Timber Grading Equipment

A-grader Introduction

[-- How A-grader works? --] [-- A-grader Tests Results --]

A new grading technology which determines the stiffness of timber using sound. Developed in conjunction with Forest Research of Rotorua, the A-grader can measure the Modulus of Elasticity on green rough-sawn, dry gauged timber, and long or short timber blocks.

This grading technology can be applied in the following applications:

 The ability to sort for stiffness prior to kiln drying saves on drying low stiffness lengths that are normally rejected later on in the sawmill.  Linking this green stiffness information back to log supply will provide very quick feedback as to the quality of the logs and expected grade recoveries.  This type of technology has been  proven on grading kiln-dried timber and can be fitted into existing timber lateral lug chain decks with relative ease.

 Many re-manufacturing companies buy finger-joint blocks to produce structural products such as studs.  The final studs then need to be graded for sale, and those that do not come up to scratch must be sold as a lower grade or used elsewhere.  A significant amount of time, energy and money is regularly spent on these rejected products. The logical approach is to grade the raw material first, culling out substandard blocks.  This dramatically decreases the reject studs at the end of the process.

 The A-grader can stiffness grade these blocks at fingerjoint production speeds so they can be sorted into stiffness groups corresponding to the final product stiffness requirements. 

 The A-grader's versatility cannot be matched.

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How does the A-grader work?

[-- A-grader Introduction --] [-- A-grader Tests Results --]

The A-grader is based on the principle of sonic resonance. Just as a bell rings when it is struck, so does almost any other physical thing with sufficient stiffness. This principle has been used for a long time to check the quality of products — like the crystal glass maker of old checking the quality of the crystal by listening to the ring that is made when a rod of the crystal glass is struck.

 Not all materials produce such a loud, clear ring as crystal might when struck, but as technology evolves so the applications to which we can put this principle grow. Fifty years ago the advent of cheap, goodquality, electronic amplifiers saw sonic resonance being regularly used to check the stiffness of concrete samples. What can be applied to concrete can be applied to timber, and in the last 20 years or so people have used sonic resonance to check the stiffness of wood in various forms, from tree stems right down to laboratory samples the size of matchsticks. The only obstacle to this has been sufficiently developed technology.

  The fundamental principle used in the A-grader is based on a sonic wave moving repeatedly from one end of the timber to the other. The sonic waves used in the A-grader are called compression waves because as they move along the timber they compress and expand the timber. This compression is very small; you can’t see it, but you may be able to feel it. It should be no surprise then that these sonic compression waves are affected by the stiffness of the timber. In fact, the speed of the wave is affected by the stiffness — as the stiffness increases the speed increases.

  Unfortunately, it’s not just the stiffness that affects the speed of these sonic waves, but also the density of the timber; as the density increases the speed decreases. This makes sense, because the heavier something is the harder it is to move around —heavier things move more slowly, you might say.

 Back to resonance; as these sonic waves bounce backwards and forwards along the timber some of the waves almost exactly overlap. These overlapping waves build up and become bigger, while the sonic waves that don’t overlap tend to cancel each other out. The sonic waves that build up on each other are resonating. It’s very much like being on a playground swing — if you push at the right time when the swing is swinging you make the swing move out more, if you push at the wrong time you make it stop.

 By looking at which sonic waves become large compared to the other waves, we can tell how often the sonic waves are bouncing backwards and forwards along the timber. Then by knowing the length of the timber, we can determine the speed of the waves. Now that we know the speed of the sonic waves, we then use the density of the timber (determined from its weight and dimensions) to calculate the stiffness. The A-grader measures both the density of the timber and speed of the sonic waves in the timber to produce a stiffness value for the timber.

 There are two technologically demanding aspects to measuring the speed of the sonic waves in timber on a chain: enerating and measuring the vibrations of the timber when it is moving on a chain, and quickly working out which sonic waves are resonating. The former requires non-contact devices or devices that track with the timber, and the latter requires cunning signal-processing algorithms and significant computer processing power.

This article was taken from Forest Research's Sawmilling Newsletter Issue 35 May 2004.  Thank you to Grant Emms and Forest Research for allowing us to use this article. 

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A-grader Test Results

[-- A-grader Introduction --] [-- How A-grader works? --]

 Please note below the results of a trial where 120 pieces of 100x50 were graded by the A-grader, then planer gauged and tested for Quality Assurance bending stiffness in accordance with AS/NZS4063:1992.

 Figure 1 plots these results along with some laboratory data. There appears to be no significant difference between the two sets of data indicating the A-grader is working extremely well.

  Figure 1: A-grader verses Quality Assurance Bending stiffness.

 

 The distribution of bending stiffness by grade is shown in Figure 2. In this Figure you can note the grades are well defined and distinctly different.

Figure 2: Cumulative Frequency Distributions for the Graded Timber

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