As we continue to review recent developments in sawmill technology in the search for ever higher lumber recovery and profit, we move on from the topic of my first column, debarking and bucking, to the next step in lumber production, primary breakdown.
Primary breakdown consists of making the first or opening cuts in sawing the log. Much innovation has been applied over the years in this area. The earliest mills, built in the 1800s, relied on a single machine center to do everything. Some small mills today still do.
Over time, the search for better accuracy and higher production spurred the development of additional specialized machines that can resaw, edge, trim, and perform other lumber manufacturing tasks more efficiently than one machine.
Primary Breakdown
Primary breakdown is carried out in many different ways, and most would agree that it is the most important step in lumber production. Recognizing the importance of primary breakdown, machine manufacturers have strived to devise ways to reach a higher LRF (lumber recovery factor) than competitors.
Here is a list of the most popular systems today:
circular scragg saw, single or twin and carriage
vertical or horizontal band head rig and carriage
sharp chain system with twin or quad band mills
canter line with circular or band saws
double length infeed (DLI) canter line with twin or quad band mills
end-dogging infeed with canters (often with circular or band saws)
· slant band head rig and carriage (27 degrees from vertical)
The degree of sophistication in these systems varies widely.
The DLI canter lines developed in the 1990s have become very popular in high output softwood mills and can have tilting and side shifting infeeds to get the most value from sweepy logs. These systems require a scanning and automatic control system to run and can have upwards of 30 to 40 servo positioners. The lumber recovery of the DLI system is the highest available today and only challenged by some of the high tech end-dogging feed lines, which are even more complex. Some DLI lines can run at speeds up to 600 feet per minute or more as mills demand more logs per shift.
Large log breakdown is usually best done by a head rig and carriage combination. This is one of the oldest methods devised, but scanning and computer optimization has improved recovery and value over the older, manually controlled machines.
In the old days, steam driven ‘shotguns’ powered the carriages along rails, but hydraulic drives have largely replaced them. Some people on principle prefer electric over hydraulic, so variable frequency AC and DC drives are also available.
For a time DC electric motor driven ballscrew positioners were state of the art for setworks, but these proved expensive to maintain and replace and have largely disappeared. The hydraulic servo positioner with temposonic type linear feedback transducer has become the industry standard just about everywhere for accuracy and repeatability. The best carriages use these hydraulic servo positioners to accurately position and taper the carriage knees. They are extremely accurate and work best if some kind of electronic scanning system is going to be installed now or in the future.
Sixty to 80 years ago, many carriages had a man riding on them to operate the log turners, spike or dog the logs, and so on. It was not a great job getting covered in sawdust and was dangerous, too. Now, by contrast, sawyers sit at controls in an insulated, air conditioned – and safe – cab.
Hardwood mills, where grade and appearance are the main priorities, still rely in large part on the skill of the operator, although suppliers of optimization technology are doing research to include defect and color data in computer control inputs.
In the 1970s, before the DLI systems arrived on the scene, the sharp chain-twin band mill systems achieved the highest recovery combined with high log count. These systems are still giving good service in many softwood mills with medium recovery factors and feed speeds up to 350 feet per minute. A few alder mills also operate sharp chains, which are usually 3 inches wide and run in machined guideways. Obviously, the chain width limits any offset by the band mill setworks to some degree.
The performance of the sharp chain-band mill systems was only really challenged by the chip-n-saw in that era, which was made by a rival manufacturer. In fact, more chip-n-saws were sold; hundreds of chip-n-saw systems were installed, especially in the Southeast to cut Southern pine.
The concept of the chip-n-saw was to convert logs to lumber in a single step by one machine. The main criticism of it was that it could chip too heavily at times. Most are still running today but have been upgraded intelligently with new infeeds and better controls and scanners, allowing them to cut full taper and half taper and achieve much higher recovery than when they were first installed.
European Methods
As a side note, European sawmills have followed a different tack. The mills sort logs (after bucking) in 60, 70 or more different size classifications, and then run a complete shift on one or two sizes without changing machine sets. The multiple in-line canter line is very popular. The first machine chips off two faces, then the logs is turned 90 degrees and the next two faces are chipped off. The resulting square cant is then passed through a circular gang edger and finished boards exit the line.
Here in
Another type of primary breakdown machine found mostly in
Progress never stops, and no doubt there will be many more innovations in primary breakdown methods in the years ahead, although sometimes it is hard to imagine how we could squeeze more out of a log than the better mills do today.
