Q. Can you please explain the role of grades in the yield of parts?
A. First, let me begin by stating that the following discussion applies mainly to hardwoods. Almost all hardwood lumber is graded using the rules established by the NHLA.
Although the rules have many requirements, as a simplification, the best grade of FAS requires that the lumber be at least 83 percent clear in one (or several), large, rectangular areas, called cuttings. This clearness is determined from the worst side of the lumber, so the other side is clearer. Appreciate that 83 percent is the minimum, so almost all pieces will have more clear area. Also, note that if you purchase this grade, the rules imply that the supplier or manufacturer has not pulled out certain sizes or certain high quality pieces, and are just selling you the lower quality pieces within the grade. In other words, you will be getting a wide range of sizes and quality, unless you have worked out a special arrangement.
The next major grade is No.1 Common (sometimes just called No.1 or called Common). This is also graded from the worst side. The minimum amount of clearness is 67 percent, but again, you should have a wide range of quality (more than 67 percent clear) and range of sizes within the grade, unless agreed upon differently.
Sometimes, when the better side of No.1 Common is very clear -- clear enough to grade FAS -- then this No.1 Common piece is graded as Select or FAS 1-Face. Keep in mind that the worst side is No.1 Common.
The next lower grade is No. 2 Common (which sometimes is called 2A when stain is a defect and 2B when stain is not counted; check the rules for each species). Again, the worst side is graded and this side must be 50 percent clear minimum.
The U.S. Forest Products Lab has developed the estimated yield of clear-2-face furniture and cabinet parts from various grades of lumber. In their work, they examined thousands of pieces of lumber. They made sure that they had a wide range of quality within the grades. They were not restrained by a cutting bill, so they began by cutting the largest pieces and then kept cutting smaller and smaller pieces from the remaining sections of the piece of lumber until the lumber was used. The technique they used was to diagram each piece of lumber and then feed the coordinates of each defect into the computer and then let the computer cut the lumber in various ways.
Their initial studies were for a rip-first mill. Subsequent studies have shown a couple percent increase with a crosscut-first mill. Nevertheless, their results are applicable today to virtually any mill with good employees and good equipment. There have been other yield data collections, including one from N.C. State University.
With today’s powerful computers, there have been many techniques developed to help a mill estimate their own specific yields for various grades, sizes, and processing configurations. Much of the work has been done by Urs Buehlmann and Earl Kline at Virginia Tech, and Janice Wiedenbeck with the U.S. Forest Service in Princeton, W.Va. Contact Ms. Widenbeck at [email protected] for the latest info.
With this background, let’s look at some effects of grade. Let me know if you have questions. You should come away with at least the understanding that yield alone does not reflect the cost.
Table 1. The yield when cutting only 30-inch long pieces. Oftentimes, No.1 Common lumber costs about two-thirds the price of FAS, yet the yield drop from 64 percent to 53 percent in less than 1/4.
Table 2. The yield of a particular cutting bill from different grades. Note that the drop in grade is not very large between grades, even though price differences are large, making the lower grade lumber, even with its high waste and lower yield, often more profitable overall. Again, notice the small yield drop between the various grades.
Table 3. In this actual cutting bill, four different grades or mixes were used to satisfy or fill the bill. Although the yield was higher with upper grade lumber, the higher price of FAs resulted in the highest overall cost (including lumber and processing costs) for the cuttings. The lowest cost was a combination of some upper grade lumber to get the larger clear pieces and then lower grade (No.1 and No.2 Common) lumber to get the shorter pieces needed. This optimum was calculated with a US Forest Service (Princeton office) free computer program.
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