|W&WP April 2003
Optimizing Operations on a Manual Crosscut Saw
By Phil Mitchell
Design of the Workstation
The crosscut saw operator should not have to expend a lot of effort in bringing full size lumber to the workstation. The operator's main job is to properly cut the lumber, which, by itself, can be very demanding. Preferably each piece of lumber is conveyed to the workstation by a material handling system or another worker. If the lumber is manually fed, the use of a scissors lift to raise the lumber package to the workstation level will facilitate the job by reducing material handling time and effort and reduce the risk of injury.
Board Inspection and First Cut
The first cut to square the end should remove end checks (if present). The end trim should be as small as possible, only 1 inch, and perhaps as small as 1/2-inch for well-dried stock containing only minor end checking. Using mirrors as described above, the operator should inspect the freshly cut end for drying checks and make an additional end trim cut if needed. However, the crosscut saw operator should not take an exceptionally long end trim in order to remove a single, long end check or split.
How much defect removal should the crosscutting saw perform? Certainly the crosscut saw should not try to remove all defects. Coordination with downstream operations will allow wane, pith, and many knots to be removed at the ripsaw or at the salvage saw operations. Defects running lengthwise, such as long splits, can be ripped out with less waste. Those defects that run across the width of the board should be considered for removal at the crosscut saw. A few rules of thumb coupled with examples may help you develop defecting guidelines at your crosscut saw.
One rule of thumb used by some rough mills is for the crosscut saw operator to remove defects that extend over 50 percent of the board width. Saddle wane extending across the full width of the board should be removed at the crosscut saw. Spike knots and multiple knot clusters, including the distorted grain around the knots, also should be removed at the crosscut saw. Distorted or sloping grain around knots may lead to torn grain or fuzzy grain in later machining.
Those operations with experienced crosscut saw operators and a strong yield orientation are able to use a more rigorous rule: if a strip of clear lumber is available between the defect and opposite side of the board that is at least equal in width to the narrowest "solid" width on the cutting bill, then the defect will not be removed at the cutoff-saw. Instead these defects will be left and removed later by the rip or salvage saws in order to recover a part.
If the crosscut saw operator determines that a defect should not be removed at his or her operation, the question becomes, "Into which length should the defect be placed?" If the defect is placed in the shortest length section, the crosscut sawyer eliminates most options for recovering a part at either the ripsaw or the salvage saw. It is often better for the crosscut operator to place the defect into an intermediate length, thereby providing the opportunity for the salvage saw to squeeze out a required shorter part.
Although it may seem to have a minor impact on yield, it is important for the operator to maximize the available clear wood by running the saw blade into the defect and removing only wood containing the defect. Doing this four times on an 8-foot board with a typical crosscut blade kerf of 1/4 inch can boost yield as much as 1%.
Cutting to Length
For each board, the most difficult-to-find sizes should be looked for first. Of course, flexibility must be maintained in the decision making process to prevent excessive yield losses within a board. For example, if taking the longest part results in excessive yield loss, it might be better to cut an intermediate and a short length.
Use a Backgauge
An example will illustrate the use of a backgauge. For an 11-foot board, the operator examines the board for defects to be removed at the crosscut saw and decides to first cut a clear 36-inch piece, designated as the red stop. The next choice to cut another 36-inch piece assumes that the defect will be removed at the ripsaw. Note the future ripping of the 36-inch piece containing the defect will yield a narrower 36-inch piece and later at the salvage operation either one 24-inch or two 15-inch pieces.
At this point, the operator has removed 6 feet from the original 11-foot board. Guided by the backgauge, the operator sees that the end of the board falls just beyond the mark that indicates the red and yellow combination (RY). The operator knows that a 36-inch (red) and a 24-inch (yellow) section can be cut with a small amount of end trim waste. Use of this system allows lumber to be defected strategically and end trim to be minimized, resulting in improved utilization of the wood.
Electronic backgauges with colored lights automatically calculate and display the location of backgauge marks. A manual system consisting of a channel bar with drilled holes, colored taper pins (golf tees, for example) to set the backgauge marks, used in conjunction with a spreadsheet program to calculate backgauge mark locations, also will work well.
Determining backgauge combinations must be done while balancing conflicting needs. The goal should be to minimize gaps between pegs on the backgauge. However, the operator must be careful when combining lengths to avoid cluttering the backgauge and complicating its use. When introducing the use of the backgauge, one approach to simplify its use is to use only two combinations of part lengths. Some combinations that include low-priority parts can be excluded even though their inclusion would more completely fill in the backgauge. The most needed lengths should be paired with each other first to make them appear more frequently on the backgauge.
Determining the Overage Allowance
One of the major causes of part loss is that parts are needed to test machine setups at the beginning of new runs. Using defective parts as set-up pieces can boost rough mill yield significantly. A second major cause of part loss is rejected parts where machining operations expose hidden defects, cause breakage or torn grain, or produce mismachined profiles, faces or edges.
More rejections occur when higher numbers of parts are needed and when more complex machining is required. End use also will affect the definition of what is defective, since hidden frame parts located on the inside of furniture can contain defects that cannot be tolerated in an exposed part that will receive a high gloss finish. Taking all these into consideration, overage allowance must consider set-up, quantity, complexity, and end-use.
Based on these factors, it is easy to see that using a fixed percentage to determine overage is wasteful. The "Yard, Kiln & Rough Machine Standard Practice Manual," prepared by Ross Associates for the Hardwood Dimension Manufacturers Assn. (now the Wood Component Manufacturers Assn.) recommended preliminary overage allowances grouped according to end use and complexity. Three different types of parts are identified: simple interior parts; simple exterior parts; and complex exterior parts.
* Simple interior parts are those in which five or less straightforward operations are performed on wood that is easily machined, and where minor defects may be tolerated. An example would be interior frame parts.
* Simple exterior parts also have only a few straightforward operations to be performed on wood that is easily machined. Minor defects are not tolerated, however, and defects are likely to occur in the machining operations. An example would be mouldings.
* Complex exterior parts are those in which a larger number of operations are to be performed and/or where part design results in fragility of the part or difficulty of machining, or where the species used is difficult to machine. An example would be a sash or a complicated crown moulding with a check-prone wood.
These groupings are used to calculate an initial estimate of the overage allowance. It is expected that manufacturers will adjust these values to fit their general production needs, and to fit specific situations that periodically arise.
The size of the overage allowance required is greatly impacted by the quantity of parts required by the production run. For small run sizes the overage allowance becomes a large percentage of the total production run. This illustrates the reason that many manufacturers are using computer technology to reduce the number of parts required for machine setups.
Bottlenecks may occur downstream from the crosscut saw such as at the straight-line ripsaws and the salvage saws. It may be tempting to control the bottleneck by having the crosscut saw operator cease production of certain parts until the bottleneck eases, but that approach will likely hurt yield. A better approach is for the rough mill foreman to smooth flow by adjusting cutting bills, lumber grade brought to the mill, lumber length and width of lumber brought to the mill. These adjustments will have a less drastic negative impact on yield.
This abridged article has discussed many of the details that can impact yield at the crosscut saw. Other considerations that should be examined can be found in the complete article available online at www.ces.ncsu.edu/nreos/wood/ and looking under "Rough Mill Operators Guide."
Phil Mitchell is associate professor and extension specialist, Department of Wood and Paper Science, North Carolina State University. He can be reached at [email protected]
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