Highland Woodworking Wood News Online, No. 191, August 2021 Welcome to Highland Woodworking - Fine Tools & Education Learn more about Highland Woodworking View our current woodworking classes and seminars Woodworking articles and solutions Subscribe to Wood News
 
The Mortise and Tenon Joint
By Samuel Colchamiro
New Jersey

Click on any picture to see a larger version.

Historically, the mortise and tenon was the most ubiquitous joint used in fine furniture, and for good reason. When executed properly, the mortise and tenon joint has significant strength and a large glue surface that ensures the joint will last for generations. Mortise and tenon joints can be found in almost every furniture piece, adjoining chair legs and rails, creating frames for doors and carcass construction, and much more. The joint is comprised of two members. The mortise - namely a slot cut in one piece of wood, and a corresponding tenon cut to precisely fit the mortise slot.

Below are two side assemblies I recently completed for a project. The carcass is joined with nothing but mortise and tenon joints.


First, I will cover the fundamental terminology necessary to understand the mechanics of the joint.

Components of the Mortise and Tenon

The Tenon is the "giving" end of the joint fitted precisely to the Mortise ("receiving" end).


Pictured above, the tenon appears on the end of a rail and will fit in the mortise cut in the leg making for a solid connection.

The tenon has several features:


The Tenon Cheek is the broad surface that makes contact with the sides of the mortise. It is the gluing face that provides strength and integrity to the joint. The cheeks meet at a 90 degree angle with the Tenon Shoulders, whose position determines the depth of the mortise and the length of the tenon cheeks.


The tenon shoulder appears above on the left, next to the tenon end. When the joint is fully seated, the shoulder contacts the rim of the mortise and is one of the few aspects of the joints that is seen when fully assembled. On a well executed joint, the tenon shoulder and opposing piece will meet seamlessly, without gap.

The Mortise, as you probably have guessed by now, is the slot cut to match the tenon exactly. It dictates the size of the tenon that will be cut.


Making a Mortise and Tenon Joint

In a production shop, router settings and the repeatability of the table saw make machine methods by far the most efficient method for producing accurate, repeatable joints with little effort. However, when dealing with one-offs and small quantities of custom joints of all different in size and geometry, utilizing hand methods can be just as efficient. As with dovetails and other joints, setting up a router jig takes time, but once dialed in, the joints will be seamless. When working with very small numbers of joints, it takes longer to set up the jig than to just go ahead and cut the joints by hand. Additionally, when dealing with many differently sized joints within a piece, a new router setting will be needed, multiplying the setup time by the number of unique joint variations that are necessary. In this case as well, utilizing traditional techniques is preferable. Regardless of your workshop proclivities, I will be covering a reliable method that can be used to produce tremendously precise joints by hand. With a bit of practice, the results are as accurate as what can be expected from a router and table saw.

Chopping the Mortise

I always fit my tenons to my mortises. Therefore, I like to begin the process by chopping the mortise to get a ballpark estimate of how large to cut the tenon later on. The first step is deciding the nature of the mortise and its size. Some mortises (through mortise and tenons) go entirely through the board whereas others have a bottom (stopped mortise and tenon). If you are cutting a through mortise and tenon, you will chop half-way from one side and half-way from the other so as to avoid blowout on either side. In this example, however, I am chopping a stopped mortise and tenon joint. Therefore, I will begin by delineating the extremities of the mortise and its dimensions.


In this case, the mortise is being chopped as a deeper portion of an existing groove that will retain a panel as pictured above. Therefore, the extremity of the mortise is the end of the stopped groove as pictured above. After establishing the end point of the tenon's width, I then darkened the entirety of the waste material to make it easy to differentiate between waste and material to be kept.

Specialty mortise chisels make the process of chopping a mortise significantly easier and more efficient.


Their broad square sides keep the chisel straight in the cut and prevent the chisel from twisting. In this article, I am demonstrating the process of chopping a mortise with a standard bench chisel. It is possible, with some finesse, to do this so long as you are careful to keep the chisel straight and only take small amounts of material off at a time. If you have the choice between spending less money on two sets of chisels (one standard bench chisel set and one mortise chisel set) or more money on a single set of standard bench chisels, I would always recommend putting the extra investment in a single premium set of standard bench chisels. Mortise chisels are a luxury but not a necessity.

Returning to the process, I establish the extreme ends of the mortise with a light chop with my chisel. These cuts will be known as "chop 1".


At this stage, I am not trying to get any depth, but merely establishing the perimeter of the mortise with crisp, accurate lines. Chopping very hard would cause the bevel of the chisel to push the tool back, compressing the fibers behind the chisel edge and effectively lengthening the mortise dimensions. Therefore, a very light tap is all that is necessary to maintain the accuracy for this type of work. Next, move your chisel over between 1/8th and 1/4 inch over from the original chop with the bevel of the chisel facing away from you. Chop hard at this point. We will call this cut "chop 2".


As you progress down the length of the mortise continue to chop moving about an 1/8th inch or less at a time from the previous cut, keeping your chisel plum. When it is time to clear the waste, clear the waste only back to "chop 2" not to "chop 1".


As you can see above, I have progressed along about 1/2" or so and removed the waste back to "chop 2". "Chop 1" is visible but has not been touched since it was defined in step 1. In the process of removing the waste, we leverage the waste out. If we had cleared the waste to "chop 1", we would be leveraging off the extreme of the mortise, damaging the crisp, clean edges and causing a gap to form. Therefore, we leave "chop 1" untouched until the very last moment, ensuring a crisp finish.


Continue to move along chopping deeper with each progressive cut and then clearing the waste back to "chop 2". The appearance within the mortise will look like a set of stairs moving from "chop 2" deeper and deeper as you progress towards the center of the mortise.

Progress like this with the bevel of the chisel facing away from you through the length of the mortise until you near the end of the mortise:


Above you can see the starting point towards the bottom of the mortise. 1/8th of an inch away "chop 2" is visible. The rest of the mortise is cleared to depth getting deeper and deeper, and I have stopped about a 1/4 inch from the end of the mortise.

Now, we spin the chisel around, and bring the entirety of the mortise to depth, before defining the final edges of the mortise. When slowly creeping up on the extremities of the mortise, the bevel should be facing towards the center of the mortise to allow for a long straight cut down to the depth of the mortise.


Pictured above, I am creeping up on the final edge for the mortise with the bevel facing towards the center of the mortise (towards me on this side, away from me if I were on my side of the mortise). For the last chop on both sides of the mortise, the chisel will sit in "chop 1" and you will tap the chisel the full depth of the mortise. This ensures that the edges are crisp, clean and perfect. Don't worry about the appearance of the bottom of the mortise, nor its sides. The most important thing is that the start and end of the mortise are crisp, and the mortise is of consistent depth.

As a quick review, define the extremities of the mortise with a knife or pencil. Tap your chisel lightly at these points. Move away from the original cuts and tap hard. Chop the waste out to depth between those two second class points. Slowly chisel back to the original extremities ensuring clean results.

Cutting the Tenon

Now that you have successfully chopped the mortise, it is time to progress to the tenon. The one thing to remember is that you can always cut a fat tenon and remove material to get a perfect fit. You can't make a tenon thicker after you have cut it too small.

Several hand tools help dial in the perfect fit and ensure a level of accuracy that is very difficult to achieve with the table saw. The key to this is the router plane. I have previously written an article with in-depth information on how to get the most out of the router plane which can be referenced by clicking here. I begin the process of laying out the tenon by setting the router plane so that, referencing the sole on the outside edge of the leg mortise member, the cutter just skims the inside of the mortise. In this case, the mortise is set in a groove, so I set the depth stop to the first side of the groove as pictured below.


This ensures that when the tenon has been cut, I can pare the material back to this depth stop and the tenon face will be flush with the leg perfectly. All this will be clear later. Next, I establish the shoulder line by taking the depth of the mortise and striking a knife line all the way around the board with a knife and square. Using the knife to define the shoulder lines will ensure that I am left with a square shoulder after sawing off the waste.


Next, I take my router plane with the cutter set to depth and, referencing off the face side of the tenon member, scribe the final cheek line using the router plane as a marking gauge.


As pictured above, I am using the router plane cutter as the marking gauge to establish the lines to saw and ultimately pare back to.

Next, I saw just beside the router plane scribe line, leaving as little material as possible between the saw kerf and the scribe line.


In the vise, I tilt the tenon away from me, sawing first down the side facing me, ensuring a plum cut to the baseline. Next, after sawing to the baseline, I begin to tilt the saw upward focusing on the top of the board.


In this side view shot, you can see that now that I have hit the baseline on my side, I begin to saw towards the top of the tenon facing away from me, straddling the blade in the existing kerf and keeping my focus on the top of the tenon. Once the kerf has been established entirely across the top of the board, I stand the saw straight up in the kerf and saw straight down.


As pictured, you can see that I am now sawing straight down from the top allowing the saw to steer itself in its own existing kerf. Once I have hit the baseline on the far side of the tenon, I stop and address the shoulder. In order to achieve essential squareness on the shoulder, I first start by using a chisel to pare back to expose the knife line previously established.

Angling the chisel downward from about 1/8" away from the line I push forward and create a v-notched groove that my saw can sit comfortably in. This groove will hold the saw blade square and ensure perfect alignment.


Pictured above is the exposed knife line with the material removed on the waste side, creating a trough for my saw kerf to sit in. Next, I saw up to the cuts I made on the cheek, being extra careful not to saw into the cheek. Next, I adjust my router plane to take a light cut and slowly work down to the depth stop that I set at the start of the process. If you don't have a router plane, you can use a chisel and slowly pare back to the marking gauge lines, taking light cuts.


After I have successfully cleared the tenon cheek to the depth stop on my router plane, I measure the width of the mortise and translate this size to the tenon, establishing a new cut line for the parallel second cheek. Then I repeat the process, cutting the cheek slightly fat, establishing a trough for my saw and cutting the shoulder square. After this, I loosen the depth stop on my router plane and slowly remove material from the cheek, checking the fit between passes. With the router plane, you can dial in the perfect fit with patience. The perfect fit for a tenon is a tight match that can be pushed together by hand without a mallet. If you are needing to use excessive mallet force to seat a joint, chances are that when you add glue to the joint and the material swells, the joint will no longer fit, and in an effort to drive the joint home you will wind up splitting the mortise and thus destroying your project. Leave the joint tight but not too tight so that you get the perfect glue joint and don't spoil your hard work.

Check to make sure the shoulder meets perfectly with the rim of the mortise. You may need to adjust the shoulder with a chisel or shoulder plane.


While there are several variations on the mortise and tenon joint, the simplicity of this standard joint makes it the optimal starting point for beginners. Combining structural integrity with the beauty of a wood on wood joint that eliminates the need for ugly nails and pocket holes, the mortise and tenon is sure to become your most versatile and commonly used joint in your shop.

I hope this has been sufficiently clear and comprehensive, but as always, if you have any questions feel free to email me.

Click here to visit the Highland Woodworking Hand Tools Department


Samuel Colchamiro is a hobbyist woodworker who lives in NJ and works solely with hand tools. He enjoys writing, and is hoping to share his woodworking expertise with others to make their woodworking experience more enjoyable. He can be reached at scolchbm@gmail.com.

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