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Photo 1
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Photo 2
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If you're doing woodworking on a shoestring budget, you'll be
happy to know that the only tools you need to cut tight-fitting mortise
and tenon joints are a square, knife and marking gauge, a fine-toothed
saw, and a couple of sharp chisels and mallet. You may also wish to use a
drill or brace and bit, to remove waste from the mortise ahead of
chopping it to final size, and of course, you'll also need to supply
some elbow grease and a healthy dose of patience.
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Photo 3
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Photo 4
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Photo 5
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Photo 6
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Photo 7
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Photo 8
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For
this example, I'll describe how to create a standard hidden (blind)
M&T joint. Other variations (through, pinned, etc.) are created by
a similar process. Start by laying out the joint on both male and
female parts. Use the square to mark the shoulders of the tenon on all
four sides, as well as the width of the mortise (Photo 1). Instead of a
pencil, use a scribing knife to make the marks (this will help when
cutting the joints later). Set the marking gauge to scribe a line that
defines the edges and ends of the tenon cheeks (the thickness of the
tenon). If both joint members are the same thickness and the M&T is
centered, you can use the same gauge setting to mark the sides of the
mortise (Photo 2).
I find it easiest to chop the mortise first, then cut the tenon and
pare it to fit. For faster mortising, use a brace or drill press fitted
with a bit that's equal to or slightly smaller in diameter than the
final mortise width. Bore a row of overlapping holes along the waste
area, stopping just short of the ends of the mortise (Photo 3) . It's
best to use a bit that leaves a flat-bottomed hole, like a Forstner
bit. Using a wide chisel, chop the waste on the sides back to the line.
I like to use a mallet to drive the chisel for the initial rough cuts,
then take the chisel in hand to do the final trimming. For the final
trim cut, set the chisel into the fine scribed line, and first just tap
the chisel and deepen the mark (Photo 4). Then, tap the chisel down to
the full depth of the mortise. To help keep the mortises' sides
parallel, you can use a scrap squared block of wood to keep the chisel
square to the stock, as shown in the photo (Photo 5). Trim the ends of
the mortise with a chisel that's the same width as the tenon.
To cut the tenon, clamp the stock vertically into a sturdy bench vise
and use a fine-toothed saw (tenon saw, backsaw, Japanese dozuki, etc.)
to carefully cut down the cheek lines to the shoulders. Use the scribed
lines to line up the saw teeth, erring to the outside of the line — it's
better to make the tenon too thick, rather than too thin. Hold the saw
so that it cuts diagonally (Photo 6) as you follow the line.
Then, tilt the saw in the other direction to complete each cheek cut.
Next, reclamp the member flat atop the bench and saw off the waste at
the shoulder on each side of the tenon (Photo 7). To make
the tenon easier to insert into the mortise, use a block plane or
chisel to chamfer the ends of the tenon slightly on all four sides.
Now carefully check the fit of tenon into mortise. If it's too thick,
DO NOT force it. Use a wide chisel to carefully pare off a little from
each cheek face as necessary (Photo 8), until the tenon fits
snuggly, but not tightly - remember, glue will swell the wood a skosh and
make the fit a little tighter.
Mortising by Machine
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Photo 9
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Photo 10
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Photo 11
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There are almost as many possible ways to chop mortises by machine as
there are power tools in a typical woodshop. One of the most popular
ways to machine chop mortises is to use a dedicated hollow chisel
mortiser or a drill press fitted with a mortising attachment. (Photo 9)
Both tools drill a square hole using a hollow square chisel fitted with
a special drill bit spinning in the center. The chisels and bits come
in different standard sizes - 1/4 in., 3/8 in., 1/2 in., etc. A fence
supports and guides the workpiece as during mortising. Overlapping the
square holes creates a rectangular mortise of any length. The
mortiser/drill presses depth stop sets the depth of the mortises.
For accurately sized mortises, it's best to cut the holes at the two
ends of the mortise first, then take overlapping cuts to remove the
material in the center area. I've found that it's good to keep an air
hose at hand, to cool the chisel and bit in between cuts — it tends to
overheat, especially when chopping deep mortises in dense woods.
The plunge router is another great mortising tool. Using a jig and
guide bushing with a router fitted with a straight bit, you can cut
accurate mortises quickly and accurately. The Mortise Pal jig, shown in
the photo (Photo 10), clamps and positions the
stock under clear plastic template that guides the plunge router's
guide bushing. Using an up-cut spiral bit makes routing a much smoother
operation, as the bit's spiral flutes cut with a smooth shearing action
that ejects chips, making the bit less prone to overheating and burning
the wood.
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Photo 12
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Photo 13
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Photo 14
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Photo 17
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Unlike a hollow chisel mortiser that produces square-cornered mortises
ready for regular tenons, a router creates mortises with rounded
corners. You must either cut round-edged tenons to match using a router
tenoning device, such as the Leigh Industries FMT (Frame Mortise and
Tenon Jig), or round over the edges of regular tenons with a rasp or a
knife (Photo 11).
Machine-cut Tenons
As with mortises, you have a choice of which machine or power tool to
use to cut tenons - band saw, router or table saw. My favorite method
uses a tenoning jig with the table saw. This jig, which can be store
bought or shop made, supports the tenon stock vertically and slides it
in a straight line over the blade (guided by either the miter slot or
rip fence) to cut the tenon's cheeks. It's usual to cut the cheeks one
at a time, rotating the stock around in the jig between cuts. (Photo
12) However, if you have a powerful tablesaw with an arbor that can
handle it, you can mount two blades (separated by a spacer that
determines the final tenon thickness) to cut both cheeks at once.
After the cheek cuts are done, use a regular miter gauge or crosscut
sled jig to make the shallow shoulder cuts that complete the tenons.
(Photo 13) For consistent tenon length, you can use the rip fence as a
stop - just temporarily clamp a wood scrap to the fence ahead of the
blade and but the tenon stock against it before sliding it through the
cut. This prevents the cheek cutoff from becoming trapped between the
blade and fence, as the blade will likely launch it across the shop
like a missile.
Although it doesn't cut as clean as the table saw, the band saw is great
for cutting tenons very quickly. You use the saw's rip fence to cut the
cheeks of the tenons first: Set the fence to blade distance, then flip
the tenon stock over between cuts. (Photo 14) Reset the
fence and use a slot-guided miter gauge to make the shoulder cuts. (Photo 15)
M&T Variations
Basic blind mortise and tenons are great joints for simple connections
between wood parts. But there are lots of variations to these simple
joints that allow them serve a wider range of applications, as well as
make them stronger and/or more decorative.
Angled Joints
Although we've focused so far on cutting square M&T joints, a lot
of the same layout and cutting processes are also relevant for angled
mortise and tenons. Angled M&Ts are perfect for connecting the
aprons and stretchers of a chair or stool to front and back legs that
aren't in alignment or are splayed. If the angle of the joints isn't
too great (say 30 deg. or less), you can chop a regular square mortise
and cut the tenon at an angled to fit into it. If you have only a few
such joints to cut, it's easiest to mark and cut these tenons by hand,
rather than setting up jigs and machines. (Photo 16)
Conversely, if you have piles of angled joints to cut, setting up the
necessary jigs and machines will save time in the long run.
Multiple Tenons
If you're joining two solid-wood parts with a M&T that are wider
than 2-3 inches, the wood's natural expansion and contraction in this
cross-grain joinery will eventually cause the joint to fail. Therefore,
it's best to create multiple tenons that fit into individual mortises.
(Photo 17) Multiple tenons and mortises are cut just like
single joints; just divide the width of the stock into mortises/tenons
that are each not wider than 1-2 inches. The joint is useful both for
joining wide solid-wood frame members or panels used in carcase
construction.
Stopped and Haunched Tenons
When M&Ts are used to form the corners of frames or carcasses, the
side of a regular tenon protrudes on one side of the joint. This is not
only unsightly, but it compromises the strength of the connection. The
easiest way to remedy this situation is to stop the mortise short of
the end of the stock, then cut a shoulder on one side edge of the
tenon. (Photo 18) Another variation is to create a
"haunched" joint: You chop a stopped mortise, then use a chisel to trim
an angled slot that extends to the edge of the stock. The edge of the
tenon is then trimmed with a chisel to match the mortise.
Wedged and Pinned Tenons
While the primary strength of an M&T joint is due to it's solid
wood-to-wood connection, the joint depends on glue alone to keep it
from pulling apart. You can increase the withdrawal resistance of a
joint by adding wedges or pins. To keep blind M&T joints together,
add a pair of wedges. Sometimes called "foxtail wedges," these are
inserted into cuts made in the end of the tenon, all the way to the
shoulder. The ends of the mortise are undercut slightly with a chisel.
At assembly time, apply glue, then insert the foxtails part of the way
into their slots (Photo 19). As the joint is driven home,
the wedges hit the bottom of the mortise and expand the tenon.
Pinning
is another way to increase an M&Ts withdrawal strength. Drill a
hole through the assembled joint and drive a pin into it. (Photo 20)
The pin can be round or square. For round pins, start with a dowel
that's just a bit larger in diameter than the hole and trim the dowel
with a block plane, leaving facets to help lock the dowel into the hole.
Through Tenons and Keys
Unlike regular blind joints, through M&Ts have a tenon that passes
all the way through and shows at the bottom of the mortised member.
Creating through M&Ts is simple: Just chop your mortise all the way
through the stock and cut the tenon as long as the width of the mortise
member (or longer - see next paragraph). For a clean joint without
tearout, rip your mortise stock 1/16 in. or 1/8 in. wider, then trim
off the bottom edge after mortising. Cut the tenon about 1/16 in.
longer than final length, then plane or sand its end flush after the
joint is glued up.
Visible wedges add a neat decorative touch to a through tenon that can
make your project look more "handmade." To add a wedge, cut a kerf
across the tenon diagonally with a fine-tooth handsaw (Photo 21). After
the joint is assembled, carefully drive a thin wedge into the kerf. For
maximum visibility, make the wedge from a contrasting-color wood. Trim
the wedge after the glue dries. A neat variation is to extend the tenon
1/2 in., 3/4 in., or more past the bottom of the mortise and add a
wedge (sometimes called a "tusk") to fasten the joint together (Photo
22). Used without glue, a keyed tenon allows you to take a
mortise-and-tenoned project apart.
Loose Tenons
Another variant of the M&T is the loose tenon joint. Like a dowel
or spline joint, a loose tenon uses a separate component to join two
pieces together: A tenon piece that's twice as long as usual fits into
a pair of mortises chopped in both halves of the joint. (Photo 23)
When glued up, loose tenons are almost as strong as regular M&Ts,
and can be quicker to create. On the down side, routing mortises on the
ends of long frame members will require either a special router jig or,
when hollow-chisel mortising, a floor model drill press and special
stock-clamping setup.
Cut the loose tenon stock into a short-grain wood strip, then cut off
the individual tenons, sized ready to insert into square-edged
mortises. If you've routed the mortises, round over all four edges of
the tenon strip before cutting it up.
The Domino System
Undoubtedly the quickest way to create loose tenon joints is with
Festool's ingenious Domino system (Photo 24). The system
revolves around a well-engineered power tool with a bit that both
rotates and oscillates side to side as it plunge cuts mortises as
quickly and easily as a biscuit joiner cuts slots. The Domino system
employs loose tenons made of compressed wood that come in a variety of
thicknesses and lengths. When these tenons are glued into their
mortises, the water-based glue swells the wood, creating a very strong
joint.
Sandor Nagyszalanczy is a furniture designer/craftsman.
writer/photographer and regular contributor to Woodworker's Journal.
His
books are available at: http://sandorsworkshop.com/Books.html