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Secondary Breakdown: Bandmills or Gangs

Alan Froome continues his discussion of sawmill ideas and methods and how they evolved, along with a review of some recent developments in sawmill technology in the search for ever-higher lumber recovery and profit.

By Alan Froome
Date Posted: 4/1/2005


(Editor’s Note: In his regular column, Alan Froome continues his discussion of sawmill ideas and methods and how they evolved, along with a review of some recent developments in sawmill technology in the search for ever-higher lumber recovery and profit. This series of columns is intended to be a generic overview of the different methods used in the sawmill industry in response to readers’ requests.)

 

Secondary Breakdown

      Following the primary breakdown stage, where the first cuts are made in the log, the next step in lumber production is to make the second cuts in the cants and slabs produced by the primary breakdown machine.

      The earliest mills, built in the 1800s, relied on a single machine center to make all the cuts, and many small mills today continue to do so. However, in striving for greater sawing accuracy and lumber recovery, many specialized machines have been developed for the secondary breakdown step. These specialized machines can resaw or gang-saw material much more efficiently than the machine center that performs the primary breakdown.

      Secondary breakdown is carried out in a number of different ways, and a variety of methods are in use. These vary tremendously in the sawmill industry. The most popular systems used today are:

• vertical bandmill linebar resaw (single, twin or quad)

• horizontal bandmill resaw (single & twin)

• slant bandmill linebar resaw

• gang with vertical arbor circular saws (straight sawing)

• gang with vertical arbor circular saws (curve sawing)

• gang with horizontal arbor circular saws (straight sawing)

• gang with horizontal arbor circular saws (curve sawing)

 

Band Resaws

      All the machines listed above are currently in use. The degree of sophistication of each system varies widely.

      For straight cants, the vertical band mill-linebar combination provides very accurate resawing capability. In some cases the linebars are fixed, and the bandmill shifts to make different lumber thicknesses; others used a fixed bandmill and a setting linebar. The results are about the same.

      Many linebar systems were installed in the 1970s. Some linebar systems in the softwood industry have as many as four close-coupled bandmills (quad) to make multiple cuts in one pass. These machines use primarily 5-foot and 6-foot diameter wheels with 100 hp or 150 hp motors for high production. Saw kerf is usually around 0.125-inch on a 5-foot machine. Some of these twin and quad bandmill systems also were equipped with separator outfeed tables that can automatically peel off the outside pieces.

      The slant linebar resaws are few in number but work particularly well in this respect, using gravity and an incline of 27 degrees to separate the boards. Vertical or slant band resaws are usually equipped with a run-around or merry-go-round conveyor feed system; the operator can circulate the cants continuously, making a cut each pass until little or nothing is left to recover.

      The main payback argument for the band resaw over the circular gang resaw is that they use thinner kerf blades; they have higher lumber recovery and are more flexible to use than a gang. A rule of thumb around the bandmill makers is that 55% of the kerf saving is recovered as usable lumber when changing from a circular saw.

      Horizontal bandmill resaws have also been in use for some time. They are ideal to resaw wide slabs and flitches, which are fed flat side down on a slatbed or horizontal conveyor table of some kind. Like its vertical cousin, both setting bandmills and setting table types are in use.

      Today many smaller sizes of horizontal band resaws are available and are in wide use, especially in the pallet and hardwood industries. These lighter, less expensive machines typically have wheels of 28 inches or 32 inches in diameter and run blades as narrow as 0.070-inch kerf. They can be used as a single machine or in multiple configurations with as many as nine or more machines in a line. Keeping everything in alignment becomes an issue, however, with large multiples of these machines.

      Some of these bandmill resaws can also be found today doing valuable work as reman machine centers towards the back of the mill building. They are used as a ‘last chance’ machine to recover saleable lumber from pieces rejected in the final grading and sorting process.

 

High Strain Bandmills

      It is worthwhile to say something at this point about the development of thin kerf bandmills.

      Thinner bandsaw kerf is only possible with some kind of high strain tensioning system to prevent the blades from ‘snaking’ in the cut. There is an amusing story about the origin of thin kerf bandsawing. Originally strain tension was provided by a simple weight hanging at the end of a lever arm. In the 1960s Canadian machinery maker Letson & Burpee (now part of USNR), based in Vancouver, British Columbia, shipped a new bandmill to a customer. The bandmill had the usual shipping blocks in place to prevent movement of the top wheel or weight arm in transit.

      The customer installed the machine and started cutting lumber. He soon found he could feed the lumber through much faster than Letson & Burpee had told him — and without the blade snaking or cracking. The manufacturer was curious and sent a service representative to investigate. He found the shipping blocks were still in place! The saw strain was much higher than normal. This accidental discovery was in fact the beginning of high strain bandsawing as we know it now.

      Many of the simple weight strain machines are still giving good service today, but more responsive air and hydraulic systems have been devised since then. They also protect the saws by momentarily reducing strain when the blade hits a knot or foreign object in the wood at high feed speeds.

 

Straight & Curve Gangs

      The alternative to bandmill resaws for secondary breakdown is the circular gangsaw. This is a machine with a cluster of circular blades running on a common arbor. It makes multiple cuts in one pass and in a short distance. Usually cants up to 8 or 10 inches high can be cut by a single arbor machine, but larger sizes usually require a double-arbor. The double-arbor gang has two banks of saws (top and bottom) to reduce kerf thickness. Careful alignment is necessary to avoid mismatch saw marks between the top and bottom cuts. Both horizontal and vertical arbor machines are popular. Guided saws are run on many machines, which allow thinner blades to be used. These machines have greater lumber recovery but require more maintenance to resurface the saw guides and other tasks. Some vertical arbor machines are installed close-coupled behind a canter line as an add-on module (without a separate operator).

      One of the most important industry innovations in recent years has been the introduction of curve sawing gangs. The first successful curve gang was built in the early 1990s. The main idea behind the patented concept is to produce longer (and more valuable) lumber from curved cants by following and sawing along the grain. This patented design uses guided circular saws on horizontal arbors that skew and slew in an arc to follow the shape of the cant. This concept was received with a lot of skepticism initially, and some lumber companies still are dubious. However, curve or shape sawing gangs have become almost standard equipment in high output softwood mills.

      Other machinery manufacturers followed with designs for curve sawing gangs, some using shifting infeeds or vertical arbor saws to achieve much the same result — more value from curved or S-shaped logs. Over 70 of these machines are now in service with some of the recent systems running at 600 feet per minute. These systems are fairly expensive compared to straight sawing machines and require scanning technology and automated, computerized controls.

      The lumber value recovered by curve sawing gangs is the highest available today for mills running large numbers of sweepy cants. It is amazing to watch one of these machines at work as the saws pivot to follow the shape of the cant. One of the machinery makers even uses the music from Swan Lake as the soundtrack on a promotional video to emphasize the fluid motion of the saws.

 

Applications

      From an application and methods point of view, circular gangs generally are capable of higher production and are found in the biggest mills. Some people suggest that a gang application is appropriate for production over 30,000 board feet a day, and in fact most large softwood mills today use gangs. Gangs also are preferred in some special situations, such as making pallets for industries that cannot tolerate sawdust — simply because they make chips rather than sawdust. Mills producing less than 20,000 or 30,000 board feet a day usually pick smaller bandmill resaws for secondary breakdown.

            The introduction of curve or shape sawing gangs has supplanted the big twin and quad bandmill resaws in many new mills or recent mill upgrades — at least at mills where sweepy logs are a fact of life. Mills with a diet of primarily straight plantation logs, however, can still benefit from linebar resaw bandmills for secondary breakdown.




 






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