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In China, 2005 was the year of the rooster. In our shop, 2005 was the year of the anvil. We built a guillotine out of framing material and dropped anvils of three weights on joints to see how they fail.
We learned a few things. First: You can get paid for doing juvenile stuff with anvils. Second: Modern PVA glues (yellow glue) are a lot stronger in end grain applications than woodworking wisdom suggests. And third: How you size the parts of your joint (thickness, width and length) has a lot to do with how sturdy it ultimately is.
Nowhere was this more evident than with the venerable mortise-and-tenon joint. Changing the thickness of a part of the joint, such as the mortise wall, could greatly weaken or strengthen the joint under the crush of the anvil.
There are some well-worn rules about how to scale a mortise-and-tenon joint, and they are worth thinking about the next time you lay out a tenon. Let’s take a look:
Tenon thickness: This one gets debated a lot, and with good reason. Traditional texts say the tenon’s thickness should be one-third the thickness of the stock being mortised (an important distinction). So if you are joining two pieces of 3/4″material for a door, the tenon should be Ã?¼” thick. If you are joining a 7/8″-thick apron to a 1-1/2″-thick table leg, the tenon should be 1/2″ thick.
Some modern texts say the tenon should be one-half the thickness being mortised , not one-third. My opinion is that this difference relates to the tools being used. If you mortise by hand, with chisels, the one-third rules makes more sense in my experience. Using a 3/8″-wide mortise chisel on 3/4″-thick material invites destruction in many cabinet woods.
But if you’ve ever used a hollow-chisel mortiser, then you’ve probably been amazed at the difference in performance between the 1/4″ chisels and the 3/8″ chisels. The 1/4″ chisel gets clogged up much more easily because its escapement is much small. Plus, the hollow-chisel mortiser doesn’t put the kind of lateral strain on your work that hand-mortising does. So a 3/8″-wide mortise works with machines.
Tenon length: The general rule is that the minimum tenon length is five times its thickness. So a 1/4″-thick tenon should be 1-1/4″ long. Of course, if you look at antique furniture, you see this “rule” violated , or maybe the furniture was made before they made the rule. Longer through-tenons are the rule of the day in much 19th and 18th century work. These are wedged tenons, generally. Check out George Ellis’s “Modern Practical Joinery” for a trip through the land of the through-tenon. Personally, I try to follow the “five times the thickness” rule for most cabinetwork. But when I’m building something that will encounter more wracking forces (such as a dining table), I go long.
Tenon width: This one is more complex. The rule in Ellis’s book is two-fold. First, make the tenon one-half the width of the rail you’re cutting it on (a 2″-wide rail would get a 1″-wide tenon). Second: If that tenon’s width would be greater than six times its thickness, then you should split it into two (or more tenons). Example: You want to cut a 1/4″-thick tenon on a 6″-wide rail. Ellis’s rule says that your tenon should be 3″ wide. But a 3″-wide tenon is greater than 1-1/2″, which is six times the tenon thickness. So you have to break that tenon into two 1-1/2″-wide tenons.
Is your head swimming yet?
This rule seemed odd to me at first. The tenons it made seemed too narrow in width, which would allow the corners of your to frame warp (or cast) over time. But when you look at Ellis’s illustrations, it makes sense. He shows all his tenons with a short haunch that runs the entire width of the work. Ah!
And what about double mortises, such as when you join a narrow drawer rail to a leg in a chest of drawers? This drawer rail is usually somewhat squarish and stout, and it doesn’t follow the rules laid out above , you don’t need a double tenon.
Some sources seem to suggest that the double tenon can be made for convenience. You might not have a 1/2″ mortising chisel for that 1-1/2″ drawer rail. But you have a 1/4″ mortising chisel (of course you do!). So making two Ã?¼” mortises that are set by the tool are easier to make than a 1/2″ mortise that you would have to make with an odd-size chisel. (It’s a theory , not much more than that.)
All this math and theory and contradiction is enough to make you want to smash something, with an anvil.
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The rule I follow, which came from observing antique furniture, regards the ratio of length to width rather than length to thickness. The tenon should always be longer than it is wide, so that when a racking force is applied, the tenon can not twist out of the mortise without breaking. In this way the strength of the wood is the determining factor, not the strength of the glue or pins. If the length of the tenon is less than its width, the joint is depending on the glue or pins to resist the racking force.
To get the most length on the tenon, through mortises were used anywhere that they didn’t show. So doors and windows were through mortised, while table legs were not.
Anywhere that a wide member, like the sill rail of a door or apron of a table, was tenoned into a narrower member, like the stile of a door or leg of a table, instead of having a single tenon wider than it was long, they used two tenons of the proper proportion. It also leaves more strength in the mortised member.
Another ratio affecting the strength of the joint is the distance from the lower edge of the tenoned member to the upper edge of the tenon. This should be less than the length of the tenon as well. Otherwise the fulcrum on which the racking force is acting (the lower edge of the tenoned member) creates a tensile force on the tenon rather than a bending force, and once again the glue and pins are stressed rather than the tenon itself.
Compromises to these rules are always necessary where the mortise is too close to the end of the stile/leg.
I still believe the rule that thickness of the tenon should be 1/3 of the thickness of the members when the members being joined are equal thickness (i.e. doors & windows). When one is thicker than the other (table apron to leg) the tenon can be up to 1/8" less than the thickness of the thinner member, or 1/3 the thickness of the thicker member, whichever is less, and as long as neither cheek of the mortise is too thin.
This of coarse, makes a mockery of modern biscuit joints and slot mortises. All they do is create a bit more glue surface for the joint.
At least that’s my theory.
Art Shaw