, and calculates the cutting profile that maximizes output.

Normally, the PC works with higher-level enterprise software. This directs the optimizing program to determine position and shape of cuts based on factors such as satisfying orders for specific sizes of lumber and boards, fluctuating prices, and so on.

Because there is little time to spare between scanning and cutting, software specialists have refined the algorithms to minimize processing time, yet produce accurate results. Major saw manufacturers, such as Coe Manufacturing and USNR, offer proprietary software, as it is now a key component of their machines. Some independent firms, such as MPM Engineering and Porter Engineering, also market software to curve-saw OEMs.

Once generated, cutting data is sent to a high-level PLC that oversees every stage of an entire saw line. The PLC, in turn, sends positioning information to the motion controller that drives the electrohydraulic axes and, in turn, sets saw-blade angles.

SETTING BLADES

Typically eight or more blades are used to make curved cuts. They are housed in what's termed a sawbox, and electrohydraulic actuators control the spacing between blades. Before curve sawing begins, the blades are positioned once for each trunk and then held in place during cutting.

All hydraulic drives involved in the cutting process must have high dynamic response, position accurately, and resist vibration and contamination from sawdust. For some axes, such as those that adjust cutting width, compact size and simple integration into the machine are additional requirements.