Date: 2008-09-10
By Gene Wengert
Q: We rip a long, narrow stile (about 72 x 3 in.) and then send it to the moulder for profiling. We test for proper MC and casehardening stress. However, more than 50 percent of the pieces have 3/16 in. lengthwise bow, just after molding or a day or two later. What are we are doing wrong?
A: Consider your stress tests. Transverse (or across the grain) casehardening stress is measured using a prong-test where the legs or prongs (cut about 6 in. long and one-quarter the thickness of the lumber) should be parallel with each other or nearly parallel. If there is transverse stress, the prongs will bend inward and lumber pieces will cup when machining.
Lengthwise warp is a different stress and will not show up using the prong test. Instead, I suggest that you cut a 24 in. long piece of wood (not near the end of the lumber) from a piece of lumber about 8 in. wide. Then rip this piece into two 4 in. wide pieces. Put the two ripped pieces back together and they should fit with any gaps being less than the thickness of a dollar bill.
Note that casehardening stress tests always require the lumber to have no moisture gradient, shell to core. Also, any casehardening stress will cause immediate warp when machining. That is the only manufacturing problem from casehardening stress.
Moisture content
In most locations in the United States and Canada, the in-use MC will be around 7.0 percent MC; maybe 1 percent MC lower in the wintertime and 1 percent MC higher in the summertime. To avoid warp, you need to avoid substantial moisture changes, so it's important that incoming lumber be at 6.8 to 7.0 percent MC. You may have to pay your kiln drying company for extra days drying to assure that they equalize the lumber properly. Special note: Always check the core MC with insulated needles to assure that it is 7.0 percent MC. As wood is a variable material, we would expect a small variation in MC from piece to piece. At least two-thirds of the pieces should be within 1 percent MC of the 7.0 percent target.
How do you test the MC? Use insulated needles and a name brand moisture meter. Pinless moisture meters are fine for scanning a lot of lumber, and are very useful if some over-drying is suspected. But any pieces that are out of range need to be double-checked with a pin meter.
Changes in size or shape that occur after machining are due to changes in moisture. That is, the moisture of the air is not equal to the moisture in the wood, so the wood changes moisture and swells or shrinks.
Plant humidity
If the lumber is dried correctly, then you need to keep it at this critical MC in storage and during manufacturing. This means the RH in storage and in your plant should run at 38 percent RH (+ or 4 percent RH). This is indeed a tough requirement, but your product (a 72-inch door) is very sensitive to MC changes.
Growth stress
Is the lumber straight and flat when you receive it? Some species, such as yellow poplar, are known to have stresses in the tree that causes the lumber to warp and cause large end splits. It will be difficult to have straight narrow pieces from growth stress lumber. If you do have warped lumber, the NHLA grading rules, if properly applied will typically lower the grade severely.
Straight grain
Wood will bow, as in your case, more if the grain is not perfectly parallel to the sides of the piece. That is, we need lumber with zero grain slope. When making narrow long products from wood, such as pool cues, drum sticks and even stiles, it would be best (maybe it is a requirement, because we know that wood will always experience small moisture changes) if the sawmill sawed the lumber parallel to the bark (also called full taper sawing) and did not use crooked logs. In the old days, sometimes that log would be split to show grain direction and then the wood would be sawn parallel to the splits. Straight grain material will have little or no risk of bowing when MC changes.
Gene Wengert, "The Wood Doctor," is presently an extension specialist emeritus at the University of Wisconsin-Madison. Fax your questions to him at 847.390.7100 or e-mail to wooddoc@uwalumni.com.
Updated: Sep 17, 2010
This article appeared in FDM, September 2008. ©Copyright 2013, All Rights Reserved.