Popular Woodworking 2005-12 № 152, страница 76

It has been said that we learn more from failure than from success. One of the first pieces of furniture I made was a coffee table with rails tenoned into the legs. I didn't know that the tenons were too large in proportion to the size of the legs until several years after the table was finished.

On the way to the bathroom one night, I accidently kicked one leg. The wood surrounding the tenon gave way, and the leg went flying. Up to that point, my work was successful; yet in an instant, a nice table was reduced to kindling. This was a joint-design lesson learned the hard way.

Much of what we know about woodworking is someone else's experience, or the embellished retelling of someone else's experience. When we find a method that works, we hang on to it and then we recommend it to others - never sure if it really is a good way or if we just got lucky.

If you want to start an argument, or at least a lively discussion among woodworkers, the relative strength of joints will almost always get one going. Like most

good arguments, this one can't be settled conclusively.

In order to add a little fuel to the fire, we decided to test 10 common joints by dropping an anvil on them. We used two sizes of anvils: 26 pounds and 54 lbs. All our joints were carefully prepared using poplar, and they were glued together with Titebond Xtend glue, which was allowed at least 24 hours to cure. The anvils were dropped from about 18" above the joint using the contraption shown at left.

We picked an extreme example not to crown a king of woodworking joints, but to examine how and why joints fail. After deliberately destroying these joints, we found ways to improve our joinery methods and produce better work.

Weaker Carcase Joints

The first joints we examined were simple butt joints, and some typical ways of reinforcing them. All of these broke beneath the 26 lb. anvil, a result we expected. What we didn't expect was how well the polyvinyl acetate glue held.

Most of us have been taught

We don't recommend dropping anvils on your furniture, but studying how and why joints fail can improve your woodworking.

that glue will not hold on end grain, and in the mitered j oint, the glue did indeed fail (below left). The grain in a miter joint is mainly short-grain to short-grain.

In the rabbet joint, however, the bond is long-grain to end-grain. Does the presence of long grain improve the strength of the joint? As you can see, the wood fibers failed beside the glue line

(below center). If you look closely at the photo, the end grain of both pieces isn't seen. Long grain from the adjacent piece is.

This was true with all of the joints we tested. From a simple butt joint to through-dovetail joints, the glue typically held to the long-grain wood fibers, even when the other component of the joint was end grain.

Long grain broke at end of tongue

Glue doesn't hold well to end-grain or short-grain surfaces, as in this miter joint. When the anvil hit, the glue line failed.

The presence of long grain in the joint increases its strength, even if the other com ponent is end grain. Here, the glue line held and the wood failed next to the joint.

Although the tongue added to the rabbet joint made it stronger mechanically, it left a weak point in the wood that broke on the joint line.

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