Grading lumber with

A LTHOUGH slightly more expen.Cl,sive than visually graded wood. machine graded lumber is increasingly popular for trusses, glulams and other engineered applications needing greater confidence and predictability in meeting design requirements. And as milling equipment becomes more high-tech, the latest grading device is an X-ray machine.

The most common type of lumber grading machine is a Continuous Lumber Tester (CLT), which directly measures the wood's stiffness, typically to provide Machine Stress-Rated (MSR) grades. The X-ray Lumber Gauge (XLG), on the other hand, directly measures the wood's density, often providing Machine Evaluated Lumber (MEL) grades.

While the volume of MEL is still low compared to both MSR and visually graded lumber, volume and types of uses are both expected to grow. Since 1993, Weyerhaeuser Co. has added XLG scanners at

Story at a Glance

New X-ray device provides improved accuracy, speed in machine grading lumber for engineered wood products Weyerhaeuser, Champion International expanding production.

its mills in Dierks, Ar.; Barnesville, Ga., Plymouth and Greenville, N.C., and plans to add equipment at its Bruce, Ms., mill later this year. In December, Champion International Corp.'s forest products operations began producing MEL at its Whitehouse. Fl.. lumber mill.

The sawmills process logs into dimension lumber, which then pass through a non-contact XLG scanner positioned directly after the planer. The XLG scans the entire width and length of each board, predicts strength and stiffness, and develops individual density profiles, at a rate up to 2,000 feet per minute, across a broad range of temperatures and moisture contents.

Spikes in the density profile indicate the presence of knots and other deviations from straightgrained lumber. Algorithms then relate density to strength and stiffness, and the XLG labels the board with a grade indicator. After the boards are presorted into specific grades, a lumber grader inspects each piece to ensure compliance with visual grade requirements (such as for wane and knots) and a gradestamp is applied.

Daily off-line tests for bending, tension and modulus of elasticity are conducted to verify the stress ratings. The mills' quality control processes are monitored by a third-party grading agency.

Developed in rhe lare 1980s, the stress grades assigned to MEL provide a wide selection that can be tailored to specific engineering applications. Listed in the Design Values for Wood Construction National Design Specification Supplement, published by the American Forest & PaPer Association, MEL grades are designated by the letter "M," followed by a number.

Examples of commonly produced M-grades are M-14, M-19, M-23 and M-24. Unlike MSR grades, MEL's numerical designation does not correspond to allowable design properties. MEL grades apply to 2x4s and wider sizes and range from an M-5 to an M-

Common MEL Grades

in M-14 and M-21, 2x5s in M-19 and M-24, and 2x6s in M-14 and M-24 grades.

According to Duane Peterson, vice president of manufacturing for Champion's forest products operations: "The XLG gives us the capability to provide lumber that helps the truss manufacturer on two fronts-it gives them product consistency which benefits design performance, and it reduces the cull rate in the truss plant."

28. For example, when comPartng allowable design strength of typical visual grades of southern pine 2x6s, M-14 closely matches No. l, while M-24 closely matches Select Structural.

MEL grades, approved bY the American Lumber Standard Board of Review, were developed specifically for engineered applications, such as trusses and glulams, where increased consistency and reliability are required. ChamPion International produces southern pine 2x4s

Plant Shndards All mills producing MEL must develop and follow a detailed plant standard which outlines all quality control procedures' leatur' ing specitications needed to produce a high quality product, including prool loads, sampling rates, and regrading procedures.

Mlll Ceilification In order for any mill to stamp MEL, the static testing equipment,.grading machine, operators and MELgrades musl be certified by the grading agenry.

Daily Quality Control Soulhern Pine Inspection Bureau procedures call for a sample to be pulled and tested at least every four hours ot produc' tion. Champiin's Whitehouse, Fl., operations test every two hours'

Calibration The Xl-G's sensors are calibrated every tour hours to ensure it correclly measures density, knot size and knot placement. Static test equipment s calibrated daily, wcekly and,annually using aluminum lest bars, proving rings and a tension calibration link'

Since MEL is relatively new and offered by only two producers, it is available in fairly small volumes, mostly in the southeastern U.S. and produced of southern pine. While there are 24 different grades listed in the National Design Specification, just seven are being produced (see chart above).

Contact local suppliers regarding what species, grades and sizes are available in vour area.

HF uu I 1000Ft