A: Here is some information about flipping pieces in an edge-glued panel in order to control cupping.
First, wood changes size or shape only if the moisture content (MC) changes. If there is no or very little MC change, there will be no issue with cupping. Apparently, you do control your moisture well and that is likely why you do not worry about cupping of tabletops or other edge-glued pieces. Rx: Get the MC at the time of manufacturing as close as possible to the final MC in use so that there will be no important MC changes.
Second, the tendency to cup is almost zero for an individual piece of lumber or for an individual stave if the rings have little curvature (when viewed from the end of the piece; that is, the end grain). Any cup would be so small as to be ignored, even when the MC changes considerably.
However, as the rings exhibit more curvature (which incidentally means that the lumber was sawn close to the center of the log; closer means more curvature), the bark side will shrink or swell more than the other side when the MC changes; this difference, face to face, means that cup results or is likely when the MC changes. Rx: Avoid lumber or strips that have high ring curvature when flatness will be critical.
Note that lumber that was located close to the center of the tree where high curvature occurs is likely to be lower grade lumber (No. 2 Common hardwood grade or lower). Such lumber is more prone to being knotty, so is not likely to be included in a clear, narrow strip. Rx: Avoid lower lumber grades when edge-gluing large panels or critical pieces that must be flat. If a lot of your individual strips have strong ring curvature, then maybe you also want to figure out why your lumber has such ring patterns; perhaps someone else has taken the good pieces of the same grade with flatter rings, so you are getting inferior quality. I have seen this before.
A wide piece of lumber from lumber sawn from today's tree is likely to have the center section (width-wise) from near the center of the tree. That is, getting wide lumber with flat rings is just not likely from today’s trees and logs in the U.S. Hence, wider lumber is prone to cupping when the MC changes. Strips cut from the edge region of this piece of lumber will likely have flat rings and will not be cup prone. Rx: Be careful when cutting wide pieces of lumber so that high curvature pieces are pulled out of production.
(Special note: If a wide piece of lumber is ripped into strips and then the strips glued into a panel in the same order as they were in the lumber without flipping, cupping tendency of the original piece of lumber and of the glued-up panel will be the same. In this case, flipping would be prudent. As the narrow, flipped pieces cup when the MC changes, one will cup upward and the next downward, creating a roller coaster effect rather than a gentle cup of the entire panel.)
If several pieces of lumber are ripped and then the ripped pieces randomly assembled into a panel, overall cupping risk is small. Some pieces will have flat rings and some curved, but the chance of several curved pieces being together would be rare indeed. Individual pieces with ring curvature will possibly cup with MC change, but it will be small indeed. (Reminder: No moisture change means no cupping in any piece.)
Fifty years ago, wider pieces of lumber were more common and grain matching was also common, so the feeling that flipping was essential may have had some merit. That is, we did not mix quartersawn grain with flat-sawn. So, by matching grain of the individual strips (which some plants did) it was likely to have more strips in an occasional panel that had lots of ring curvature. Rx: Let’s not use old procedures on today’s raw material and with today’s processing unless we verify that the rule still applies.
The glossier the finish, the more noticeable any defects will be.
Most furniture wood, solid or glued, can expect 1/2 to 1 percent change in width seasonally due to a 3 percent MC change annually. There is no change in length. This is indeed small.
However, it is common to see the MC of purchased lumber a bit on the wet side. This means that the moisture change and resulting size change after the piece leaves your shop and goes to the customer can easily be 2 percent, especially in the winter. Rx: Get lumber no higher than 6.5 percent MC in the wintertime; maybe 7.5 percent MC summertime.
Special note: Humidifying a shop above the humidity level of the customer is not the best idea. Although such humidity will stop shrinkage in lumber that is a bit high in MC, it will not stop the shrinkage that occurs when the customer gets the item. Oftentimes the customer will have 30 percent relative humidity or lower in the wintertime which is 6.0 percent MC or lower.
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Q: We are having a problem with shrinkage. We make furniture, but someone else sells and delivers it. This person claims he did everything correctly, including opening the furniture wrapping (we wrapped the furniture with shrink-wrap and it was fairly well sealed) and letting it acclimate to the house climate. When the customer moved in, they said the furniture looked really wonderful, but within a week, it started to warp, open joints and crack in a few places. We are so careful to keep our plant at 40 percent RH and check the MC of the lumber. This is frustrating! Can you help?
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