Q. We made a table out of an 8 inch wide piece of lumber and I can guarantee that the top was flat when we shipped it, but now the customer has said that the individual pieces are cupped. Indeed, when I inspected it, he is correct. We were careful to make sure that all the pieces had the grain running the same direction (heart side down) to give a nice look. We did screw the top to the metal frame but the screws pulled out. What advice can you give?
A. First, it is important that you recognize that wood does not cup unless its moisture content changes. We might argue that the customer’s office is too dry or that the wood we used was a bit high in moisture. In either case, or a little of both, the wood pieces will cup slightly. So, you need to pay more attention to matching the MC when you make a top with the MC that it will have in service.
On an expensive table, measure the MC of every piece. After manufacturing, you should probably use a large plastic bag to keep the moisture from changing during storage and shipment.
Bottom line: With no or very little MC change, there will be no appreciable cup of the individual pieces. Note that a good finish will also slow any moisture changes which also reduces warping; slow and small moisture changes are “absorbed” by the wood without warp.
Another factor: By aligning the grain, you do achieve a striking grain pattern, but you also accumulate cup across the entire width. Therefore, with wide pieces, we do suggest alternating the pieces up and down.
Incidentally, it is worth noting that the amount of cup in an 8 inch wide piece is reduced to 1/4 (not just 1/2) if you were to rip the piece into two 4 inch wide pieces. This is why we see so many panels made with 2 inch or 3 inch wide strips.
Cupping forces are fairly large, so it is very difficult to restrain cupping with frames and screws.
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