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Destroying 10 joints taught us shocking classes about joint design, wooden failure and the tenacity of recent glue.
Editor’s word: This article initially appeared within the December 2005 concern of Popular Woodworking, when Robert W Lang determined he actually felt like smashing issues with an anvil.
It has been stated that we be taught extra from failure than from success. One of the primary items of furnishings I made was a espresso desk with rails tenoned into the legs. I didn’t know that the tenons had been too massive in proportion to the scale of the legs till a number of years after the desk was completed.
On the way in which to the toilet one night time, I accidently kicked one leg. The wooden surrounding the tenon gave manner, and the leg went flying. Up to that time, my work was profitable; but straight away, a pleasant desk was lowered to kindling. This was a joint-design lesson realized the laborious manner.
Much of what we find out about woodworking is another person’s expertise, or the embellished retelling of another person’s expertise. When we discover a technique that works, we dangle on to it after which we advocate it to others – by no means certain if it actually is an efficient manner or if we simply bought fortunate.
If you wish to begin an argument, or at the very least a energetic dialogue amongst woodworkers, the relative power of joints will nearly all the time get one going. Like most good arguments, this one can’t be settled conclusively.
In order so as to add a little bit gas to the fireplace, we determined to check 10 widespread joints by dropping an anvil on them. We used two sizes of anvils: 26 kilos and 54 lbs. All our joints had been fastidiously ready utilizing poplar, they usually had been glued along with Titebond Xtend glue, which was allowed at the very least 24 hours to remedy. The anvils had been dropped from about 18″ above the joint utilizing the contraption proven above.
We picked an excessive instance to not crown a king of woodworking joints, however to look at how and why joints fail. After intentionally destroying these joints, we discovered methods to enhance our joinery strategies and produce higher work.
Weaker Carcase Joints
The first joints we examined had been easy butt joints, and a few typical methods of reinforcing them. All of those broke beneath the 26 lb. anvil, a consequence we anticipated. What we didn’t anticipate was how effectively the polyvinyl acetate glue held.
Most of us have been taught that glue is not going to maintain on finish grain, and within the mitered joint, the glue did certainly fail. The grain in a miter joint is principally short-grain to short-grain.
In the rabbet joint, nevertheless, the bond is long-grain to end-grain. Does the presence of lengthy grain enhance the power of the joint? As you may see, the wooden fibers failed beside the glue line. If you look carefully on the photograph, the top grain of each items isn’t seen. Long grain from the adjoining piece is.
This was true with all the joints we examined. From a easy butt joint to through-dovetail joints, the glue usually held to the long-grain wooden fibers, even when the opposite element of the joint was finish grain.
In the housed rabbet joint the wooden broke neatly throughout the joint line, leaving a skinny layer of lengthy grain intact. The break continued throughout the quick tongue that held firmly to the top grain of the 2 adjoining surfaces. This joint was theoretically stronger mechanically, but it surely reveals our second essential discovery: The wooden will break on the weakest level inside or close to the joint.
This is essential when designing joints. The sizes of the parts of the joint are essential, however so is the quantity of wooden subsequent to the joint. The mortises and tenons on my espresso desk weren’t dangerous, however there wasn’t sufficient wooden left past them for the joint to remain collectively underneath stress.
The biscuit joint above held collectively the place the biscuit was in touch with the wooden, however the wooden itself broke out on the quick grain on the fringe of the biscuit. The weakest level wasn’t on the joint; it was simply past it.
Stronger Carcase Joints
As we anticipated, field joints and dovetail joints had been considerably stronger than the primary group of joints. They every survived the impression from the smaller anvil with minimal harm. With the bigger anvil, the dovetail at high gave manner however the two items didn’t fully separate (high). Notice that the glue held on to the end-grain surfaces, and that the wooden fractured on the quick grain of the tails. The joint opened up about midway, the place the tails and pins wedged in opposition to one another.
The parallel surfaces of the field joint above, with out the good thing about the dovetail’s wedging motion, unfolded like a hinge. Once once more, failure occurred from the wooden fibers on the glue line, not the glue itself.
The final box-type joint we examined was a miter, strengthened with cross-grain splines. This held in addition to the dovetail underneath the 26 lb. anvil, but it surely broke when the heavier anvil fell.
Stout Frame Joinery
We then turned our consideration to joints generally seen in tables and chairs versus packing containers and instances. In explicit: mortise-and-tenon joints, and a few of the strategies which were developed as options similar to dowels and pocket screws.
One of the large questions we had was how pocket screws would evaluate to mortise-and-tenon joints. In this check, the pocket screws did effectively underneath each anvils.
While this was spectacular, I’d be reluctant to make use of pocket screws as an alternative of mortises and tenons in all functions. As the standard joint fails, the tenon pulls away from the mortise step by step earlier than letting go fully.
With pocket screws, when the joint is harassed, the screw holds on to the wooden till the wooden breaks, leaving one thing that doubtless can’t be repaired. The long-term efficiency can also be a priority. In many antiques with screwed joints, annual shrinking and swelling crushes the wooden surrounding the screw, defeating the screw’s capacity to carry.
In testing our drawbored mortise-and-tenon joint, the 3⁄8“-thick tenon was nearly half the thickness of the 7⁄8“-thick items being joined. The quantity of wooden remaining subsequent to the mortise was skinny sufficient to be fractured on impression.
A bigger joint was then examined, with a 1⁄2“-thick tenon in 13⁄4“-thick materials. The tenon was pulled tightly along with a 1⁄4“-diameter oak dowel. The thicker parts, together with a tenon lower than one-third the thickness of the inventory, produced a joint of nice power.
A dowel joint of the same-size parts, nevertheless, didn’t maintain collectively. The dowels held when the grain within the dowel was parallel to the grain of the wooden (beneath proper). In the opposite course, the place the grain of the dowel was 90˚ to the grain of the wooden, the glue joint failed and the dowels popped out of their holes.
Seasonal wooden motion can also be a long-term concern with dowel joints. As the wooden and the dowel increase and contract, the dowel and the opening change form from spherical to oval. Over time, solely a tiny space of contact between the dowel and the wooden stays.
Designing for Time
Nearly any joint can work short-term, but when your sights are set increased than that, think about not solely how you’ll be a part of your parts, but in addition the sizes and scale of the parts.
In getting ready these joints for testing, I used sizes and proportions that I’d usually use. Most of those joints I’d nonetheless make the identical manner. If a biscuit or rabbet location was modified in location or proportion, the failure would doubtless nonetheless happen, however on the opposite aspect of the joint. But I don’t suppose I’ll ever once more make a mortise that’s greater than a 3rd of a chunk’s thickness.
Understanding what the wooden will doubtless do, and what to anticipate from a glue in any given scenario is a lesson effectively value going to a couple extremes to be taught, although you are able to do this with out an anvil.
When you strive a brand new technique, make some follow joints and see how effectively they maintain collectively. Sam Maloof as soon as tossed one in every of his early chairs from the roof of his storage simply to see how sturdy it was. He realized one thing about his joinery strategies that day. And that’s the identical lesson right here: Our successes emerge from the splinters of our failures.
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