Mortise and Tenon for Leg and Rail

The heart of joinery, like that of carpentry, is the proper mortise and tenon joint. It makes our tables and chairs, our doors and windows, with mechanical and aesthetic refinements for each application. We'll take them in turn, starting with the table.

At each corner of a chair or table stands a post. When we make doors and windows, we call these posts stiles, but since we're starting with tables, they're legs. We'll cut mortises in the legs to receive the tenons from the horizontal pieces. These horizontal pieces are called rails, a word that spans chairs, tables, doors, and windows.

We're not joining equally sized pieces, so the thickness guideline of more than a third and less than half doesn't apply. Moreover, in a table leg, two rails intersect at right angles within the leg, the mortises intersecting as well. This makes the tenons each lose some depth to the other. That's why the mortises in the leg are shifted farther toward the outside of the leg, rather than being centered—to give the tenons a deeper reach and equalize the strength of the mortise. You can readily judge the right size and placement for the joint by drawing various options on paper. You know what size mortising chisels you own, and you know the sizes of the wood. Draw a cross section of a leg and see how strong various widths and placements look.

The four legs of the table are alike in size and placement of the joints. Indeed, the more alike you can make them, the better, so start by laying them out all with the same measurements made at the same time. Clamp them all together, face side up on the bench top, square across, and mark them all with the same strokes of the knife or pencil.

Much of the stress and shock on the end of the mortise will come while you're cutting it and test fitting the tenon. That's why it's customary initially to cut the legs, or any piece getting a mortise near its end, about an inch longer than it needs to be. The extra wood helps strengthen the leg against splitting.

Short single dovetails join drawer rails to a table leg...

... and long single dovetails will connect the legs to this candlestand pedestal.

... and long single dovetails will connect the legs to this candlestand pedestal.

The beveled ends of the rail tenons meet within the intersecting mortises of a table leg.
Unless you want the faces of the rails to sit flush with the legs, reset the mortising gauge after laying out the legs.
Score across the grain and saw the shoulders of the tenons.

After you have assembled the piece and the glue has set, you can trim these protruding "horns" flush to the rails.

The length of the horns may vary, so align the bottoms of the legs and measure up from there to find and mark the height of the legs. Square this line across all four pieces and measure down from it to mark the setback and the width of the rails, again squaring the lines across. The setback keeps the mortise closed and the joint strong, but the greater the setback in the leg, the more must be taken away from the tenon. One-third the width of the rail is the maximum setback I'd consider. Separate the legs and check the square of the lines on each piece, and then carry the lines square around.

The beam of the mortising gauge has two teeth, one fixed in the beam and one movable. Set these teeth to match the width of your chisel and adjust the beam in the fence to place these teeth where you want them on the leg. Ride the fence of the mortising gauge on the faces of the leg to leave parallel lines for the mortises on the back sides. The faces may be the prettiest, but the backs take the joints, and must be true and square.

The legs are now laid out for their mortises, but don't start cutting yet. First, lay out and cut the tenons on the rails. (The tenons-first habit may save you from a potential error later on when making panel doors.) On rectangular tables, do the gang layout for rails in opposite pairs, face edge up. Separate the pieces and check and square the lines all around. If you use a fine pencil at this stage, each line will get checked again when it's scored for sawing.

Set the rail in the bench hook with the tenon to your right, face edge toward you. Stick your striking knife or chisel into the line at the corner of the face and edge, slide the try square up to touch the knife, and hold it there. Now strike the line with the knife across the face, followed by a second knife cut to make the V for the saw.

To score the back side, flip the face edge away from you and place the knife in the far corner of the back side and face edge. Slide the try square up to the knife and then hold it, either with your hand, or squeezed between the back stop of the bench hook and the face edge of the wood. Strike the back line. You now have scored lines across the face and back, and penciled or lightly scribed lines across the edges.

You have established where to start the saw for the tenon shoulder—now you have to know where to stop. If you were to use the same setting on the gauge to lay out both the mortises and the tenons, the faces of the rails would be perfectly flush with the legs. If that is what you want, go ahead and scribe the double lines all around the tenon ends. More often, though, for looks, you want the face of the rail to sit at least 1/8 inch back from the face of the leg—enough offset to look deliberate and not like a mistake. In this case, the mortise gauge is still correctly set for the width of the chisel, but the distance from the fence has to change before you scribe the lines.

Set the rail back in the bench hook and start the backsaw in the V-groove. Keep checking the far side and stop sawing when you're supposed to. Now turn the tenon upright and decide whether to saw the cheeks or split and pare them. The broader the cheeks, the easier splitting looks. This is especially true if you don't have one of the larger sizes of backsaw, and your large ripsaw seems too coarse for the job.

When splitting, work your way back with several splits so you can see how the grain runs. Splitting always needs smoothing and trimming with a plane or a broad paring chisel. Paring chisels are thin bladed and tang handled—made to be pushed, not hit with a mallet. Hold the rail in the bench hook as you slide the paring chisel, bevel up, across the grain, taking shavings until you just touch the lines left by the mortising gauge.

In narrower tenons, you can finish the surface entirely with the saw, using the same touch-but-leave-the-line accuracy. When you clamp the rail in the vise for sawing, lean it away from you so that you can saw diagonally, watching the lines on the side facing you where the saw goes in and on the end grain where the saw comes out. Turn the piece about and saw from the other side. Slow down as you bring the saw around square to meet the shoulder kerf and finish the cut. Save the sawn-off waste piece for wedges and pegs.

As it is now, the mortise setback requires that you saw away the outer portion of the tenon. This leaves the mortise strong, but means that up to a third of the connection between leg and rail is just a butt joint with no resistance to twisting and no glue surface. Light could even shine through it if the leg shrinks back.

Here's where you decide if you want to leave your tenons bare shouldered or haunched. The haunch is a little stub, square or sloped, that you leave above the tenon to fit into a reciprocal opening above the mortise. A regular haunch is square, and always present in door work to fill the groove cut for the panel. The marginally stronger sloped version is called a secret haunch because it diminishes to nothing and can't be seen once everything goes together.

We still have not cut the mortise or sized the tenon. Which do you fit to the other? In door work, part of the tenon is taken away when you plow the grooves for the panels. Cut the mortise first and you may forget or be surprised by the amount of wood removed by this groove and end up with an undersized tenon. But, working the tenons first, any loss to the groove would be immediately revealed. Making the tenon first also permits you to work by superimposing the completed rail tenon on the leg and confirm the position of the mortise. You already have the guidelines on the legs; the superimposition is just a further check.

Align the edges of the rail tenon with the lines on the leg and transfer the setback dimension. Use a rectangular cabinet scraper laid flat on the tenon and pushed against the shoulder as a square to run this line down the tenon. Saw the waste away, leaving a haunch if you wish. Now lay the tenon across the leg with the shoulder pushed flush. Confirm that the lines for the mortise are good and get ready to chop with the chisel—or bore with the auger.

On long, deep, or large mortises, removing much of the waste with an auger is a great help. On smaller, narrower mortises, the auger holes just slow you down. A table leg mortise is right on the line between auger helps and auger hurts. Since we have two intersecting mortises to cut, we'll do both, starting with the auger.

On broad tenons, split back to the line using the chisel as a wedge...

... then pare the cheeks smooth with the beveled edge of the chisel facing up.

... then pare the cheeks smooth with the beveled edge of the chisel facing up.

Some like to bore their mortises and pare out the waste.

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But chopping with the mortising chisel is usually cleaner and faster.

If we were going to do the whole of the mortise with auger and paring chisel, we could just hold the leg in the vise. But even using the auger, it helps to establish the mortise with a light pass of the mortising chisel. So, set the leg on the bench top under the holdfast, and set your chisel across the grain about 1/8 inch in from the ends of the mortise. Make a light tap with the mallet, driving the mortising chisel in about 1/8 inch. Walk the chisel down the mortise about another 1/8 inch, bevel facing the uncut wood and tap it in again. Work your way along, not going for depth, just making a clean recess to start in.

Find an auger bit smaller than the width of the mortise and set it at one end of the mortise, ready to bore. Set a try square next to the bit and true it up before you begin. A bit of masking tape on the auger or counting the turns will help you gauge the depth. Stop short of the space shared with the intersecting mortise. If you were to undercut this mortise, subsequent chiseling might splinter away the inside corner and weaken the leg.

With the first auger hole complete at one end of the mortise, move to the other end and bore the next one. Let each subsequent auger hole slightly overlap the previous one. You'll need to increase the spacing if a small auger in soft wood keeps getting pulled off track into the overlapping hole. In a perfect world, the final hole takes out the wood between two existing holes and overlaps them equally. Make it a game to adjust the overlaps as you bore along so that this comes out right.

When the boring is done, the mortise is a mess. It's the paring chisel's job to clean it up. With the bevel to the middle of the mortise, start slicing your way back through the webs of wood left by the auger. Until the sides are pared back to the line, the mortising chisel can't do much more than act as a gauge for the width. Were you to drive it in, it could wedge the webs apart and split the leg open. The mortising chisel gets honest work only at the ends of the mortise. Even then, leave that last 1/8 inch until after the other, intersecting mortise is done.

This second mortise is going to be all chisel work. First, for cleaner work, sink a shallow mortise with light cuts the same way we did earlier. The bevel always leads—that is, as you work away from a previously cut area, the bevel faces the uncut area. This orientation causes the chisel to ride back as you drive it down, and forces the chip out behind it.

Set the leg on the bench top under the holdfast. You may also want to put a hand screw clamp around the leg as extra insurance against splitting. Some shops use a special mortising stool, a low, stout sawhorse with a broad top. There you can sit on the work to hold it as you mortise. You can hop up on the workbench and sit sidesaddle on the work just as well.

Most workers like to get to the bottom of the mortise as quickly as possible and work out from there, so that's what we'll do. Position the chisel across the grain in the middle of the mortise with the bevel facing the right. Drive it in. Pull out the chisel and turn it so that the bevel now faces left. Set it less than 1/4 inch away from the back of the previous cut and drive it in again, pushing a chip to the middle. Repeat this left and right beveled chopping, until you have excavated a V to almost the full depth of the mortise. Throughout this, the bevel has been facing the walls of the deepening V and the flat side has faced the middle.

Once the V is to the bottom, it gives the chips a place to go as you work outward to the ends of the mortise. (Starting with a single auger hole and expanding it is another option.) I usually work without a central cut, and march up and down the mortise with a series of deepening cuts. In any case, working with the mortising chisel alone is smooth, precise, and efficient. Soon enough you'll come to the bottom. We have preserved the last 1/8 inch of the ends of the mortise to give us a shoulder to lever against. Now we can turn the chisel so that the flat side leads and work our way back into the ends, squaring them down to the bottom. You can check the squareness of the mortise end by holding the flat of the chisel or the edge of a ruler against it and comparing its inclination with a try square.

If you have left haunches on the tenons, cut the places for them to fit above

Working carefully everything should fit square and flush. If one of the tenon shoulders shows a gap, you could use your backsaw to kerf it in. Bring Pare the tenons to a miter. The sloping stub the framework together, squared up as desired. Set the backsaw flush against above the tenon is the secret haunch. the mortised piece and saw the ill-fitting tenon shoulder, taking it back one even saw kerf. Hold the joint from shifting as you saw the tenon's other shoulders in like fashion. Now all the shoulders of the tenon are at least one even kerf away from the mortised face. Knock the joint in and it should close flush. Repeat if needed. Of course, this kills any fine surface, but there are times and places where it's handy.

Finally, you can drawbore and glue the joints. Positioning the pegs is a matter of judgment best developed from observation. If you set the pegs too far from the shoulder, the leg may shrink back to them. Set them too close to the shoulder, and the leg can split, so give yourself at least a 1/4-inch setback.

In joinery, you always need to be careful about splintering. When you bore the peg hole through the cheeks, stick a scrap in the mortise and on the far side. This will make a clean cut and help the auger run true.

It's easy to mark the tenon for drawboring if you're using a screw-tipped auger. Just stick the tenon in the mortise and push the auger back through the hole in the cheek, leaving the impression of the screw on the tenon. Pull the tenon back out and bore its hole, offset up to a quarter of the peg diameter toward the tenon shoulder. Splintering in the tenon can also weaken it greatly, so back it up with a scrap, and clamp the sides if your auger screw might split it. Prepare the way for the final peg with a tapered iron or hardwood drawbore pin to burnish and pull up the joint between the table leg and rail.

the mortise. The tenon should make a good push fit. As when chopping the mortise, the leg will resist great pressure along the length of the grain but may be easily split by pressure across it. As you test fit each tenon, reach into the intersecting mortise with a pencil or scratch awl and mark where to bevel off its end. Shear off the bevel with the paring chisel and test the fit of the three pieces together.

Mortise Gauge
Foxtail tenons take a one-way trip into the undercut, stopped mortise.
Keyed, through tenons are called tusk tenons in joinery, but they lack the stubby, hidden shoulder of the carpenter's joint.
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Woodworking Tools and Installation Tips

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