Wedged Mortise and Tenon

Wedged Mortise and Tenon 5F00 lead Wedged Mortise and Tenon

Wedged Mortise and Tenon

This joint will
never loosen!

By Tom Caspar

 

Tap, tap, tap. The wedges go
home, the glue squeezes out and a
big smile lights up your face. “This
joint isn’t coming apart for a hundred
years,” you say. “It’s as solid as
a rock!”

Making a wedged mortise-andtenon
joint is richly rewarding. Once
you understand how it works (see "How the Joint Works," right), you can’t help but
admire the joint’s elegant simplicity.
It also sends a message. A wedged
joint says to one and all, “This was
made by a skilled woodworker.”

Where could you use a wedged
joint? It’s a candidate for any joint
that receives a lot of stress. A table
base such as the one shown here,
is a good example. Pushing or leaning
on the table might slowly force
a standard joint apart, but wedges
keep this joint locked together.

The wedged mortise-and-tenon
joint isn’t difficult to make, but you
should have some experience making
standard mortise-and-tenon joints
before tackling it.

 

Tools required

To make this joint, you’ll need a tablesaw,
drill press, plunge router, chisel and
a bandsaw. If your mortise’s width is 5/8"
or more, like the mortise I made, you’ll
need a 1/2" dia. top-bearing flush-trim
bit. If the mortise is more than 3/4" deep,
you’ll need a bottom-bearing flush-trim
bit (see Source, below). For a mortise
less than 5/8" wide, you’ll need a straight
router bit and a fence or jig for your
plunge router.

 

Rout the mortise

Before you begin your project, make a
prototype joint (see “Designing Your
Wedged Joint,” right).

It’s good practice to start with the
mortise for any type of mortise-and-tenon
joint. It’s easier to fine-tune a
tenon to fit a mortise than the other
way around.

This is a through-mortise, meaning it
goes all the way through the workpiece.
My favorite way to make a fairly large
one is to remove most of the waste on
the drill press and then use a plunge
router and template (Photo 1). This
method works particularly well in thick
stock, because it makes a mortise with
absolutely straight walls. That’s important
for appearance’s sake in a through
joint, because you can clearly see from
the outside how well the mortise and
tenon fit together.

Make the template from plywood or
solid wood by gluing four pieces together.
The inner two pieces are the exact
width of the mortise, but their overall
length is unimportant. The outer pieces
must be long enough to allow room for
clamps. Space the inner pieces apart by
the length of the mortise.

Before you start routing, use the template
to draw the mortise on the workpiece.
Drill out most of the waste using
a Forstner bit that’s 1/16" to 1/8" smaller
than the mortise’s width. Make overlapping
holes to remove as much wood as
possible. Rout the mortise (Photo 2).

 

Taper the mortise

Tapering the ends of the mortise
requires a razor-sharp chisel; there’s no practical
way to do it
with a router.
You must
use a chisel
to square the
ends of a routed
mortise anyway;
so tapering isn’t that
much extra work.

Make a 1-1/2" to 2"
thick block to guide your
chisel. Cut one end square. Cut
the other end at the angle you’ve
chosen for tapering the mortise and
wedges. I’ve found that a 3-degree angle
works well.

Use the guide block’s right-angle
end to square the back of the mortise.
Chop about one-fourth of the mortise’s
depth. Turn the workpiece over and
position the block a short distance
away from the end of the mortise
(Photo 3). The exact distance depends
on the mortise’s depth. You’ll want the
taper to extend approximately three-fourths
of the way down the mortise.
On a 3-degree taper, shifting the block
1/16" from the mortise’s ends results in
a taper about 1" deep.

 

Make the tenon

Make the tenon any way you want. I use
a tablesaw tenoning jig to cut its cheeks,
a bandsaw equipped with a fence to rip
its top and bottom sides and a tablesaw’s
miter gauge to cut all four shoulders.
The tenon’s length is up to you; it
can be flush or stand proud of the joint.

Fit the tenon to the back, untapered
side of the mortise. It should be no
more than a paper thickness smaller
than the opening. If your tenon stands
proud, chamfer its end using a block
plane or file.

The next two steps are unique to this
joint: making the strain-relief holes and
sawing kerfs for the wedges. Start by
marking and drilling the holes (Photo 4).
Their location and diameter determine
the flexible strips’ thickness. In most
woods, such as the white oak I’m using
here, I drill 1/4" dia. holes centered 1/4" from the edge. This makes the bending
strip a flexible 1/8" thick. Holes
that are only 1/8" are commonly used
for this joint, too, for types of wood
that bend easily, such as maple and
ash.

For the saw kerfs, draw lines that
connect the holes to the tenon’s
end. Traditionally, the kerfs go to a
hole’s center, but I aim for the hole’s
inside edge (Photo 5). Looking headon
at the completed joint, I believe
this divides the tenon into more
pleasing proportions.

 

Saw the wedges

Make wedges using the tablesaw
(Photo 6). This method allows you
to cut a precise angle and fine-tune
each wedge’s thickness. Make a
wedge blank from straight-grained
wood. I prefer one that contrasts
in color from the tenon. Make the
blank about 3/4" thick and as wide
as the mortise.

Tilt the blade to the guide block’s
angle. Here, it’s 3 degrees. Raise the
blade to make wedges that are about
1" longer than the tenon.

For a trial cut, position the stop
block so the thin end of the wedge
is the same thickness as the tenon’s
kerfs. Clamp the blank to a tall fence
using a wooden handscrew. (A
wooden clamp protects your blade
from damage if you accidentally
place the clamp too low.) Flip the
blank around to cut a second wedge.
Remove the blank and crosscut the
wedges by hand or on the bandsaw.
File chamfers all the way around the
wedges’ thin ends.

 

Test the wedges’ fit

Push the tenon all the way through
the mortise—without glue, of
course. Tap in the wedges, but not
too hard (Photo 7). If they’re too
skinny, cut them shorter or adjust
the stop block and saw new ones.
If your wedges become stuck, pull
them out using locking pliers. The
wedges should go in as far as possible
but not be so long that they hit
bottom before fully spreading the
tenon. Marking the bandsaw kerf’s
length on each wedge will help you
prevent this problem.

 

Assemble the joint

When everything is ready to go
together, you only have to put glue
on the mortise’s long sides and the
tenon’s cheeks. Clamp the joint so
the tenon’s shoulders are tight to the
mortise. Then brush glue into the saw
kerfs and the mortise’s tapered spaces.
Tap in both wedges and clean up the
glue squeeze-out. Saw off the wedge’s
excess length after the glue dries. Use
a file or low-angle block plane to level
the wedges flush to the tenon.

Click any image to view a larger version.

Wedged 2D00 MT 5F00 how 2D00 the 2D00 joint 2D00 works Wedged Mortise and Tenon

How the Joint Works

Here’s a cutaway view of a wedged mortise-and-tenon
joint. Driving in the wedges forces the tenon to flare
into a fan or dovetail shape. The mortise is tapered to
match the angle of each wedge. Like a dovetail, this joint
can’t pull apart after the wedges go home.

This tenon has two unusual features: saw kerfs that create
flexible strips and holes that disperse the strain that the
wedges create. The wedges cause the strips to bend; the holes
prevent the bend from splitting the rail.

Wedged 2D00 MT 5F00 1 Wedged Mortise and Tenon

1. Make the mortise before you cut the tenon. I use a shop-made template,
a drill press, plunge router and two flush-trim bits to make large throughmortises
(Photo 2). The hole in the template is the exact size of the mortise.

Wedged 2D00 MT 5F00 2 Wedged Mortise and Tenon

2. Here’s a cross section
of the mortise
in various stages
of completion. You
make it in four steps:
1. Drill out most
of the waste.
2. Follow the template
with a short
top-bearing flushtrim
bit.
3. Using the same bit,
remove the template and rout deeper.
4. Flip the workpiece and finish the mortise
with a bottom-bearing flush-trim bit.

Wedged 2D00 MT 5F00 3 Wedged Mortise and Tenon

3. Using an angled guide block and chisel, taper the mortise’s ends into
a flared shape. The taper leans 3 degrees from square. Make the taper
about three-fourths the depth of the mortise. Turn the mortise over and
square the remaining corners.

Wedged 2D00 MT 5F00 designing 2D00 wedged 2D00 joint Wedged Mortise and Tenon

Designing Your Wedged Joint

Each part of a wedged joint must often be
tailored to fit the joint’s size, intended strength
and type of wood. Make a prototype following
these steps:

1. Substitute a notch made with a dado set
for the mortise (see “How the Joint Works,”
page 2). Taper both of the notch’s sides by
angling the miter gauge.

2. Make a full-size tenon. Observe how well the
flexible strips bend. You may be able to use smaller
strain-relief holes or no holes at all.

3. Experiment with the notch’s angle. The
wider the taper, the stronger the joint. My taper is 3 degrees, but you can
increase it up to 8 degrees.

4. Test the bend. My flexible strips are only 1/8" thick opposite
the strain-relief hole; so they bend easily. Depending on the
wood, this thickness can be increased to 1/4" or so
to improve the joint’s appearance.

Wedged 2D00 MT 5F00 4 Wedged Mortise and Tenon

4. Cut a tenon to fit tightly into the back of the mortise, where there’s no taper.
On the tenon, draw a centerline directly opposite the point where the mortise
begins to taper outward. Drill two strain-relief holes all the way through the tenon.

Wedged 2D00 MT 5F00 5 Wedged Mortise and Tenon

5. Saw kerfs in the tenon to receive the wedges. This creates strips that can flex
without breaking. I aim for the inner edge of the hole so that the kerfs don’t
end up too close to the tenon’s edges.

Wedged 2D00 MT 5F00 6 Wedged Mortise and Tenon

6. Cut extra-long wedges on the tablesaw. Tilt the blade 3 degrees—the same angle
as the guide block you used to taper the mortise. Crosscut the wedges from the
blank with a bandsaw.

Caution: You must remove the blade’s guard for this cut. Be careful.

Wedged 2D00 MT 5F00 7 Wedged Mortise and Tenon

7. Test-fit the wedges without glue. You have to get their thickness just right to completely
flare the tenon before the wedges hit bottom. Adjust the tablesaw setup
until the wedges are the right size. You’re ready for gluing.

 

Sources

Note: Product availability and prices are subject to change.

MLCS Woodworking, mlcswoodworking.com,
800-533-9298, 1/2" Pattern/ Flush-Trim Bit, 1/4"
shank, #16509; 1/2" Flush-Trim Bit, 1/2" shank,
#17803.

This story originally appeared in American Woodworker July 2006, issue #122.

big 5F00 fcnawjul06 Wedged Mortise and Tenon

July 2006, issue #122

Purchase this back issue.

 

 Wedged Mortise and Tenon

No related posts.

Comments are closed.