A hardwood Log Grading Handbook :
Veneer, Sawlogs and Other Log Classes Some hardwood logs are reserved for the production of veneer – thin sheets of wood that are peeled or sliced directly from logs. These logs are usually very high quality, with few if any visible defects. Veneer logs may also be judged on color, growth rate and amount of sapwood versus heartwood. Veneer logs are higher value than sawlogs. More information on veneer, and the grading of veneer logs, can be found in the publication Factors Affecting the Quality of Hardwood Timber and Logs for Face Veneer.1 Sawlogs are those that are sawn into hardwood lumber and the grading of sawlogs is the subject of this handbook. These logs are also called “factory” logs. Logs that are not of sufficient quality to be a sawlog may be used for cutting pallet stock or railroad crossties, where appearance is not important, or for pulpwood. This handbook deals only with hardwoods. Softwoods such as pine and spruce are gen- 1Cassens, D.L. 2004. Factors Affecting the Quality of Hardwood Timber and Logs for Face Veneer. Purdue University publication FNR239. Available for download from http://www.ces.purdue.edu/extmedia/ FNR/FNR-239.pdf 2 erally used for making structural products, for example the studs used for framing a house. Softwoods are graded using different rules that focus on strength-reducing defects. Log Scaling Scaling is used to predict the amount of lumber that will be sawn from a log. The amount of lumber is measured in board feet, where one board foot is 1” x 12” x 12”, or any dimension with the same volume of wood. For example, a board 2” x 6” x 8’ contains 8 board feet. There are different log rules that are used for scaling in different regions and for different products. The three most common log scales are the Doyle rule, the Scribner rule and the International ¼” Rule. By tradition, the Doyle Scale is the most commonly used scaling rule used in Tennessee (Table 1.)2 The Doyle Rule often underestimates the yield of lumber for small logs and for modern, more efficient sawmills (e.g. band mills). This underestimating can lead to “overrun”, when the actual yield is more than the scaling predicted. Figure 1 shows a comparison of the Doyle scale to the International ¼” Rule, which more accurately predicts the true yield. Log prices are normally different depending on the rule used, reflecting the differences in the scale resulting from each rule. Thus, the 2For more information on scaling, and conversions among the log rules, see Understanding Log Scales and Log Rules by Brian Bond. University of Tennessee Extension PB1650. Download available at www. utextension.utk.edu/publications/pbfiles/Pb1650.pdf
average log value will be the same regardless of the scaling rule that is used. Special rules for certain species or products are used in some locations. For example, some buyers use a “cedar rule” for predicting the lumber yield from eastern redcedar logs, which are small and irregularly shaped. Species is another important factor in determining log value. For example, top grade walnut, cherry or hard maple logs can be worth two- or three-times as much as hickory, soft maple or poplar logs of the same size and quality. This is because the products made from those species are worth more and there are fewer top quality logs of the high-value species available. Species differences may Figure 1. The predicted lumber yield using the Doyle scale compared to the International ¼” rule. The International Rule is fairly accurate for all log diameters, while the Doyle Rule underestimates the true yield of lumber for smaller logs. The actual yield of lumber from a scaled log will vary with mill technology, wood species and other factors. Log Diameter (inches) 10 15 20 25 30 405060708090 100 International 1/4″ Rule Overrun Predicted Lumber Yield (percent) Doyle Scale be stated explicitly in some log grading rules. Other log grading rules grade all species of logs in the same way, with buyers paying different prices for logs of the same grade but of different species. There are regional differences in pricing of logs and lumber products, because the wood quality of certain species is thought to be superior in some areas. For example, cherry logs from the northeastern United States are believed to contain higher-quality lumber and thus higher prices are paid for cherry logs from that area. There are also local variations in log prices. For example, if there are many sawmills within a reasonable hauling range of a timber harvest, this can increase the demand – and thus the price paid – for logs. Finally, prices paid for logs can vary substantially with fluctuations in demand due to seasonal changes and overall economic trends.3如无特殊说明，文章均为本站原创，转载请注明出处。If there are no special instructions, the articles are original, if you want to use or reproduce, please indicate the original source www.plywoodinspection.com,If you find that our articles infringe on your copyrights and interests, please email us at email@example.com in time and we will delete it at the first time.