Popular Woodworking 2004-11 № 144, страница 42

Pictured is a dovetail joint (top left), a mortise-and-tenon joint (bottom left), a mortised-and-tenoned cope-and-stick joint (above) and a long-grain-to-long-grain edge joint (right). All four of these joints provide plenty of long-grain surfaces for gluing. Look closely at the callouts to see where you should apply glue to each joint.

magnification (use a jeweler's loupe or a photographer's slide loupe) you'll see that the wood resembles a handful of plastic drinking straws. Those straws served as the tree's plumbing to transport sap. Sliced lengthwise the straws will create a strong glue bond. But when sliced and joined at the ends, a weak bond is the result. The solution is to use construction that joins long-grain. Both the mortise and tenon and dovetail joints do just that, which is another reason that they're still used for the finest furniture.

In contrast, dowel joinery is weak because there is very little surface area for glue and most of it is end grain.

Cope-and-stick joinery created by matching router bits is another example of weak j oinery. Although cope-and-stick joints make long-grain contact, there is very little of it. When a heavy wood or glass panel is added and the door framework is suspended from a pair of hinges, the joints undergo a lot of stress. A better so

lution is to use a mortise and tenon at each corner of the frame. The length of the tenon should be two-thirds to three-fourths the width of the stile to which it's joined. This provides plenty of long-grain surfaces for gluing.

Too Much Surface Area?

Remember, joinery is often used to change directions such as when joining the sides of a box at 90°. This can often introduce cross-grain construction problems. When wood is joined and glued cross-grain, there is potential for one of the pieces to split. (Wood expands and contracts across its width during seasonal changes in relative humidity.) The solution is to reduce the surface area and/or to avoid glue application to certain areas.

For example, broad surfaces such as a tabletop or hinged lid of a slant-front desk are kept flat with breadboard ends. Tenons are cut on the tabletop, which are fit into mortises on the breadboard ends. But while this solves one problem,

it creates a new one - cross-grain construction. The solution is to use an odd number of tenons and apply glue only to the center tenon. The remaining tenons are assembled without glue and held in place with a wooden pin. The hole in the tenon to accept the pin is slotted, which allows for expansion and contraction.

Another example of a potential cross-grain construction problem are tabletops. Tabletops shouldn't be glued to the base that

supports them. The glue bond will hold tight and cause the tabletop to split. Instead, a mechanical fastening system that allows for wood movement should be used.

As a general rule, I put glue on any long-grain surface of a joint. For example, when gluing up a mortise-and-tenon joint, I put glue on every part of the tenon and mortise that is long grain. I don't put glue on the end-grain walls of the mortise. I also don't put glue on the bottom of the mortise. See

Pinned mortise-and-tenon joint

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Popular Woodworking November 2004