Mortise and Tenon Joint

Joining wood together in a way that is attractive and secure is the most challenging part of woodworking. It’s usually possible to tell the level of skill of any woodworker, by looking at the quality of their joinery. Experienced woodworkers will join pieces together with no gap left, while those without much experience will not only leave gaps, but avoid making projects which require tight joinery.

Good joinery is a result of careful attention to detail, more than anything else. Laying out the joint carefully is essential to making that clean joint. This not only means having the right tools to use, but also marking on a clean, smooth surface. Avoid marking on the crosscut end of a board, as it comes from a factory. Instead, clean that up by cutting it on the table saw or with a miter saw, ensuring a smooth surface that is exactly at 90 degrees to the surface of the boards.

Why Mortise and Tenon Joints?

The mortise and tenon is actually the oldest known method of wood joinery in existence. Part of that is because back when it was invented, the fasteners we use today were not in existence. It was necessary to join wood parts together with a method that maximized the benefits of the hide glue they were using. A mortise and tenon joint does this, by providing a large amount of long-grain surface in contact between the two pieces.

For any adhesive to be effective, it needs to be applied to the long grain of the wood. While adhesives applied to the end grain will provide some adhesion, the joint is considerably weaker, because the adhesive easily breaks away from the ends of the wood fibers. That’s after soaking in, so that there isn’t much adhesive at the joint.

A mortise and tenon is actually the strongest type of joint that can be made, because it is the wood itself that is making the joint. The mortise (a square or rectangular opening) and the tenon (a pin cut to fit into the mortise) provide positive, solid contact; both for gluing and for mechanical leverage.

People tend to think of fastener joints as being the strongest, mostly because the fasteners themselves are made of metal. But it is the interface between that metal and the wood that is the weak point, not the fastener itself. It may take some time, but that interface can open up, leaving less and less contact between the two, to the point where the joint fails.

Another thing that makes a mortise and tenon strong is that the tenon from the rail is actually going into the mortise on the stile, so that there is a fair amount of wood supporting the weight. While the thinner tenon may not be as strong as the rest of the rail, it is still stronger than any other type of joint.

Besides strength, the big advantage of a mortise and tenon joint is the ability to pull the pieces up close to each other, without a gap. This makes for much more attractive furniture and other projects. While it will definitely take some practice to get to the point of being able to make mortise and tenon joints which don’t have any gap, it’s worth the effort for how it improves the quality of the job.

It Starts with the Layout

Because precision is required, it’s important to start with a precision layout. The tools used for this are critical, as poor quality layout tools can’t provide a high quality layout. A combination square usually provides the best option for this, as the ruler portion is graded for small fractions of an inch and the adjustable crossbar allows for using the square to set a particular depth. Along with that, a fine-point mechanical pencil should be used, for the thinner line it creates, rather than a ballpoint pen or carpenter’s square.

The mortise and tenon can be made any size that fits within the limits of the wood being used. However, it is typical to make the tenon 1/3 the thickness of the board, assuming both the rail and style are the same thickness. In cases where the stile is thicker than the rail, such as when the stile is a chair or table leg, the tenon might be as much as 1/2 the thickness of the board being used as a rail.

Height-wise, the mortise and tenon is typically sized to leave ½ inch of shoulder at the top and bottom, although in the case where narrow dimension boards are being used, such as a 1”x 2”, that would only leave a ½” tenon. So the shoulder would be cut down to ¼”, providing for a tenon that is one inch long.

As for depth, the mortise and tenon can be any depth desired, all the way to the point of going through the stile. However, in most cases, the depth is held to somewhere between 1/3 and 1/2 the thickness of the stile. The tenon is cut 1/16” to 1/8” shorter than the depth of the mortise, to leave room for glue.

Keep in mind that whatever you do, the strongest joint will come from the largest mortise and tenon you can reasonably fit into the project, without making the sides of the mortise so thin that they become weak.

When laying all this out, it’s a good idea to do the layout of the entire joint at one time, starting with one part and then carrying the dimensions over to the other. This allows the ability to check dimensions against each other. Even so, it can be helpful to use the cut pieces as a gauge for setting up power tools, thereby ensuring accuracy.

Methods for Cutting the Mortise and Tenon

There are a number of different methods that can be used for cutting a mortise and tenon, which we will look at. Depending on the tools available, the joint being made and the skill of the woodworker, any of these may be used in combination.

Always start with a clean cut, square end. Never assume that boards which come from the lumberyard are square. It’s best to cut them square, just to be sure.

Method #1 – Cutting by Hand

The original way of cutting mortise and tenon joints was by hand, using nothing more than a mallet, chisel and back saw for the tenon and the same mallet and chisel, along with a bit and brace for the mortise. This requires some skill with these tools, but will still produce an excellent mortise and tenon joint.

To make a joint in this manner, lay out the joint on both parts. Then cut the shoulders of the tenon with the back saw, taking care to keep the saw aligned with the marks, not wandering off them, and not go too deep. The saw can also be used to cut the sides of the tenon, down to the cuts made for the shoulder.

While cutting with a back saw will provide a relatively clean cut, it is going to be necessary to clean that cut up with either a chisel or plane. A shoulder plane or rabbet plane will probably be needed, as that is the only kind of plane where the edges of the blade are flush with the sides of the plane, allowing the cut to be made all the way into the corner.

In this case, the mortise is started by drilling out the majority of the material with a bit and brace. The mallet and chisel can then be used to finish out the mortise, cleaning up the sides and corner. While not all that hard to do, the challenge here is in fitting the chisel into the mortise and keeping it vertical, so that the sides of the mortise make good contact with the sides of the tenon.

Method #2 – Using a Drill Press

The simplest addition to that hand cutting method is to use a drill press, rather than using a bit and brace to hog out the majority of the material in the mortise. Using a drill press, rather than a hand drill, helps prevent the problem of the mortise not being perpendicular to the surface of the stile.

There are drill bits made just for cutting mortises. These look like a drill bit encased in a square box. The idea is that the corner of the box, which are ground like a chisel, will square out the corners, while the drill bit cuts out the majority of the material. However, these drill bits require a mortising attachment on the drill press, so that the outside box will stay straight and not spin around with the bit. Nevertheless, the cost of the attachment is a worthwhile investment, if a lot of mortises are going to be cut.

Method #3 – Using a Table Saw

A table saw can come in handy for cutting tenons, especially if it has a dado blade; but it can’t be used to cut the mortises. The table saw’s ability to set the blade to a very precise height allows for very accurate cutting of the tenon. All that’s needed is a sled to go with the saw, to ensure a good miter on the cut.

It’s a good idea to use a gauge for setting the saw blade’s height, if available. If not, then the saw’s blade can be set visually to the layout marks on the board. However, the teeth on most table saw blades are not square ground, so a visual height adjustment might be difficult. Therefore, it’s a good idea to cut a test piece a little short, measure it, then bring the blade up to the finish height.

Due to the teeth being staggered, even on a carbide tipped saw blade, the surfaces of the tenon will have to be cleaned up with a chisel or plane. A smooth surface is necessary for gluing, as wood glues are not good for gap filling.

mortise, tenon, joints
Mortise and tenon joints, John

Method #4 – Using a Router Table

Probably the most versatile power tool for making a mortise and tenon joint is a router table. Both the mortise and tenon can be cut with the router, with almost no cleanup to be done by other tools.

For cutting the mortise, a straight sided bit, the width wanted for the mortise is used. The fence is set up so that the space between it and the outer edge of the bit is the same as the distance desired between the mortise and the edge of the board. A piece of tape placed on the table, marked to show the forward and back edges of the bit. Similar marks on the side of the stile will show where the mortise begins and ends.

To use this setup, it’s advisable to make several cuts, removing no more than 1/8” of depth on each pass. Start the cut where the marks on the stile and tape align and end it when the other mark on the stile aligns with the other side of the blade.

As an alternative to this, without using the router table but just using the router, a template can be made of the mortise to be cut and clamped to the rail. Adding a stop on the back side of the template, to help align it straight with the side of the stile is helpful. Then cut out the mortise in several stages, making the cut deeper each pass.

Cutting the tenon on the router table is much like cutting it on the table saw, with the exception that the router bit spins in the other axis. In order to cut it effectively, either a miter gauge or a sled is going to be needed, in addition to the fence. The fence will determine the depth of the tenon; the height of the bit will determine how much material is being cut off, leaving the shoulder.

Most router table miter gauges are not very effective, so it’s actually much easier to do this with a simple sled. All that’s needed is a piece of plywood that is determined to have an absolutely square corner between the fence and the forward edge. Attach a strip of wood vertically to this edge, to provide support to the rail when it is on edge. It is not necessary to add a strip to go into the table’s T-slot, as long as the sled is held snug up against the fence.

In all cases, when using a router table to make the cuts, it’s a good idea to make a test cut first, checking the setup. Cuts should be made in stages, so that the bit isn’t cutting off too much at one time. Heavy cuts, rather than light ones, are more likely to go wrong.

Since the corner of the tenon will be square and the corners of the mortise will be rounded, one or the other will need to change. It’s easier to cut the corners of the tenon, rounding them, than it is to square out the corners of the mortise. This can either be done with a chisel, making a chamfer cut or with a file, rounding the corners.

An Alternate to Mortise and Tenon

Due to the difficulty of perfecting the technique of making mortise and tenon joints, two alternatives have been developed; doweling and biscuit joining. Before talking about doweling, I will have to say that biscuit joining doesn’t really do the same job as a mortise and tenon, even though it is touted as doing so. The problem is that the biscuit doesn’t fit snugly in the slot cut by the biscuit cutter. Since wood glue is not gap-filling, this means there is a reduced amount of surface area contact, reducing the strength of the joint. The actual strength comes from the wood to wood bond of the rail and stile.

On the other hand, dowels are an effective alternative for use in place of a mortise and tenon. While the dowels are not as strong as a mortise and tenon, providing a smaller cross-section, they do provide adequate glue surface for a solid joint. They are also faster and easier to install, if you have a doweling jig for drilling the holes.

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