Slippery kiln-dried wood

What could make dried wood so slippery that it is difficult to machine? August 12, 2001

Question
We dry red and white oak to 8 - 10% MC for laminated trailer floors. The plant tells us our lumber is slippery to handle with the forklifts and also hard to feed into the AEM abrasive planer because the drive wheels spin on it. They say lumber from other suppliers is fine. Why could this be?

Forum Responses
This can happen with lumber that is actually over-dried. So, how do you measure the MC? If you use a pin meter, you can get readings of 6.5% MC and yet the true MC can be as low as 4% MC. Also, after drying, do you equalize and condition? It is the surface MC that is critical when machining lumber, not the average or core MC. Please post any additional information or ideas based on what I have said here.

Gene Wengert, forum technical advisor

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From the original questioner:
We use the weight method with sample boards and most of the time we double-check it with a surface meter. We air-dry our lumber and endcoat it as well. Quality control at the plant also uses a surface meter. We equalize if the moisture content spread is too much and always condition. Two-thirds (200,000') of our kiln capacity is in new American kilns so we can hold them where we want them. We will check out our MC again.

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What is your final setting in the kiln? Try one load at 160 F and 115 F (DB and WB) or maybe even 160/117.

Are your samples including several pieces that represent the driest lumber in the load?

As a general statement, never skip equalization, even if the sample boards are within the range desired (no more than 2% MC apart, assuming you have both the driest and wettest), as the surface is drier and the ends are drier.

Drier lumber is much stronger than lumber over 7% MC. As a result, drier lumber machines with more difficulty and requires more energy to feed.

Gene Wengert, forum technical advisor

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From the original questioner:
We are doing shell and core testing. I just heard from the plant it isn't as slippery on rainy days.

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This sure points to over-dried lumber--check the operator to make sure that he/she is looking for the driest as well as the wettest.

Gene Wengert, forum technical advisor