Q: We have just begun processing a little cypress for moulding and find it to be an excellent wood. Our supplier insists that the lumber should be approximately 12 to 15 percent MC. Indeed, the wood seems to be fine when we run it. I hate to admit it, but I heard from another magazine that lumber (no species given) should be no higher than 7 percent MC. We are asking you to be the ultimate decision maker here.
A: Appreciate that my short comments here are generalizations and may not apply to all cases. However, they certainly apply to most cases.
Your supplier is giving you very good information. Cypress (and in fact, the following applies to most softwood lumber as well) is hard to machine without some serious splitting defects if it is much drier than 11 percent MC. I suggest a target of 10-12 percent MC, with very strict limits on 10 percent MC. Actually, a few MCs as high as 13.0 percent MC would be fine. However, I think that 15 percent MC is a little bit too high. I am sure that if you measured the MC of every piece of lumber (a practice that is worth the time and effort) that you will indeed have a few pieces at MCs under 12 percent. In other words, your supplier may indeed have a range of 10 to 15 percent MC, a little too big.
The production problem is how to dry the wettest pieces without over-drying the driest pieces. One solution is to equalize at 10 percent EMC in the kiln when the driest piece reaches 10 percent MC. (Perhaps 11 percent is a better equalization setting when the driest piece reaches 11 percent MC.)
Two key concepts: Do not try to overdry the wood and then bring it back up to a higher MC. If you dry to 7 percent MC and then come back to 12 percent MC, it machines like it was still 7 percent MC rather than like 12 percent MC. Do not go over 160 F in the main part of the schedule or you will produce "softer" wood when machining. Although it is really not softer, it just seems that way.
As probably everyone already knows and understands, when you machine wood at this higher MC level, there will be, if it is used indoors, some shrinkage as it dries to its in-use MC. This shrinkage (which is small in most softwoods, compared to oak and other dense hardwoods) must be accommodated in design and manufacturing.
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