Log specifications, grading and pre-processing

Log specifications, grading and pre-processing

A successful veneer operation depends on three main criteria:
◆ a supply of suitable logs;
◆ the use of suitable processing and manufacturing techniques;
◆ an effective sales and marketing program

In order to sell veneer logs, a transferable and comparative grading system is needed. This gives buyers confidence that the veneer logs meet their manufacturing and quality requirements. The purpose of grading veneer logs is to sort material into groups that match the best utilisation and price/value category. This price/value relationship is set not only by the aesthetics and physical properties of the wood, but also by the demand for particular qualities and species. Most veneer peeling processors buy logs based on log volume, log quality and species.

Factors affecting the veneer quality of peeler logs can be attributed mostly to species, genetics, silviculture, growing conditions and tree health. They include age, growth rate (slow and uniform generally equates to more desirable quality),
stem form, knots and limb-related defects (size and location), decay and insect damage. It is important that there is an exchange of information between managers of veneer production facilities and forest managers to provide an understanding of forest management practices that impact on the yield and quality of peeler logs.

The three main criteria used to assign a value to a hardwood peeler log destined for veneer production are species, available volume and grade. Some species are more prized than others due to their rarity, ultimate suitability for a particular  end use, or reputation in the market. Available volume considers not only the volume of recoverable veneer from a log but also the scale and long-term supply potential of suitable logs from the forest. Grade refers to a combination of log attributes (beneficial or non-beneficial) that will determine a log’s suitability for veneer production.

Log grading systems vary internationally although the principles are essentially
the same. The process aims to:
◆ identify grade limiting defects;
◆ measure and/or assess those defects;
◆ determine other important aspects such as size and species requirements;
◆ assign a grade to the log.
Grading a log destined for veneer production entails evaluating the log quality and hence the quality of the veneer that can be expected from that log. Log grading is based on a visual assessment of specific log features that are set out for
each grade classification. Ideally, the grading rules should be easily understood so they can be applied quickly and accurately. However, it should be noted that different interpretation of the rules can lead to small differences of opinion and sometimes written rules are not adequate to describe all scenarios. Then it is for the experienced   grader to use their discretion.

Factors to consider when grading a peeler log are:
◆ the dimensions of the log (length and diameter);
◆ the form of the log (the degree to which the log deviates from a true cylinder)
including sweep, taper and ovality;
◆ the presence of defects in the log (external and internal)

Dimension and form
Some definitions relevant to the dimension and form of peeler logs are as follows:
◆ diameter—the small end diameter with the bark removed;
◆ length—the nominal length specified plus a trim allowance;
◆ volume—the calculation of log volume;
◆ sweep (or bend)—a lengthwise curvature of a log (a trend away from the true cylindrical form), the extent of which may cause a peeler log to be discarded;

taper—loss of diameter along the length of the log resulting in an elliptically shaped log.
Sweep is determined by measuring the maximum distance (a) along the curved side of a log when a line is extended between the log ends (of length L). Log sweep (S) can be calculated in various ways such as the ratio between the two
measurements, expressed as a percentage using:S (%) = × 100 a/ L
Any trend away from a true cylinder is considered a general sweep defect. Sweep  for grading purposes can encompass taper as well (Figures 3.1 and 3.2)

Special thanks to the original source of https://www.aciar.gov.au/publication/guide-manufacturing-rotary-veneer-and-products-small-logs

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