Popular Woodworking 2009-10 № 178, страница 38

aseball bats, utensils, turned items and small carved objects are usually made from one piece of wood. But most woodworking proj ects are made under the premise of "some assembly required." Face it: Most everything we make with wood involves one or more boards joined together. Without something to tie those boards together there is no gain in structural integrity.

There are several ways to secure boards together. The first is with a mechanical fastener such as nails, screws or bolts. Mechanical bonds are point-specific and once the fastener is removed the bond is lost. The second way is worked j oints, where interlocking or fitted wood piecesj oin together. Worked joints include dovetails, mortise-and-tenons, dowel joints and housed joints. The third way to j oin wood is to use some kind of glue. When properly used, most modern glues are stronger than the wood they are used on. A fourth way to join wood is to combine two or more of these methods.

Glue is the most often used means of j oin-ing wood and by far the most misunderstood. Cutting a dovetail is a learned skill. Gluing two boards together is much easier. But in the long run it's the glue between those dovetails that locks them together and it's the glue that will continue to hold them together throughout time.

If the glue fails you can bet that the j oint will fail. However, if the joint fails it might not mean that the glue has failed as well. I believe that understanding how to glue wood together is an inherited trait - we j ust know how to use it. Nobody ever te aches us how to put the stuff on or explains how it works.

And the two most common factors of gluing two pieces of wood together are almost never considered: the wood and the glue. Before you grab a bottle of glue for your next project ask yourself these simple questions. How should the wood be prepared? What glue is best for this specific task? What is

the correct procedure for applying glue, and how can I avoid gluing problems and correct them if they happen?

Prepare the Wood for Gluing

Any wood that is to be glued together should have the same moisture content and be at the same ambient air temperature. Good practice is to place both the wood and glue in the same environment for at least 24 hours before machining and gluing to allow both the wood and glue to acclimate. This will take place automatically if you store everything in the same relatively warm place.

The moisture content of the wood when glued should be the same moisture content that it will have once the glued pieces are put into service. The average moisture content for interior woodworking is around 8 percent,

Prepare your wood, pick the right glue and prevent problems.

BY MARc ADAMs

but that can vary a few points depending on where you live. Exterior wood averages between 12-18 percent moisture content. The higher the moisture content, the more challenges there are with curing.

How wood is machined before gluing is especially important. Mating wood surfaces should be smooth, flat, have parallel surfaces and be free from damage to make good contact. Machined wood, such as wood processed on aj ointer or planer, has a rough uneven surface when viewed under a microscope. When two pieces of machined wood are brought together the surfaces only touch

at certain places and can easily be pulled apart. However there have been times when I've handplaned the surface of two pieces of wood and set them on top of each other and had a difficult time separating them (this is called "specific adhesion").

Imagine the effectiveness of a glue joint if you have a perfectly cut surface of the mating parts. At one time I was told that you should roughen the surfaces of the mating parts, but this can damage the wood cells and add dust fibers to the pores and hinder the wettablilty of the glue. A smooth, knife-cut surface is best for bonding.

Glue manufactures recommend that you glue your wood together when it is freshly cut. A good rule is to try to cut and glue in the same day. The longer the wood has to set before being glued, the more opportunity there is for damage, contamination and moisture changes at the surface. Gluing wood sooner rather than later also minimizes the chance for any distortion in the wood. A clean and dry surface is essential.

It's a good idea to make a trial assembly. All joints and parts should fit together without excessive clamp pressure. If you have to force a joint together you will be gluing stresses into the project that may cause the j oints to fail. A test run will ensure that your clamps are open and ready and that you have thought through the gluing sequence.

It's a good idea to use small blocks of wood between the clamp and wood to protect finished and exposed surfaces and disperse the point pressure.

Selecting a Glue

More than likely you have in your shop a half-empty bottle of yellow glue with no cap on it. I guess the crusty glue around the sp out

serves a purpose alter all. Don't worry, this qualifies you as a typical woodworker.

However, I have to ask a lew questions: Is yellow glue really the best choice when it

GLUES : PART 2

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