Q: We make table tops and dresser tops among other items. What is the reasonable percentage of pieces of lumber that could or should be over 7.5 percent MC if we order lumber '6.0 percent to 7.5 percent MC'? Can we reject the load if there are 5 percent over the limit?
A: There are really two issues here. First, we need to examine the problems you might have if you use lumber that is over 7.5 percent MC. Because your product is a large, flat piece, we can expect to see a significant problem with wetter pieces. One wet piece might end up in four or five table tops. If one of the tops misbehaves, I don't have to tell you that the repair is costly. So, I suggest that you would be wise to measure the MC of the incoming lumber - every piece - using an in-line moisture meter. The cost of $10,000 or so will be paid back in short order, especially if you buy KD lumber instead of drying it yourself. (One plant here in Wisconsin went from 1,000 rejects in the winter down to 3 when they eliminated wet lumber with an in-line meter.)
The second point involves the purchase of lumber. Are there many kiln drying operations that can guarantee the MC of the lumber they provide to you? I have yet to see a supplier run an ad that will give you 1,000 BF of lumber free if they include any wet (over 7.5 percent MC) lumber with their order. Why won't they do this? Is it because the risk is too high? In other words, are they passing on the risk of wet lumber to you? Are they asking you to assume the risk instead? (I know one manufacturer of kiln dried lumber that does check its MC "with a fine tooth comb" because they found out it was financially beneficial for their customers to do so. I think more will be doing so in the next few years.)
If you do get an in-line meter, then you just return the "wet" pieces to your supplier and get a credit. They should be willing to do that, because I am sure that they told you that they don't have any wet lumber in the first place. So, what you are returning is lumber that is okay; you just measured the MC wrong. But maybe you should take this returned lumber and write my phone number on it - the next purchaser of that wood is going to need some help-big time!
In case you are into SPC (Statistical Process Control), you might argue that you could just sample the MC of incoming lumber. It is true that from 30 random measurements we can estimate the MC and the spread of MC quite accurately. But the in-line meter does more than that; it actually tells you which pieces you need to eliminate. Once the wets are gone, then your average MC will be lower and variation of MC less - this means higher quality!
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