110149042.pdf

Double Mortise and Tenon

Improves Joint Strength

Add structural integrity to delicate furniture parts

CRAIG

VANDALL

STEVENS

he mortise-and-tenon joint and its

many variations have long been a

preferred method for joining two

pieces of wood at a right angle or close to

it. One of my favorite versions is the dou-

ble mortise and tenon. I use it to increase

the strength of a joint on relatively small

furniture parts, such as those on the free-

standing room screen I built last year (see

FWW #141, pp. 70-73). I knew the screen

frame would be subject to some flexing in

daily use, and I wanted to be certain that it

would hold up to the stresses.

You'll often find the double mortise-and-

tenon joint on chairs and window sashes-

projects in which structural integrity on

visually delicate pieces of wood is essen-

tial. By doubling the surface within a wood

joint, you can greatly improve the joint's

strength without increasing the size of its

parts. What follows is an account of how I

design and execute this joint. To cut the

mortises I use a simple plywood jig and a

plunge router equipped with an end-mill

bit. For the tenons I use a combination of

tablesaw, bandsaw and chisels. Others may

prefer another technique, such as doing

the job entirely by hand or using a mortis-

er or drill-press setup. Any way you choose

to cut it, the benefits of employing a dou-

ble mortise and tenon in your work are

worth the extra effort required.

Prepare the stock

with the end use in mind

I always start milling the lumber for a pro-

ject several days ahead of time, then set it

aside to stabilize. Initially, I flatten stock

with a jointer and rip the individual pieces

a bit oversized with the bandsaw. (I use a

bandsaw rather than a tablesaw most of the

Cut the mortises as deep as

you can without going all the

way through. The deeper you

make the mortises, the

stronger the joint will be.

Minimize the space

between the two tenons,

but make it wide enough

to be cleaned out easily.

Use a double mortise-and-tenon joint

when strength is an issue, as it is in

the freestanding room screen shown

above. By doubling the surface area

of a glue joint, you can greatly

improve its ability to hold together

and to withstand stress.

Tenon thickness

will depend on

the size of the

workpiece.

time because it's a safer and quieter ma-

chine, and it produces less waste.) When

the wood has stabilized, all of the pieces

can be rejointed on two adjoining faces to

flatten out any springback that has oc-

curred and then brought down to their fi-

nal thicknesses with a planer. It's always a

good idea to mill some extra stock for set-

ting up the joinery and to use as backups if

you make a mistake along the way.

With furniture parts that will eventually

be sized differently, I prefer to mill all of

the stock to the same thickness, complete

the joinery and then bring the thinner

pieces down to their final sizes with the

thickness planer or a handplane. For ex-

ample, on a conventional table, you can

mill the legs and rails first to an equal thick-

ness, then cut your mortises and tenons.

After that, send the rail pieces through the

planer again to make them thinner and

provide a step-back from the surface of the

legs when they're joined together.

sible to maintain a strong joint. At this

stage, having in hand a good sketch of the

joinery detail is especially helpful.

Estimate the amount that each face will

be stepped down and then experiment

with different tenon sizes until you have a

layout that will be strong without creating

any weak areas in the joint. I leave the

space between the tenons at least a little

wider than the narrow in. chisel that I

use to clean up that area. On the outside of

both tenons, I'll often leave only a narrow

shoulder, about in. wide, which allows

some leeway in deciding the thickness and

spacing of the tenons.

Mark and cut the joinery,

starting with the mortises

After layout, transfer the width and length

of the tenons to the mortise workpieces

using a marking knife. If you have to cut

more than a few mortises, make a story

Tools for multiple

marking. A story

stick and a marking

knife add accuracy

and reliability to a

repetitive process.

Think through the layout first—I lay

out the joinery dimensions for the tenons

first. The tenons need to be as long as pos-

stick from a straight scrap of wood and

tack a cleat onto one end. Hook the cleat

over the appropriate end of each work-

piece, then transfer the mortise locations

with the marking knife.

As I mentioned before, I lay out the

tenons first, but when it comes time to cut

the joints, the mortises come before the

tenons. It's important that the two mortises

line up with each other and be cut square-

ly. To cut mortises, I use a simple plywood-

jig design based on one that Tage Frid used

(see FWW #82, pp. 52-55).

The jig holds the workpiece in place

while a four-flute center-cutting end mill

mounted in a plunge router accurately cuts

the mortise. The router fence simply rides

against the outside of the jig. Clamp stop

blocks to the jig to create mortises of iden-

tical length, and use a chisel to square up

the ends of the mortises.

To fit a double mortise-and-tenon joint

successfully, focus on properly fitting the

outside cheeks of the tenons before deal-

ing with the inner cheeks. Think of the first

setup as fitting an extrathick tenon into an

extrawide mortise. Chop away the wood

separating the two mortises on one of the

practice pieces and use this practice mor-

tise later when you're setting up the table-

saw to cut the tenons.

A SIMPLE JIG FOR ROUTING MORTISES

This jig has an important feature. Adjustable stops on this mortising jig limit the distance the

router can travel and keep the length of all of the mortises consistent.

Two tenons are not twice the work—A

tablesaw will cut the two tenons very accu-

rately, and you can use a test piece with the

tenons marked and drawn on the end of it

to set up the cut.

A sliding cutoff box really helps achieve

consistent results in crosscut work, and a

shoulder cut along the cheeks is a good

place to start. Raise the blade so that it's

slightly below the pencil line representing

the tenon cheek. A marking-knife tick on

the side of the workpiece indicates the

length of the tenon, based on the depth of

the mortise. The length of the tenon should

be in. or so shorter than the depth of the

mortise to ensure a snug fit and to allow

room for excess glue that gets pushed into

the mortise during assembly.

A stop block keeps the shoulder cut con-

sistent as you rotate the workpiece. After

you cut the first two cheek shoulders, the

blade height will probably need to be

changed to cut the other two sides. And

here's another secret: Before cutting these

two adjacent faces, put a piece of masking

tape on the end of the stop block to bump

Help with the handwork. A thick block of

wood clamped firmly to the workpiece serves

as a guide to chisel the ends of the mortises

true and square.

Steady as she goes. The router base sits

firmly on the top of this jig as the router fence

indexes the location of the mortise.

away the workpiece slightly. This will keep

the sawblade from nicking the previously

cut shoulders.

With all four shoulders cut, clamp a

straight piece of wood to the tablesaw

• fence. This auxiliary fence should be

around 5 in. high or higher; check it with a

square to ensure that it is 90° to the saw

table. With this setup, one hand slides

along the top of the auxiliary fence, hold-

ing the workpiece firmly in position, while

the other hand helps push the workpiece

through the cut. Both hands are kept safely

away from the blade. Set up a clean, sharp,

ripping blade to cut just below the shoul-

der cuts. Adjust the fence to ait the practice

workpiece a little proud of the outside of

the tenons.

When you use this method, the waste

piece falls away from the action rather than

being trapped against the fence. Use a

steady feed rate to move the workpiece

through the cut, then rotate and cut the op-

posite side. Ideally, with the first pass, the

tenon will be too fat to fit into the practice

mortise you prepared earlier. Readjust the

fence and repeat the cuts until the practice

pieces go together with no sloppiness, us-

ing only hand pressure.

By fitting the outer cheeks first, there's no

guessing whether it's the outer or inner

cheeks that are preventing a nice fit. After

you cut all the outer tenon cheeks, you can

reset the fence to cut away the space

between the double tenons. Sneak up on

the final fit, readjusting the fence until the

tenons fit nicely into a pair of mortises.

The bandsaw makes short work of cut-

ting the tenons to their proper width.

Again, use the layout marks on your set-up

pieces to determine the location of the

bandsaw fence, and clamp a stop block to

the fence to prevent the blade from cutting

into the shoulders. Start a little wide,

bumping the fence until the tenon is the

same width as the squared-off mortises.

Use a chisel to clean up the corners of the

shoulders, and take care to avoid damag-

ing the adjacent shoulders. You'll need a

narrow chisel to fit between the tenons and

pare away the waste. To make the final as-

sembly go more smoothly, you can use a

file or a knife to cut a slight chamfer on the

ends of the tenons.

TABLESAWN TENONS

A higher fence for

safety and stability.

The extra height of the

auxiliary fence provides

a firm surface to press

the workpiece against

and helps keep hands

away from the blade. Us-

ing scraps for testing the

fit, make sure the two

outside cheeks fit snugly

into a mortise before

proceeding with the cuts

for the inside cheeks.

Almost there. Tweaking

the final fit requires fre-

quent checks with the

set-up scraps before

you can complete the

final cuts on the work-

pieces. A snugly fit

joint will go together

smoothly with moderate

hand pressure.

Craig Vandall Stevens studied woodworking at

the College of the Redwoods. Today he makes

custom furniture in Sunbury, Ohio.

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