The Anvil Test

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

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

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 accidentally 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.

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.

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.

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 above.

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 that glue will not hold on end grain, and in the mitered joint, the glue did indeed fail. 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. 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.

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

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

In the housed rabbet joint the wood broke neatly across the joint line, leaving a thin layer of long grain intact. The break continued across the short tongue that held firmly to the end grain of the two adjacent surfaces. This joint was theoretically stronger mechanically, but it reveals our second important discovery: The wood will break at the weakest point within or near the joint.

This is important when designing joints. The sizes of the components of the joint are important, but so is the amount of wood next to the joint. The mortises and tenons on my coffee table weren’t bad, but there wasn’t enough wood left beyond them for the joint to stay together under stress.

The biscuit joint above held together where the biscuit was in contact with the wood, but the wood itself broke out at the short grain at the edge of the biscuit. The weakest point wasn’t at the joint; it was just beyond it.

Stronger Carcase Joints
As we expected, box joints and dovetail joints were significantly stronger than the first group of joints. They each survived the impact from the smaller anvil with minimal damage. With the larger anvil, the dovetail at top gave way but the two pieces did not completely separate. Notice that the glue held on to the end-grain surfaces, and that the wood fractured on the short grain of the tails. The joint opened up about halfway, where the tails and pins wedged against each other.

The parallel surfaces of the box joint above, without the benefit of the dovetail’s wedging action, unfolded like a hinge. Once again, failure occurred from the wood fibers at the glue line, not the glue itself.

One thought on “The Anvil Test

  1. keithm

    Can’t we just all get along?

    There are a number of factors in “joint strength,” some of which you mention and some you don’t.

    I am always skeptical of the vendor that says their dowel joints are stronger than M&T. Of course, they are testing “young” joints without several seasons of cyclical humidity.

    There’s also the factor of “soft fail,” making a repairable joint and “hard fail” making a catastrophic failure that is not repairable. Is it better to have dowel joints that are easily repaired every 10-15 years, or pocket hole screws that last 25 years, then are not repairable? Will you even want the piece in 25 years, or not?

    Stress is another concern. The anvil is a concentrated short duration stress. Like dropping a table off the back of a truck onto its legs or kicking a leg in the middle of the night.. What about dragging a table across a carpeted floor or Fat Uncle Joe leaning back on the rear legs of a dining chair every holiday after desserts?

    There are also different types of stress
    – Shear strength — parallel the the glue line. This is the most quoted ATSM test.
    – Cleavage — perpendicular to the glue line, spreading it apart
    – Peel – ditto but with one adherand bending, like a veneer
    – Tension – pulling a joint apart in a direction perpendicular to the glue line
    – Racking – the type of test demonstrated by your anvil test

    Not all glues or joints have equal strengths in these various types of stress.

    Fit of the parts also pays a factor in how well a glue works, thus joint strength. Some fill gaps well, some don’t.

    And of course, there’s a large majority of people that think, “Biscuits are for alignment only — they don’t add strength.” I don’t happen to believe that both from _real_ research I’ve read and anecdotal experience from Jim Stack before and while he wrote his book on the subject.

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