Gang Ripping Crooked Lumber:
How big is the problem?
by: Dr. Fred M. Lamb
Gang-ripping-first has steadily replaced the traditional cut-and-rip line as the primarily configuration for lumber cut-up operations. The reasons for this change are varied, but they usually relate to yield, cutting bill, production technology and throughput issues. (This will be a topic for a future article.) However, gang-ripping-first is somewhat more sensitive to lumber characteristics - especially those related to shape, defect types and defect distribution pattern.
The increased use of the lower grades of lumber (especially 2A and 3A Common) in today’s grade mix provides a real challenge to the gang-rip-first configuration. (I often comment that today we are making furniture and cabinets from wood that would have been in a mine prop or railroad tie 15 years ago.) These lower grades have a greater variability in lumber characteristics - especially those that are more detrimental to the gang-rip-first configuration. One of the more important of these lumber characteristics is crook or side bend.
Crook
Crook (the edge-wise deviation or distortion of lumber) is one of the four major categories of warp. (The other three are cup, bow and twist.) Crook is sometimes called side bend.
Crook usually occurs because one edge of the board shrinks longitudinally (that is, along the grain) more than the other edge. This causes the board to bend or "crook" in the direction of the side with the largest shrinkage. Since "normal" wood does not shrink much along the grain, crook is a resource problem of how the wood was formed in the tree, from where in the tree the wood was sawn and how these factors are impacted by poor processing practices.
Causes of Crook
Crook is the result of many factors, the most common of which are listed below:
- Tension wood: The reaction wood formed on the upper side of a leaning hardwood tree is called tension wood. One of the characteristics of reaction wood is an increase in longitudinal shrinkage. When the tension wood is located on one edge of the board, this increase in longitudinal shrinkage results in crook.
- Juvenile wood: The first-formed wood around the center of the tree (around the pith) is usually considered to be "abnormal" in many properties. A useful estimate is that the first five to 20 rings from the pith can be considered as juvenile wood. The actual amount of juvenile wood varies with many factors such as species and site, but this is a reasonable approximation. Juvenile wood also has the property of excessive longitudinal shrinkage. Therefore, if juvenile wood occurs along one edge of a board, the result will be crook.
- Sawing patterns: Sawing patterns that place the juvenile region of the log on one edge of the board contribute to the problem of crook. Figure 1 shows the ends of two boards with the pith area at or near one edge. Both of these boards would have the tendency to crook. By observing the end of a board and noting where the pith and juvenile wood areas are in relation to the edges of the board, you can estimate the potential of that board to crook.
- Spiral and diagonal grain: The grain angle (the alignment of the fibers) can deviate from straight grain, in relation to the long axis of the board due to spiral grain in the tree, or diagonal grain, resulting from poor sawing practices. In either case, a slope of grain in the board can cause an increase in warp, especially twist and crook.
- Defects: Large defects and the associated grain swirl and slope of grain also contribute significantly to the potential for crook. The lower grades of lumber have a greater risk of coming from the more defective center of the tree, and thus are more likely to contain reaction wood, juvenile wood, large defects and substantial grain deviations. Therefore, the lower grades of lumber are more likely to exhibit excessive crook.
- Stacking and sticking practices: Poor stacking and sticking practices alone do not cause crook, but they can aggravate the potential and thus make the crook more severe, whereas good practices will help minimize the severity of crook, but can not correct the problem.
- Drying practices: Too slow a drying rate at too high a humidity can aggravate all warp including crook. The damage is usually in the early stages of drying, from the first 1/3 to 1/2 moisture loss from green. Dry surface of drier lumber that has been re-wetted can also exhibit excessive warp including crook.
The Amount of Crook in Lumber
The U.S. Forest Service in Princeton, West Virginia, developed some preliminary data that offers an insight into the amount of crook in today’s lumber. Figure 2 shows the amount of crook in a sample of red oak lumber of all grades. Note that about eight percent of the boards had crook of one inch or greater. Figure 3 shows the amount of crook of one inch or greater by lumber grade. As expected, the lower the grade, the higher the proportion of boards with crook.
The Impact of Crook on Yield
What little data there is in the open literature strongly indicates that crook has a significant effect on yield in a gang-rip-first operation. For those operations with a good, effective salvage station and a typical No. 1 and 2A Common grade mix, an average estimate is about three to five percentage points loss in yield due to crook. If an operation has no salvage station or one that is operating poorly, then the losses due to crook can be as much as 10 percent or greater. An effective salvage station can help lessen the effects of crooked lumber. However, a significant amount of crooked lumber places an excessive load on the salvage operation. If it is not capable of handling the additional burden effectively, yield will suffer. The lower the grade mix and the narrower the boards, the greater the impact of crook on yield. Gang-rip-first operations cutting a very low-grade mix of lumber without salvage operations would have the greatest yield losses from crooked lumber. However, all gang-rip-first operations suffer some impact from crook.
What To Do
I generally suggest that operations seriously consider crosscutting all boards with one inch or greater crook before gang ripping. There are others who suggest that the limit should be set much lower, that is, all lumber with 1/2 inch or more of crook. This lower limit may be reasonable for those operations running twin lines (that is, both a gang-rip- first line and a standard cut-and-rip or crosscut-first line). However, for the operation with only a gang-rip-first line, this lower crook limit may not be a workable option to start out with. The amount of lumber involved and the space requirements may be too much. However, crosscutting all boards with one inch or more of crook is a workable option, and doing at least this much will have a significant impact on yield improvement. At the same time, it will not be such a large volume of material that it will be overwhelming, and it also keeps the space requirements to a manageable level.
Once this is accomplished, then the operation can decide for the future how it will handle crooked lumber - pre-sorting, twin lines, manual crosscutting, flying saw arrangements or other alternatives. One way or another, for gang-rip-first operations, crook must be addressed. The situation will not get any better in the near future. Crooked lumber is here to stay - for awhile at least.
Fred M. Lamb is Professor and Extension Specialist, Department of Wood Science and Forest Products, Brooks Forest Products Center, Virginia Tech Blacksburg, Virginia.
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