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Sawlines: Edging, Trimming, Grading, Sorting
Edging, Trimming, Grading, Sorting
By Alan Froome
Date Posted: 7/1/2005
(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.)
Following the previous articles focusing on methods used for primary and secondary breakdown, which produce boards and cants, the next steps in lumber production are to cut off the waney edges, trim to length, square off the ends, and sort the lumber. Grading also comes into the sorting process. For those unfamiliar with the term wane, it refers to a rough edge of a board, cut from the outside of a log.
Edging is sometimes carried out today as an integral part of the primary and secondary breakdown process. Many canter lines chip off the wane in the same machine that breaks down the logs into cants and boards. This follows a general industry trend to automate lumber manufacturing and reduce labor in the mill.
However, for the purpose of describing the basic process, we will assume that edging is carried out on a separate machine with its own operator. This is in fact the case in most mills around the country anyway. We will also assume that the machine is not equipped with a scanner-optimizer at this point.
The basic edger either saws or chips off the outer edges of rough sawn boards to square them and size them to the correct width. The advantage of a chipping edger — versus a saw edger — is that the feathery waste material on the outside of a board is converted directly to chips, which can be directed down to a belt conveyor. Saw edgers are more common, but the edgings they produce break easily and are something of a problem to convey to a central waste wood chipper.
A note about saw edgers in general: they are normally single arbor machines, either a top or bottom arbor with a pair of guided or unguided circular saws. Bottom arbor is usually preferred as an oversize piece cannot get wedged under the arbor, which can happen with a top arbor machine. Some kind of setworks is used to shift one or both saws.
The direction of saw rotation is another variable to consider when selecting a new edger. Climb cut or conventional cut? Climb cut machines rotate the blades in the same direction that the lumber is fed. Tests by Forintek Canada have demonstrated that climb cut edgers produce considerable power savings and far less sawing variation (80%) compared to conventional machines.
But — and it is a big but — as far as safety, climb cut machines can eject the lumber at high speed from the outfeed side of the machine, a potential cause of injury or worse to personnel nearby. As a result, conventional cut machines are much preferred, with the saws rotating against the direction of feed. This applies to both top and bottom arbor types.
After the board edges have been squared to the correct width ( 2 inches, 4 inches, 6 inches, 8 inches, etc.), the lumber needs to be trimmed to the correct length (6 feet, 8 feet, 10 feet, 16 feet, etc.). Some defects also can be cut out at this stage in the process.
This is another machine center where scanning and optimization has been applied in recent years, which has enabled higher piece counts to be achieved. We will leave this subject until later in the series and focus on the more generic types of trimmers.
Where high through-put is not an issue — but cost is — a ‘push-pull’ or Canadian trimmer can do a good job. This is a popular method used in Scandinavia, too, where all logs are fed into the mill oriented one way, either small end or large end first.
The Canadian trimmer comprises 2 circular saws mounted on either side of a lugged transfer deck. There may be one or two operators, and the boards are pushed or pulled to a series of marks ahead of the saws so that the protruding end is cut off as it passes transversely through one of the 2 saws. This is part of the grading process as well, as the operator stands or sits close to the saw, an ideal spot to see any defects. A realistic piece count with this type of trimmer is around 30 to 40 pieces per minute. For very small production numbers, a simpler lineal system can be used where the boards are conveyed end-first past the operator, who uses a cross-cut saw to trim the ends. A saw that swings up and across in an arc, with the pivot below the conveyor, is called a hula saw.
For higher piece counts, a multi-saw trimmer is used. Lumber is conveyed transversely by lug chains across a deck, and an even–ending device pushes them to a length stop; selected saws drop down from above the deck to trim the ends. In a dimension mill, the saws are spaced 2 feet apart and mounted on a bridge structure spanning the deck. Other mills cutting metric lengths have saws spaced accordingly.
Earlier multi-saw trimmers used a single line-shaft and one motor to drive all the saws, but most have since been upgraded to use a separate motor and drive for each saw. This has reduced the down time needed to replace blades and drive belts. Many of these machines are now equipped with scanners and computers as piece counts have continued to rise. Around 90 pieces a minute is now possible. Earlier manually controlled systems were slower and positioned the operator in a booth high above the deck so he could see the entire board passing below.
The latest scanner software can ‘see’ many of the common defects (more on
After the board has been edged and trimmed, the next task is to sort the pieces by grade, size and length. There are a number of ways to do this, principally by green chain, tray sorter or drop sorter.
The green chain is the oldest method used and is still very popular where labor is available at reasonable cost. This can be an arduous job if large sizes are cut at the mill (2x12 green hemlock, for example). In earlier times, the lumber was spread out on a table and sorted by hand. Later the table became a moving transfer deck with chains carrying the lumber past a number of workers who stand on both sides of the deck; each worker is assigned to pull off certain sizes and lengths and stack them on carts or skids.
Quality grading takes place at the same time. The number of grades varies from mill to mill. These typically include shop grade, clear, standard and better, and so on. Since the green chain is a manual and visual method of sorting lumber, grading can vary widely. Roving government mill inspectors endeavor to reduce grading variables by periodically checking production against American Lumber Standards (ALS).
As its name implies, with this method rough lumber is dropped into a series of slings or counter-balanced pockets. This is the preferred method of rough lumber sorting in high production applications although it is not the best for planed and finished lumber.
With a drop sort system, lumber is unscrambled and singulated on to transverse lug chains, where a sorting and grading decision is made. Sensing devices, such as photocells or a scanner, make the sorting decision for each piece as it passes by. Each piece is then picked up off the chains by an overhead J-bar conveyor, which carries it along the sorter and crops the board into the correct bin.
A drop sorter system may have 70 bins or more — some duplicated to provide storage capacity. As the bins fill, they are lowered; the lumber spills on to a deck at the base, and the boards are conveyed to a stacker.
Tray sorters are very popular in planer mills rather than sawmills. They handle the lumber more gently, and the boards are not dropped, so the planed edges and faces should not be damaged. In a tray sorting system, the lumber moves up an inclined deck and is directed to slide down skate rolls to trays, where each sort class is accumulated. When enough is collected to make up a package, the individual sort is transferred to a stacker for later packaging.
As mentioned above, lumber grading is part of the overall sorting process. Grading standards are maintained by periodic visits by ALS inspectors. The trend in softwood dimension mills now is to produce fewer different grades. Most construction lumber is sold in only one or two grade mixes rather than in several separate grades.
In a modern mill, one or more graders inspect each board as it passes by on a lugged chain deck. Many mills have two graders, and they inspect alternate pieces. They mark a grade decision on each piece with chalk or another marker. In many mills, the mark is ‘read’ by a scanner,
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