PROBLEM: Racking is hard to prevent when gluing up carcases and other large assemblies. Clamp jaws that reach beyond corner joints can cause sides to bow inward.
SOLUTION: Loosen clamps, then clamp across longest diagonal until diagonal measurements match. Avoid bowed sides by using blocks to focus clamping pressure over corner joints.
A dry or "starved" joint won't hold adequately. On the other hand, a slight amount of glue squeeze-out is good insurance, but anything more is both messy and wasteful.
Edge-to-edge joints are commonly used for producing broad, flat surfaces such as tabletops. When I'm arranging my stock, 1 don't bother with alternating the annular rings. Instead, I arrange boards for the best color and grain match, lidgc-to-cdgc joints between two long-grain boards arc stronger than the wood itself, so you don't need dowels or biscuits for strength.
To prevent the panel from bowing up or down under clamping pressure, 1 alternate the clamps under and over the workpiece. (See top photo, page 38.) vSpace the clamps so clamping pressure will be spread evenly over the entire glued area—a clamp every 6 to 12 in. is usually adequate, depending on the width of the boards you're gluing up. Narrow stock may require more clamps. You can use fewer clamps by inserting wide pieces of scrap between the work and the clamps to spread clamping pressure over a broader area. (Sec drawing.)
Remember that you need only enough clamping pressure to close the joint. Too much pressure can crush the fibers and cause internal stresses in the wood. After the glue cures, these stresses remain in the assembly and can eventually loosen joints. Also, too much force only serves to squeeze most of the glue out of the joint and weaken it. Once I've spread the glue, I start applying clamping pressure at the center and work toward each end. While clamping, I align the faces of the boards as I go, by lifting or pushing down on the boards at their ends.
In leg-and-rail construction you're usually dealing with offset surfaces, where a thin rail joins a thicker leg. To avoid twisting or racking the joint, it's important to apply pressure directly in line with the joint, centered on the width and the thickness of the rail. (See drawing.) After I clamp the joint, I use a straightedge to make sure that the inside surfaces of the legs are in the same plane. Then I check for square by measuring the diagonals on the outside corners. If the legs on your particular project don't have outside tapers, you can measure across the top and the bottom of the assembly. When the measurements are identical, you know the assembly is square.
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