Q: I'm looking for acceptable practices of testing the moisture content of KD lumber. At the beginning of 2007, we tightened our incoming lumber pass/fail parameters on moisture content. We no longer use average MC; we have read and learned through our own tests that high MC pieces, even when the average is good cause problems. So, our new standard today (we use it only in the heating season as we believe a little wetness in the summertime is OK) is to test eight samples from an incoming load of KD lumber; if any of the eight samples fall outside our acceptable parameter of 8.25 percent MC the load is rejected. As you might expect, our suppliers are concerned. On the other hand, rejects in the plant have dropped. Please give me your opinion on fair practical guidelines for testing incoming lumber moisture.
A: First, I do agree that it is more likely that the pieces high in MC will result in a loss of quality in manufacturing.
Second, if you can eliminate the pieces that are too wet, you will have an improved process and improved quality (less complaints) for the finished product.
Third, overly dry pieces also will present special problems, so to achieve "No Wet Pieces" you must be careful that your suppliers do not over-dry. This means you also need to check for overly dry pieces, perhaps using a non-pin type moisture meter.
Fourth, the moisture meter itself will put in some variation in the MC readings. In other words, if all the pieces were exactly 7.0 percent MC, you would not get 7.0 percent MC when checking the meter. You will find some variation and it is even possible that the average meter value will not be 7.0 percent MC either. The meter must be well maintained, working properly and operated properly. (Two examples: With a pinless moisture meter, you can get a high MC reading if the piece is slightly more dense than normal for the species, even though the MC is actually OK. With the pin meter, you can get an acceptable reading if the wood is cooler than room temperature.)
Fifth, though it seems that your new standard is a step in the right direction, but rejecting the entire load might be a little severe. I suggest that if you find a piece that exceeds your standard, you might want to check the same piece in a different location to confirm the high reading. If it is still too high, then take another eight samples. If the second eight are OK, then I would be tempted to accept the load. You could even take another eight samples to be sure.
Sixth, the overall technique you are addressing is called Acceptance Sampling, which is part of Statistical Process Control (SPC). There are some good books about SPC. My favorite is by Hy Pitt, "SPC for the Rest of Us". A community college also is likely to have classes to help you develop a rigorous sampling program so that you will not reject loads falling or accept loads that should be rejected. In other words, with a little effort, you can develop a program for MC quality that will give you and your suppliers' confidence in your assessments. I strongly urge you to get this additional information and training. Do not delay.
Seventh, the best way to control MC is to measure every piece of lumber using an in-line MC meter. Although the meter is expensive, the quality improvement in manufacturing and performance after manufacturing (for example, no open glue joints or warping) will pay for this capital cost. However, the in-line meter, as with all moisture meters, is not measuring MC but is measuring an electrical property of the wood. So, you need to understand this and be prepared to double-check any wet MC readings with a pin-type MC meter. If you cannot afford an in-line meter, then sampling is the next best technique, but the results or conclusions will be subject to a small level of uncertainty. The number of samples helps control this uncertainty.
Q: What is the purpose of the small grooves on the back side of flooring and of door jambs and similar items?
A: I have heard several guesses as to the correct answer, including that they are used for ventilation (they are too small for that), that they reduce the shipping weight (not really that much benefit), and that they are used to help bend the pieces flat across a bumpy surface (they are not deep enough to make the piece bendable). The truth is that they are an attempt to relieve any stresses (especially drying stresses, which are also called casehardening). The front face has wood removed, which would cause the piece to cup immediately, but the grooves relieve the stress, if any, on the back side so the piece stays flat.
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