Lots of Options

There are as many variations of the mortise and tenon joint as there are tool manufacturers out there. From the decorative wedged through tenon to the simple, single shoulder variety, you’ll find a different version for every possible application.

As someone who builds custom fur­niture in a hand-tool-only wood shop, I understand how important it is to get familiar with this joint and these meth­ods. Even if you have a garage full of power tools, the mortise and tenon, in all of its variations, is something you should practice. You’re bound to need one sooner or later.

For the furniture designer, the chal­lenge is deciding what type of mortise and tenon to use. Which application makes the most sense? Which version is the strongest? What will look the best? Before we can answer, we must first understand the factors that give this joint its strength.

The long grain area of the tenon cheek meeting the inside of the mor­tise is the most important aspect to its strength. The glue bond between these two surfaces gives the joint longevity. The mechanical aspect adds additional strength to an already solid joint.

A Good Starting Point

When deciding on the dimensions for any mortise and tenon, it’s a pretty safe bet to go with the rule of thirds. An example for ¾” material would be a ¼” thick mortise cheek with a ¼” thick tenon flanked by another ¼” thick cheek (see figure 1). Although a good place to start, this is not always going to give us the best results. Some may argue that when the joint needs only to withstand a downward force, a stronger joint may be one with a thicker tenon and thinner mortise cheeks. Keep in mind that a thinner outside mortise cheek means you run the risk of damag­ing the piece either while cutting it out or test fitting the components. If you only need to protect against a down­ward force, the rule changes slightly; make the tenons thickness the same as the thickness of the two outside cheeks added together. Theories aside, for someone starting out the rule of thirds is good practice.

The width of the joint is usually dic­tated by the size of the project, but any time you have a wide tenon, we should divide the area into sections, thus cre­ating multiple tenons. Remember the strength is com­ing from the amount of long grain glue sur­faces we have. The more tenons we have, the more surface area can be glued.

While we’re on the topic of tenon width, let’s look at a com­mon mortise and tenon application where a table apron joins a leg. We should be weary of the distance we begin the mortise down from the top of the leg. Good practice is leaving some space and starting the mortise down a little bit. If we were to place it too close to the top of the leg then the small area above would be fragile and we’d run the risk of blowing out the wood fibre in the end grain.

Speaking of this fragile area at the top of a table leg, let’s look at a haunched mortise and tenon, another slight varia­tion of the joint.

A haunched tenon is much more stable than one that isn’t; the haunch act as training wheels, keeping things from twisting and wracking through the years. The grooves or dados that run up the inside of a door rail and stile to hold a panel or something similar in place, would be exposed in the top of the door, adding a haunch will fill in that void. (See figure 2a)

If we’re making a small scale cabi­net door for aesthetic reasons we may not want to see the haunch at the top of the finished piece; if this is the case we can employ what’s known as a sloping haunch (see Figure 2b). This adds all of the same benefits in strength but keeps the lines of the piece a little cleaner.

Now that we have some guidelines to help us determine the thickness and width of the joint, we must consider depth. If you’re using a mortising machine or a drill press, you may be able to make deeper mortises but keep in mind that most machines will bottom out around 2 ½” to 3″ deep. When chopping mortises by hand, the only real concern is keeping the joint intact without self-destructing things while we work. If we think again about where the joint gets most of its strength from we can say the deeper the better, within reason of course.

Let’s again use a table as an example. If we chop out really deep mortises then we run the risk of removing too much material as well as having the tenons bump into each other, causing problems. If you look at some history books and find some examples of aprons meet­ing legs, you’ll often see the two apron tenons actually meeting within the leg cavity. They were keyed or stepped so they actually pass each other or mitred to meet in the middle. Some careful consideration when laying out join­ery is essential while you design your own pieces; just try to con­sider where the joint gets its strength from and work your way out from there.

Get Down To Business

Now that we’ve looked at some examples of the mor­tise and tenon and have given some thought to the theories behind them, let’s take a look at my own methods for mak­ing this joint.

As mentioned, I work in a hand-tool-only wood shop but the methods described could easily be adapted to a power tool shop as well.

To begin, let’s gather up the tools we’ll need and go over to the sharpening station. Make sure your tools are sharp and tuned properly for best results.

We’ll use a quality marking gauge, a square and a striking knife, a drill bit the exact size of the mortise and, finally, a mortise chisel, also the same width as the mortise. This method of using tools and cutting irons that are the same overall widths is known as slaving. This means we would use a ¼” iron to plough out a groove, then fol­low with a ¼” drill bit to remove the waste and finally a ¼” chisel to clean up the ends.

First Up – The Mortise

The mortise is always the first step in the process and I first establish its length. I’ll clearly mark the top and the bottom of the mortise with nice deep scribe lines using a small square off of the same ref­erence face of the work piece. From here I use a Glen Drake marking gauge with a wheel cutter on it. If you use a pin style gauge I’d recommend you spend some time and file it to a fine, knife like point. You’ll have less chance for tear out and a cleaner scribe line to work to. I’ve also noticed some people mark only one side of the mortise, allowing the chisel’s width to dictate the opposite side. I find I get cleaner results by taking a few extra seconds to mark deep, crisp lines completely around the entire perim­eter of the cavity. I’ll also go ahead and scribe a deep line perfectly down the cen­ter of the mortise; this will help register my drill bit to begin the excavation.

With the mortise laid out I’ll begin the process of removing the waste. To do this, I choose a brace and a bit. Start the first hole just shy of one end; we’ll clean up the tiny waste area later with a chisel. I’ll measure my desired depth on the bit I’m using and wrap a small piece of painters tape around it to control the depth. If you find it difficult to maintain a straight, square hole while you’re bor­ing, a small square or wooden block can assist you. Continue removing the waste with the drill then use a chisel to clean out the small areas of remaining waste. If we’re fitting a tenon that’s 1 ½” deep, it’s good practice to make the mortise a little deeper than needed – ⅛” is plenty. This allows somewhere for glue or debris to go during assembly. For extra deep mortises, a special purpose chisel can really come in handy for levering out the wood chips. I have a dedicated swan neck chisel that’s perfect for this appli­cation. This may be a luxury, but when you’re chopping a lot of mortises it can really be a time- saver. With the mortise cleaned up we can move ahead to saw­ing the tenon.

Cut the tenon to fit the mortise

Carefully lay out the shoulder lines around the perimeter of the work piece, then measure and mark the tenon cheeks then clamp the piece in your vice. Because this is a ripping cut, down through the end grain, a suitable rip saw is needed. Something to consider before you get to this point is the depth of your back saw. If you’re trying to saw a 3″ tenon with a saw that only has a 2 ½” saw plate, then the back of the saw will bottom out before you reach the required depth. In the past when I came across this problem I would finish off my cuts with a Japanese-style Ryoba saw. These are relatively inexpensive and have both a rip tooth and cross-cut pattern. They don’t have a spine so the depth of cut is irrelevant. When beginning the cut, try to leave a bit of fat on each side of the tenon; this logic is that it’s easier to remove a bit of wood when fitting the pieces than it is to put some back on!

Carefully rip down to your shoulder lines, being careful not to cut past them. This would show in the finished piece and you’ll always regret it. If this is a simple two shoulder tenon you can use a bench hook or mitre box to secure the work piece as you cross cut the waste off. Be sure to leave the scribe line for fine tuning. If it’s more elaborate and has four shoulders, saw the remaining two shoulders next. With the sawing complete, it’s time to clean down to our scribe lines – a razor sharp paring chisel is my tool of choice. Using the scribe line as a guide, I carefully place the tip of the chisel and remove the waste. Again, you have to be careful not to cut past the line, as this will leave gaps and spaces in the finished work. Try your luck at dry fitting the joint.

If you left some extra meat on the sides of the tenon, it should be a little too snug so it’s back to the bench hook to pare down the sides. A shoulder plane or rabbeting block plane will be the per­fect companion. Whatever tools you have on hand will probably work just fine if you are familiar with them and can control them. Just be sure to take light shavings and check the fit as you go. A properly fitted joint should be able to go together with hand pressure alone. If you have to use a mallet to seat the joint, you’re not there yet. Don’t force it home. Pare off a little more waste, being careful not to remove more than necessary. The best way I can describe a well fitting joint is that it is one where I can push the pieces together and hold them upright at a 45° angle. The tenoned piece should be able to hold true with­out falling out or sagging. If it moves at all the joint is too loose. So what do we do if this occurs? A simple remedy is to glue thin shims back on to the sides of the tenon. I say the sides of the tenon because if we were to shim only one side and re-fit the piece it will be off-center. When the shims are dry, return to the paring stage to get that perfect fit. Take a moment and relieve the sharp edges of the tenon. This will make assembly a little less stressful and allow some room for excess glue.

These methods are just one way to make a traditional mortise and tenon joint. There are dozens of mortise and tenon varieties and probably even more methods of execution. If you ever have doubt when designing a piece of fur­niture, you can refer to text books and magazines, check working examples or ask fellow woodworkers. If you take the time to produce a proper mortise and tenon joint it will reward you with a life­time of trouble-free use.

Happy shavings!