Popular Woodworking 2004-11 № 144, страница 41

Better Glue Joints

Your joints will last for decades if you know how to apply your glue.

Much of woodworking is joinery: An edge-to-edge joint is used to join two or more boards to create a tabletop, dovetails are carefully cut and fit to create a box for a chest of drawers. And the corners of a door frame are joined with a mortise-and-tenon joint.

However, whether it's a simple butt j oint or a complex interlocking joint, glue is typically used to hold everything together. And if you've ever been asked to repair a piece of cheap, factory-made furniture (it's often called curb furniture - it's used for a few years and then set out on the curb) you'll see that it's typically the joint that has failed rather than the wood (assuming real wood was used).

Yet modern glue is strong stuff. In fact, try this experiment: Glue two inexpensive poplar boards together edge-to-edge with ordinary yellow glue and allow the glue to dry overnight. The next day break the boards apart and you'll find that the wood will break instead of the joint. You see, when well-crafted, even a simple edge-to-edge joint is stronger than the surrounding wood.

Interlocking joints, such as dovetails, and the mortise and tenon, are incredibly strong even without glue. Add glue during the assembly and these joints can last for decades, even centuries.

So as woodworkers, how do we make certain that the joints that we carefully construct and glue together will not loosen and fall apart in just a few short years? Let's take a look at what makes a good glue joint.

Mating Surfaces Must Touch

Yellow glue won't add strength in a gap; the mating halves of a joint must make contact. Before gluing and clamping two boards to make a tabletop, I align them to see if the edges make contact. If the edges are convex (they touch in the middle and are open on the

ends), a weak bond will result. Although you can squeeze the joint shut with clamps while the glue dries, the joint will always be in tension and will likely pull apart within a short period of time.

One theory is that an edge-to-edge joint should be sprung. In other words, the edges should be slightly concave. The idea is that as the tabletop experiences normal changes in relative humidity, the ends will shrink faster. (Remember, end grain absorbs and releases moisture at a faster rate than the other surfaces in a board.) A "sprung" j oint will apply more pressure at the ends and keep it tightly shut.

My experience has been that a sprung joint isn't necessary. As long as the edges make contact, the glue bond will be strong. This

holds true for other types ofjoints, too. The mating parts of an interlocking joint should be in close contact. In fact, dovetails and a mortise and tenon should have a "friction" fit. When expertly crafted, they should assemble with moderate hand pressure or light blows of a mallet. To test the fit, I assemble the joint without glue to see if it holds together.

If you're using a lot of clamp pressure to close a joint, there's probably something wrong with the joint. Clamps should only be used to hold a joint in position until the glue sets; they shouldn't be used to close a poorly crafted joint. In fact, when assembling dovetailed drawers and casework, I typically don't use clamps. I just apply glue to the mating surfaces, tap the j oints together with a mallet and set the assembly aside until the glue dries.

Grain Plays a Role

Glue bonds best to long grain; end grain will bond but the j oint will be very weak. Simply put, you can join two boards edge-to-edge but not end-to-end. If you examine the end grain of a board under

by Lonnie Bird

Lonnie is the author of "The Complete Illustrated Guide to Shaping Wood" (The Taunton Press) and teaches woodworking. You can learn more about his classes online at lonniebird.com.

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