The Much-Heralded Mortise And Tenon

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This article is from Issue 20 of Woodcraft Magazine.

By John English

End grain is not the best gluing surface, so it’s fortunate that Chinese artisans in the twelfth century perfected an ancient joint that today allows us to maximize gluing surfaces while also permitting a more mechanical approach to joint making. In short, a finger (called a tenon) is milled on the end of a workpiece, and this fits into a hole (mortise) in a mating workpiece. In the typical mortise-and-tenon joint, all four edge and face surfaces of the tenon (as opposed to end grain) slip snugly against the walls of the mortise. Add glue, and/or some mechanical fastener, and you create a powerful bond for project parts.

Of course, early on, Chinese craftsmen used mortise-and-tenon joinery to construct large buildings rather than coffee tables or cabinet doors, but the principle is the same regardless of scale. On page 22, you’ll find two complementary mission tables that let you try this tried-and-true joinery firsthand. But before going there, stick around for a closer look at one of woodworking’s hardest working joints.

A powerful joint with many faces

Mortise-and-tenon joints come in variety of forms (see Fig. 1) but there are two main types: stopped (or “blind”) A, and through B. In the former, the tenon dead-ends inside the mortise; in the latter, it goes all the way through. The standard stopped-mortise joint in modern furniture is relatively simple. It can be hand-cut, milled with a combination of power and hand tools, or created entirely with machines. And in addition to glue and a nice fit, a tenon also can be secured in a mortise with pins, wedges, or tusks.

Open mortises C (sometimes called bridle joints, but more often lap joints) occur when one wall of the mortise is missing. Sometimes, the tenon in such a joint is dovetailed D and revealed for aesthetic reasons. Open mortises are not true mortise-and-tenon joints, but they are easier to make and serve well in applications where the joint doesn’t have to deal with stress coming from an angle.

A pinned mortise E is nothing more than a standard joint with dowel pins running through the tenon to lock it in place. Sometimes the pins are tapered or square, but usually not. Often, they are of a contrasting species to create a pleasing look.

In a wedged-tenon joint F the through mortise is slightly flared toward the outside (bottom of the hole), and there are one or two splits in the end of the tenon. After the joint has been assembled, one or more wedges are glued and hammered into the split, forcing the tenon to expand and fill the flared mortise. A round hole drilled in the tenon at the end of the split will stop it from telegraphing past the joint and traveling along the part (as with a chair rail, for example). The degree of taper depends on the density of the wood species.

A wedged tenon can be made in a stopped mortise too, but it’s more difficult because the math is challenging. This is referred to as either a blind-wedged tenon G or a fox wedge. In a stopped (or “blind”) mortise, the wedge must be exactly the right length or it will be either too loose or too tight. In a through mortise, the wedge can be trimmed after the joint is closed.

In a tusk-tenon joint H the tenon protrudes quite a long way through the mortise. It is then secured with a tapered key that is driven through a hole in the tenon. One edge of the hole is actually inside the mortise, so the wedge pulls the joint tight as it is driven.

Loose tenons I (also known as floating or feathered tenons) are becoming more and more popular in the machine age, as they can be made quickly and with little effort. To create this joint, two identical mortises are plowed with a router bit, leaving the ends round. A piece of stock is then milled with round edges to fit, and this is the tenon. Festool has recently introduced a new tool called the Domino Joiner that makes quick work of this joint. It’s fairly easy to mill with a standard router, too, while providing reliable strength and longevity.

A haunched tenon J has a smaller tenon beside the main one. Sometimes, haunches are square, while at other times they’re triangular. On a part with two or more tenons, there can be a short haunch joining them, to add strength.

The built-up mortise K (sometimes called a boxed mortise) is a product of the age of superior glues. It is laminated in three layers around the tenon, so there is no chopping involved. The tenon can be straight or dovetailed L.

Sizing the joint

The key to the mortise and tenon is to create as much gluing surface as possible. Sometimes, that means revealing more than one tenon. If the tenon is 2" or wider, it’s a good idea to make multiple tenons as in Photo A. This also avoids the slight possibility of problems due to wood movement. In extreme humidity, the tenon could expand enough to split the mortise.

Tenons are normally longer than they are wide. In general, a tenon should be about 1/3 the thickness of the stock for the mortise (not the tenon!). That means that the sidewalls of the mortise are about the same thickness as the tenon (Fig. 2). For 1" thick stock, a 3/8" mortise is a good choice.

The length of a stopped tenon should be an absolute minimum of four times its thickness, and hopefully more (six times is probably best, but not always practical). If the tenon is ¼" thick, it should be somewhere around 1 to 1½" long. If in doubt, longer is better. If the tenon is to be haunched, the haunch should be about 1/3 the length of the tenon. By the way, one of the primary reasons that people use haunches is to accommodate the grooves for door panels in door frames.

Sizing the width of a tenon involves a choice. It’s a lot easier to make the shoulder the same width all the way around, but it may not always be the best choice for strength. If your mortise is at the end of a ¾" thick stile, you might want to leave a little more material there than ¼". Doubling it to ½" is probably wise. If the mortise is halfway along the stile, then shoulders that are even all around are a sound choice.

Tenons need to fit properly in their mortises. If they are too tight, they can crack the walls of the mortise. Too loose, and the joint will fail under stress. Lay out the mortise first, as a tenon is easier to adjust (more accessible) than a mortise. A precision square and a sharp marking knife will leave a thin, accurate line. You might also consider a wheel marking gauge (see the Buying Guide) for easier, one-handed marking. Both are ideal for scoring across grain, where a clean, crisp edge is essential for accuracy. The stock must be square. With a gauge that has one wheel, scribe the two lines for the long edges of the mortise by running the gauge across the best face of the stock two times, moving the fence for the second pass as shown in Photo B. This guarantees that the mortise is parallel to the best face (just in case your stock isn’t the same thickness).

Creating double tenons when they will be more than 2" wide adds edge-grain gluing surface, and avoids expansion problems.

When laying out the sides of a mortise, draw both lines off one face of the board, in case the workpiece isn’t the same thickness throughout.

HOW TO CUT CLEAN, SQUARE MORTISE

With your mortise locations marked, begin cutting them to shape using either of the following proven methods. 

  • By Machine: 

For most mortises, a hollow-chisel machine (or a mortising attachment kit for the drill press) has only two variables to set after choosing the bit size. (See the sidebar “Hollow-Chisel Mortising Machines and Accessories,”  as well as the Buying Guide.) These are the locations of the fence and the depth stop. Setting the depth is done before setting the fence. The mortise should run about 2/3 of the way through the stock for a stopped mortise, but this might not be possible with some hollow chisels. They have a taper which limits the depth of cut. You can mark your stock and hold it beside or behind the cutter, and then adjust the stop visually, as shown in Photo C. It’s a good idea to go with ½" increments when deciding how deep a mortise should be. That way, when you cut parts that have tenons to length, the math is a bit easier.

To locate the fence so that you chop a mortise in the center of the stock, you’ll need a piece of scrap material the same thickness as your workpiece. Eyeball the location of the fence and lock it down. Make a plunge cut, and then rotate the stock 180°. Move the workpiece about 1/8" left or right and make a second cut as shown in Photo D. This movement makes it easy to see how much the fence must be adjusted to center the mortise in the edge of the stock.

Many pros seldom concern themselves with centering mortises. They work from a common face (if the parts are to be flush to one face), and any differences that might occur are then always related only to one face.

With the fence and depth set, chop a mortise in the scrap stock, and then crosscut it to reveal the geometry of the mortise as shown in Photo E. This is your template for the tenon setup. Now you can chop your mortise in the actual workpiece. Note that the bottom of the mortise won’t be absolutely flat, and this is fine. Tenons are often cut about 1/16" shy of the mortise depth, as the joint relies far more on the glue along the sides of the tenon than on its end. It’s better to have a tiny gap than to have the tenon a hair too long, which will prevent the joint from closing. On a through mortise, work from both sides equally or you’ll have tear-out on the exit side.

When setting the depth of a mortise, be sure to take into account the taper on the hollow chisel or it will crush the well-defined corners.

To check if a mortise is centered on a stile or rail, make a cut, flip the board end-for-end and make a second cut a little offset. Any difference will be immediately obvious.

With the mortiser set up, chop a mortise in a piece of scrap and then cut off one end to make a template for your tenoning jig.

  • By Hand: 

For anyone who doesn’t have access to a hollow-chisel mortising machine (or a mortising jig for a drill press), there is another option. You can do it the old-fashioned way. Mark your mortise, drill out most of the waste with a Forstner bit as in Photo F, and clean up the edges with chisels and a mallet as shown in Photo G. The drill bit’s diameter should be about 75% of the smallest dimension of the mortise (usually the width). A clamped-on scrap fence is often used as a guide to make sure that the chisel is perpendicular to the top of the workpiece. This is especially useful when chopping tapered mortises for a wedged tenon.

While ordinary bench chisels will work to pare the sides of a mortise, there are special chisels available (see the Buying Guide) that are far better suited to the task of actually cutting mortises. Their shafts are thicker, to resist the levering action required to tease waste from the bottom of a deep mortise, and a high square shoulder keeps them properly aligned in the mortise while cutting. A steeper bevel lets you work with more force while not having to worry about a fragile cutting edge.

Whatever chisel you use, it must be as sharp as possible to slice through end grain without taking an uninvited detour. And if you’re chopping a through mortise, work from both faces toward the middle. That will preserve the edges on both openings, and it will minimize any drift from vertical.

Begin manually chopping a mortise by laying it out and then removing much of the waste with a slightly undersized Forstner bit chucked in the drill press.

To make a perfectly perpendicular cut on a through mortise, support the chisel with a clamped-on auxiliary fence.

HOW TO MACHINE SNUG-FITTING TENONS

There are several ways to create a tenon. The most popular methods are to use a tenoning jig on a table saw, a dado head, and miter gauge on the table saw, a sled on a vertical router table, or a miter gauge on a standard router table. You can also use a special handsaw, but that method is a bit dated and takes a lot of practice to get it right. Maintaining a vertical cut with a freehand saw takes practice. Machines are more predictable (although the saw is more fun). 

  • The Tenoning Jig Approach

This accessory can be shop-built or commercially available (see the Buying Guide). It slides in the miter gauge groove of the table saw and holds the workpiece in the same plane as the blade. Before using the jig, the saw blade must be set at exactly 90° to the table. If it’s a hair off, the tenon will be tapered. The jig also must hold the work at 90° front to back for most work (and can be used on mitered tenons as well). 

First, secure the work in the tenoning jig, and use the scrap piece you saved earlier from the mortise setup to lock the jig’s position side-to-side as shown in Photo H. This determines the thickness of the tenon. Now, raise the blade to the length of the tenon and make your cheek cuts (Photo I).

Next, define the shoulders by using the saw’s miter gauge. Some professionals do this the other way around, defining the shoulders before the cheeks. However, that leaves a piece of narrow waste standing on its end, waiting to fall into the blade and shoot across the room. Raise the blade to the depth of the shoulder to make all four passes as shown in Photo J. You may have two different setups here if the shoulders are not even all around. Clamp a stop block to the fence to avoid trapping offcuts between fence and blade. Lock the fence in the right position, and use the block to register all four cuts, so they line up properly. For the edge shoulders that determine the width of the tenon, it’s a good idea to nibble away a bit of the tenon on each edge as shown in Photo K, and then test the fit in the mortise as shown in Photo L. Then move to the bandsaw for the final cut. See Photo M.

Use the scrap template saved earlier to locate the clamping face of the tenoning jig on the table saw. Note that the blade is aligned with the mortise wall.

Check that the blade is at a 90° angle before making the two tall cheek cuts, or the tenon will be tapered.

By using the table saw’s miter gauge to make the short shoulder cuts after the tall cheeks have been cut, the waste lies flat on the table instead of falling into the blade.

As you define the tenon shoulders, nibble a little off the ends. This allows you to dry-fit the tip in the mortise, before making the final cuts.

When dry-fitting, try not to wiggle the tenon from side to side as this will crush some fibers and cause a sloppy fit.

When you're satisfied with the fit, remove the rest of the waste on the bandsaw. A wide (resaw) blade makes for a straighter cut.

  • The Dado Head Method

If the saw’s miter gauge is set at precisely 90° and a sharp dado head is available, you can create tenons without a special jig. Clamp a stop block to the fence to avoid kickback, and lock the fence in the correct position. Then simply nibble away the waste until the tenon is revealed as shown in Photo N.

  • Tenons with a Router Table 

For both this and the dado head method, it’s a good idea to score the lines of the tenon in the stock using a square and a sharp knife to establish clean lines and eliminate tear-out on the back end of each cut. On a horizontal (standard) router table, use a miter gauge to guide the part across the cutter. Usually, several passes will be required to nibble away the waste as shown in Photo O. It’s a good practice to make each pass no more than 1/3 of the diameter of the bit being used. A hinge mortising bit (see the Buying Guide) is an excellent choice here, as it has wider carbide flutes and an enlarged center relief for chip clearance.

The tenon should fit tightly into the mortise, but not so tightly that it might split the sides. One should be able to dry-fit it and still take it apart without wiggling it. If the tenon is too tight, clamp it in a vise and work gently on the cheeks with a medium file. Be careful not to remove too much material. If the fit is sloppy, mill a new tenon. 

Tenons can be revealed on the table saw using a dado head and a starter block that avoids kickback, in case the end of the workpiece drags against the fence.

The safest way to cut tenons on a traditional router table is to clamp the work to a miter gauge fence and make several passes over a mortising bit.

ASSEMBLING THE MORTISE-AND-TENON JOINT

Apply glue to all five surfaces of the tenon, and use a brush to distribute an even coat within the mortise. Tap the tenon in place and, when properly seated, use a clamp to apply pressure until the glue cures. I like to wipe off excess glue with a damp cloth, but some pros hold to the theory that it tends to dilute the glue, and spread it into the wood. Instead, they allow the excess to skin over, and then lift it off it with a chisel.

If wedges are used, the edges of the mortise must be chopped at a slight angle. Use a block of scrap under one end to elevate the part to about a 5° angle as shown in Photo P. Drill two holes near the base of the tenon as in Photo Q, supporting the outfeed side with scrap to avoid tear-out. On the bandsaw, cut two kerfs in the tenon as in Photo R, and stay with the bandsaw to cut a couple of small wedges from the edge of a piece of stock that’s the same thickness as the width of the mortise. During the glue-up, drive the wedges evenly to keep the tenon straight in the mortise. Don’t drive one of them all the way in before starting on the other as shown in Photo S. After the glue dries, cut the wedges close to their bases and then sand them flush. When finished, a wedged tenon serves as a strong, eye-pleasing wood joint.

For pins in a pinned tenon, drill from both sides and try to meet in the middle. This will leave clean holes on both faces. If a dowel plate is available (see the Buying Guide), it will deliver dowels that are exactly the same diameter as a drill bit, and perfectly round. 

On tusk tenons, make sure there’s enough wood beyond the opening for the tusk (also called a key or a wedge), so that the tenon doesn’t split when the tapered tusk is driven home. The tusk and the outside wall of the hole must be cut at exactly the same angle. Make the tusk about 1/32" narrower than the width of the hole, and cut the taper on the bandsaw using a simple jig that can ride against the machine’s fence. One cut will deliver the tapered edges on two tusks. Matching and contrasting species both look good in this application. Clean up the edges with a file, or use a bench plane that is locked upside-down in a bench vise (use a push stick for this approach to save your fingertips). Tusks can be glued in place, but usually they are not. They can be a simple taper, or the tusk heads can have a decorative profile.

To make slightly tapered through mortises for wedged tenons, simply raise one end of the work and
dress the edges of the opening
at about 5°.
The first step in cutting the slots for a wedged tenon is to drill a couple of holes for the slot to dead-end in, or they might split the part under pressure.

Bandsaw the two kerfs for the wedges. One wedge won’t spread the tenon to the gentle dovetail shape that locks it into the mortise.

During glue-up, gently tap the wedges into their kerfs using a short length of dowel and a hammer or dead-blow mallet.

Hollow Chisel Mortising Machines And Accessories

Over the last few years, several quality mortising machines and drill press mortising accessories have come on the market aimed at the serious home craftsman, and they are reasonably priced. Here’s a rundown of few well-known brand names.

General International’s tilting/swiveling head model 75-050T M1 is a cast iron mortiser with a gas head cylinder and a 9" chisel stroke. It comes with sleeves for both 5/8" and 3/4" chisel shanks, and the really cool feature is that the head swivels and tilts 180° for mortising at an angle (think splayed legs and chairs). The 75-050T weighs in at just over 100 lb, has a 1/2 hp motor, and it comes with 1/4", 5/16", 3/8", and 1/2" chisels.

Another benchtop model worth a look is the Steel City 25200. Built for left- and right-hand woodworkers, it also runs on a ½ hp motor. In addition to its rack-and-pinion head (up and down motion), it also has a rack-and-pinion fence system with roller bearing work supports that hold the workpiece firmly against the fence, and a couple of pull-out table extensions that give you 35" of support. On top of all that, the Steel City machine has a five-year factory warranty. It, too, includes 1/4", 5/16", 3/8", and 1/2" mortising bits and chisels. It ships at 88 lb.

A good entry-level unit is Jet’s model JBM-5 Benchtop Mortiser. Like the others, it has a ½ hp motor and a rack-and-pinion head. While the unit ships with only 1/4", 3/8", and 1/2" chisels it does have a removeable safety toggle switch that prevents accidental start-up.

 Delta makes a nice rig that converts a drill press into a mortising machine. It’s a little less expensive and is designed to fit most Delta and Sears models and any other drill press with a 25/8", 2", 17/8", or 11/2" diameter quill. The Delta fence bolts to the drill-press table and includes four chisels and bits (1/4", 5/16", 3/8", 1/2").

Like any other edge tool, don’t forget to keep your bits sharp. A simple hand-held sharpener is available for honing mortising bits and should be the first accessory you buy.

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