Q: Have you heard of this problem and do you have a solution? We have a nice multiple-drum-type sander. We have noticed that occasionally the sander will leave scratches on the finished surface, which we must repair. It appears that the last belt actually has the scratch in it. We use this machine for sanding solid lumber, composites before laminating fine veneer and for veneer with a composite or lumber core.

A: I will take an educated guess at what is happening.

First, I think that the scratches are caused by a very hard material scratching the belt and removing abrasive from a small location on the belt.

Second, wood does not have any really hard abrasive material in it, except for some tropical species. Therefore, the wood itself is not creating the scratches. Third, some composites, such as particleboard, can have a little bit of trash (like small rocks, dirt and so on) in them. This trash is called mineral and is often measured by burning a small sample and determining the amount of ash left. Clean wood is all carbon, oxygen and hydrogen, so there would technically be no ash after burning. So, ask your supplier for ash content measurements on your boards. Obviously, they would have to do this on each shipment. (And they usually do this test themselves for quality control; they may not give you the results unless you ask.)

Third, glue also can become very hard and scratch the belts. To avoid having the glue line scratch the belt in one spot, a glued panel is often fed at a slight angle to the belt, thereby distributing any wear over a larger section of the belt. Have you tried this? You will also want to make sure that the panels themselves are not always fed in the same position, but vary slightly in position from side to side, to avoid developing a wear pattern.

Note: I assume that you have high quality belts and do not try to clean old belts that are damaged already.

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