Reply by JBrow

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Posted on Building shelves, need advice

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1368 posts in 1335 days

#1 posted 04-19-2017 03:57 PM


I am guessing that the explanation for the currently cupped panels is either moisture or a jointer fence not set perfectly square to the jointer table. Like jerryminer, I doubt that leaning the panels against a wall, assuming exposure to air on both faces, caused the cup. However, whenever I lean panels against a wall, I like to set the end of the panels on some scrap wood stickers to keep the panel off the concrete floor.

If the cupping is due to moisture, there would be more moisture on crowned face versus on the opposite face (concave face). Assuming both faces were exposed to air when leaning upright, I wonder whether the position of clamps kept in position for so long might be at least part of the explanation. If the clamping forces were not perfectly parallel to the panel faces, slightly more clamping force could have existed along one face than the other face. These unbalanced forces could have compressed the wood fibers a little more along the face where clamping pressure was slightly greater. This could in turn reduce the ability of the wood fibers to absorb and possibly even release moisture at the same rate as the opposite face. If this guess is correct, then removing the clamps when the glue was fully cured (about 24 hours) could have prevented the cupping. I also suppose that this is why people often recommend alternating the positioning of the clamps; one on top, the next on the bottom, then next clamp back over the top of the panel.

If the problem is due to a moisture imbalance, the concave side (the dryer side) of the panel could be wetted with water and left to soak into the wood. A dripping-wet clean rag placed on the concave surface for a few hours could be enough to flatten the panel though careful monitoring of the panel would be required. If too much moisture is introduced, the cup could reverse to the opposite side.

With some luck, enough moisture would penetrate into the drier wood fibers, causing the drier wood fibers to swell and in so doing, cause the panel to flatten. Once moisture levels on both faces are equalized, keeping the panels stickered, as shown by jerryminer, would help maintain the moisture balance.

As an aside, if the cup is due to moisture, I doubt that positioning the flattening cauls (or as you said splines) edge-wise rather that flat-wise, would have made much difference. Moisture induced forces in wood are significant; enough to split granite.

If the jointer fence is not set perfectly square to the jointer table, a panel could appear to have a cup when in fact it is a more like a segment of a polygon. This can be checked by placing a straight edge across the width the each of the planks on the crowned side. Planks that are themselves flat suggests a problem with the jointer set up; fence set out of 90 degrees to the table. If this is the case, then there are several method for flattening the “cupped” panel, none of which are very good solutions. The easiest as already suggested is fasten some rails that would act to flatten the panel. However, since the cup occurred despite flattening cauls while the panel glue-up cured, this rail-flattening method may not work very well.

To avoid problems with jointer set up, the adjoining edges of planks that will be glued together can be jointed so that upside face of one plank is held against the jointer fence. The second plank is jointed with downside face against the jointer fence. This produces complementary edge to face angles that sum to 180 degrees at the joint line and thus produce flat panels even if the jointer fence is not set at a perfect 90 degrees.

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