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The dado is prime-choice joinery. It
follows that hoary adage of woodworking, “Use the simplest joint
that will work.”
It certainly works. The dado
joint has a centuries-long history of use in cabinetmaking, furniture
making, just in woodworking in general.
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The cross-grain cut is the dado, the
long-grain cut is the groove.
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It definitely is simple. A dado is a
square-walled, flat-bottomed channel cut across the grain of the
wood. (When it runs with the grain, the channel is called a groove.)
You cut a dado or groove into one board, and the end of a mating
board fits into it. One well-placed, properly sized cut makes the
joint.
The dado does not have to be deep to
create a strong joint. In solid wood, 1/8" is deep enough, 1/4"
in plywood, MDF, or particleboard. Just this shallow channel helps
align the parts during assembly, and the ledge it creates is enough
to support the weight of a shelf and everything loaded on it. The
dado also prevents the shelf from cupping.
The one stress it doesn’t resist
effectively is tension. It doesn’t prevent the shelf from pulling
out of the side, in other words. Only glue or fasteners can do that.
Because all of the gluing surfaces involve end grain, the glue
strength is limited.
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For the optimum joint strength, a
dado's width must be just a few thousandths greater than the
thickness of the mating piece. The dado on the left is obviously too
wide, the center dado acceptable (though just a skoshe wide), while
that on the right is dead on.
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Keep in mind that a snug fit of mate to
dado is essential for a strong joint. The cut must be to just the
right width. The bottom needs to be smooth and flat, the sides
perpendicular. Glue won't compensate for a rough, ill-sized dado. Get
the fit right.
How to cut a dado? A dado cutter in a
table saw hogs out through dadoes in no time at all. No question. But
the router has a lot of pluses that compensate for its lower cutting
speed.
One is that its cuts are precisely
sized -- the diameter of the cutter determines the width of the dado.
Another is that the cut is invariably clean and square -- no raggedy
bottom. Yet another is the router's ability to cut stopped dadoes.
In the end, the router is the one tool
that will handle all the dadoing and grooving you'll do in
woodworking. No workpiece is too big or too small. Any router will do
-- large or small, fixed-base or plunge.
Typical
Step-by-Step
To avoid problems, be as methodical as
you can in setting up a cut. The process includes installing a bit in
the router, making the depth-of-cut adjustment on the router,
securing the workpiece, and positioning a guide. The trick in routing
-- as in all woodworking -- is to get the setup right (if not the
first time, at least before you actually cut the good wood).
Choosing
a bit
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Cut dadoes with ordinary straight bits.
If you have a choice, select the bit with shortest cutting edges. The
3/4" bit on the left and the two 1/4" bits on the right are
ideal; the other two will do the job, but shorter would be better.
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You cut dadoes and grooves with
straight bits. That's pretty simple. You don't need any kind of
specialized dado-cutting bits: The commonplace, conventional straight
bit -- the double-flute -- will give excellent results for all but
the most troublesome workpieces.
A variant of the double-flute straight
that is valuable for dadoing plywood is the shear-cut straight.
This bit's cutting edges are at a
slight angle (usually about 3 degrees) to the bit’s axis. A
down-shear bit slices down on a face veneer, yielding a cleaner edged
cut.
Straight bits are made in an enormous
variety of sizes. Cutting diameters range from 1/16" to 2",
with flute lengths from 1/4" to 2-1/2". Even metric sizes
are available.
So how do you choose? The rule of thumb
has long been: Use the shortest length that'll do the job. Dadoes
seldom exceed a 1/2" depth, so a bit with the 3/4"-long
flutes -- a size common in bit sets -- is plenty long. Shorter is
better if you do a lot of dadoing. For stopped dadoes, make sure you
are getting bits that have plunge-cutting ends. Most do these days,
but check before you buy.
Setting
the cut depth
With the chosen
bit secured in the router's collet, the next step is to set the cut
depth. The conventional wisdom is that the router is a
trimming machine, that you should waste no more than 1/4" of
stock per pass. Given that 1/4" as about as deep as a dado ever
needs to be, you're set for a one-pass job.
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| Set the bit's cut depth however you
prefer. Two good approaches are shown here: with a small square and
with thickness gauge of the desired thickness. |
There's no one
depth-setting technique you have to use. Hold a small square or rule
against the router base beside the bit. Or adjust the bit's top flush
with the surface of a setup shim. Use a setup gauge.
However you
choose to do it, set the cut depth, and lock it.
Marking
the cut location
Laying out the cut (or cuts) is
straightforward. Of course, you want to locate the cuts so your
project can be assembled in proper alignment. But you also want to
lay out -- locate -- the cut in a way that makes it easy to position
the guide fence accurately. And you want to minimize the measuring,
squaring, and markup.
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Here's the dado I want to cut all laid out. But elaborate marking isn't necessary for routing it. The red center line (or
a similar mark delineating one of the shoulders) is the only layout you need.
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A sensible way is to mark the center
line -- and just the center line -- of the intended cut. The guide
fence then is offset a distance equal to the radius of your router's
base. The width of the cut itself is irrelevant with this layout. And
the offset distance is the same to either side of the line.
An alternative is to mark just one edge
of the intended cut, and offset the fence from that mark with an
easily made setup gauge. (See
"Setup Gauge," below.)
Setting
a guide fence
Choose your guide fence with
appropriate care. A scrap is fine, but be sure it is perfectly
straight, with a square edge for the router base to reference. It
must be wide enough and stiff enough to resist deflecting when the
router bears against it and wide enough that the clamps securing it
to the work will be clear of the router as you rout the dado. It
should overhang the work on both ends so the router can be braced
against the fence before the bit engages the wood and again, after it
emerges from the cut.
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A guide fence is a must, and you've got a variety of choices. I prefer my shopmade T-square (center), which usually can
be secured with a single stout clamp. Just a straight scrap works fine, but requires two clamps. A Tru-Grip is an excellent option.
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You can use a manufactured
straight-edge clamp like the Tru-Grip. While separate clamps aren’t
needed with such a device, you still need to align it with a square,
because it won’t square itself to an edge. You’ve got to check
the alignment.
A shop-made T-square,
accurately assembled, is a great dadoing guide. It will square
itself, and all you have to do is clamp it. (See "T-Square,"
below.)
Before clamping the fence to the work,
let's take a time out to review router dynamics. You want to harness
those dynamics to keep the tool under control. Here's the key: All
routers spin a bit in the same direction -- clockwise. You want to
move the router counter to the bit's rotation, so you move the
router counterclockwise.
Try this little exercise. Stand a
plunge router on a board, switch it on, and plunge the bit into the
wood. Now let go of the handles. It will just stand there, wailing,
perhaps lazily rotating (clockwise, of course!).
Regrasp the handles and push the
router. The hole will elongate into a groove as the bit cuts a path.
But the router won't move in a straight line, your best efforts
notwithstanding. Because you are pushing, the router will veer to
your left. Pull it; it will veer to your right. In both cases, the
router is veering counter to the clockwise spin of its bit.
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Cutting freehand with a straight bit will demonstrate to you where to locate a guide fence to achieve a straight cut. The
router will always tend to veer counterclockwise.
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That counterclockwise veer is what you
must exploit. Position whatever guide you are using where the router
will bear against it as it is moved. In the case at hand -- guiding
the router along a stationary fence -- the fence must be on the
counterclockwise side of the router's path. If you will push the
router, position the fence on the left. If you will pull it, position
the fence on the right.
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Use a square to ensure your fence is properly aligned as you position and clamp it.
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So, position and clamp your fence.
Measure from the desired cut location, to the correct side, and
position your fence. Align it with a try square, and clamp both ends
to the workpiece. Feed the router left to right along the fence (or,
to put it another way, counterclockwise).
Making
the Cut
Don your earmuffs and safety glasses.
Plug in the router. Set the base on the work, its edge against the
fence, the bit clear. Switch on the router and advance it across the
work, keeping it against the fence.
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Cutting a dado:
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Begin with the router switched off. Set the router base on the work, its edge against the fence, the bit clear.
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Begin with the router switched off. Set the router base on the work, its edge against the fence, the bit clear. Switch on
the router and move it briskly along the fence.
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When the bit clears the work at the end of the cut, switch off the router and wait for the bit to stop spinning before lifting
the tool from the work.
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Feed rate is significant. Beginners
tend to be hesitant about moving the router through a cut, feeding it
haltingly along the fence. They tend to move the router much too
slowly. That prolongs the operation and degrades the cutter quickly.
The goal is to get a good quality cut without overtaxing the router
and the bit. With experience you develop a "feel" for
pacing and loading. Without conscious thought, you weigh the power of
the router, the configuration of the bit, and even the hardness of
the stock. With experience, you develop a feel for how fast you can
go.
When the bit emerges from the
workpiece, ending the cut, switch off the router and let it wind to a
stop before lifting it off the work.
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Blowout occurs as the bit emerges from the wood at cut's end. A scrap clamped to the work's edge prevents damage to it;
instead the blowout affects the scrap.
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One caveat: When the cut is finished,
suppress the urge to return the tool to the starting point without
retracting the bit or without switching off the power. While you may
get away with the move, the risk is that the bit will snag, pull the
router away from the fence, and mess up your dado.
Set the tool aside and take a look at
your cut.
Any time you rout across a board's
grain, you're going to get some degree of splintering. Most common is
blowout -- a ragged flap of wood fibers that the emerging cutter
folds out rather than cutting off. This flap can tear along the edge,
marring it. The best remedy is avoidance: clamp scrap strips to the
edges. The blowout mars the scrap, not the workpiece.
Another bit of splintering sometimes
occurs along the surface edges of the entire cut -- I call 'em
"fuzzies." A few swipes with sandpaper remedy the fuzzies.
Don't view this as a sign of a dull bit; they simply are symptomatic
of cross-grain cuts.
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Rout across the grain, and the wood fibers right at the surface tend to shred and lift, leaving fuzz along each side of the
cut. Light sanding removes them.
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A more malign form of the fuzzies
afflicts plywood. Cutting a dado across the grain of plywood's face
veneer often lifts splinters that extend beyond the dado's edge,
leaving ugly divots. A down-shear cutter may prevent this
splintering, because the cutting edges push down on the veneers as
they slice.
Customizing
the Dado Width
Remember when I said that getting a
snug-fitting joint is essential to joint strength? This dual-fence
jig is a cheap but effective way to guarantee that snug fit. You
build it to
use with your router (or one particular router if you have more than
one) and a specific bit, say at 3/8" or 1/2" dia. bit. (See
"Dual-Fence Jig," at left.)
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The dual-fence dado jig makes dadoing virtually foolproof. The router is trapped between fences and can’t veer
off-course, regardless of your feed direction. Even better, the width of the cut can be adjusted easily to perfectly match the thickness of material the cut is for.
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The jig has two fences, one of which is
glued and screwed to two crossbars. The crossbars are perfectly
perpendicular to the fixed fence. When either crossbar is tight
against an edge of the workpiece, then the fence will be at a right
angle to that edge.
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Guided by my jig, any router with a 6" dia. base and a 3/8" bit, cuts dadoes from 3/8" wide (left) up to 1 1/4" wide
(right). Best of all, it cuts dadoes customized to the imprecise and typically varying thicknesses of plywood.
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The second fence -- a duplicate of the
fixed one -- is bolted to the two crossbars so it can be moved toward
or away from the fixed fence. When the two fences are as close
together as they can be, the router baseplate will just fit between
them and it'll cut a dado the width of the bit you've selected to use
with the jig. Adjust the movable fence to set the width of dado that
can be cut. With 1-1/4" of adjustability, you can, for example,
cut dadoes between 3/8" and 1-1/2" wide.
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The gap between the fence bases represents the cut width. Pinch a scrap or two of the working material between them
to set up the jig. The cut you make will match their thickness perfectly.
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The best way to set that adjustable
fence: insert a couple of pieces of the material you want to house in
the dado between the two fence bases. Pinch them tight, and tighten
down the movable fence. Tug the samples out from between the bases,
and the jig is ready to be positioned on the work. When you cut the
dado, its width will match the thickness of the pieces.
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With the jig, routing any but the narrowest cuts requires at least two passes. The first is made referencing one fence
(since I'm pushing my router away from me here, it's the fence on my left).
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Pull the router back, referencing the opposite fence, to make a second pass. If the cut is wider than 3/4", additional passes are required.
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Rout a blind dado from mark to mark with a plunge router. Align
the bit by eye on the correct side of the start mark (top),
and plunge the spinning bit into the work. Advance the
router through the cut, halting as the bit approaches the stop mark, and
retract the bit.
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Any width but the minimum, you rout in
two -- or more -- passes. Guide the router initially along one fence,
then guide it back along the other fence. Because the two fences trap
the router, you can't miscut the dado, regardless of the direction --
clockwise or counterclockwise—you move the router.
Stopped
dadoes
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Dadoes can be (top to bottom) through, stopped, or blind.
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When a dado or groove doesn’t extend
completely from one edge of a board to the other, it’s referred to
as a stopped cut or a blind cut. It can begin at one edge and end
before it reaches the other (half-blind), or it can begin and end shy
of either edge (full-blind).
You can make these cuts easily with a
fixed-base router, but face it, this work is exactly why we have
plunge routers. The chances for botching the job are much reduced.
Lay out the dado, indicating the cut’s ends. Make sure you can see
your layout marks, that the router won't obscure them. Set your
fence, position the router, plunge the cutter at the starting mark,
rout until the bit aligns with the end-of-cut mark, and retract it,
ending the cut.
If you are wary of cutting mark to
mark, use stops attached either to the workpiece or the fence to
physically corral the router. Screwing stops to the fence makes it
easy to duplicate stopped dadoes at multiple locations.
Routing
two sides at once
Making cabinets? Try dadoing both sides
of a cabinet at one time by lining them up side-by-side. This ensures
that the dadoes will line up, that the cabinet and its shelves will
be square when they're assembled. Just as important, this trick that
saves time, since you only set up the guide fence once.
Be sure you book-match the sides as you
set up; that is, abut either the front edges or the back edges (not
the front edge of one side to the back edge of the other). Align the
top and bottom edges very carefully, and clamp them together so they
can't shift out of alignment.
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Screw a stop to a T-square if you need to produce identical stopped dadoes in a stack of casework parts. Move the
T-square from one part to another, and the stop moves too.
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Dadoing bookshelf sides? Clamp them edge to edge, and rout both at the same time. Here I've protected the outer edges
from blowout by inserting strips of MDF between the clamps and the workpieces.
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Setup Gauge
A scrap of plywood or hardboard is all you need to make a setup gauge. With it, you offset the guide fence from a layout mark speedily and accurately.
Rip a 6"- to 12"-long strip to match the radius of your router’s baseplate edge. Position the piece overhanging the workbench edge and tack it down with a couple of brads or with carpet tape. Butt a fence against the edge and clamp the fence to the bench. Guide the router along the fence, cutting through the stock. Pry up the gauge and remove the brads or tape. You now have a gauge to position your fence or T-square when using that router-and-bit combination.
To use the gauge, measure and mark one edge of each dado. Align one edge of the setup gauge with the mark, and butt the T-square against the other edge. The T-square’s crossbar will ensure that the fence is square to the edge. The setup gauge ensures that the fence is the proper distance from the dado location.
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Using the setup gauge is as simple as aligning one of its edges with the layout mark, then butting the T-square against
the other edge. The T-square’s crossbar will ensure that the fence is square to the edge.
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Rout a blind dado from mark to mark with a plunge router. Align
the bit by eye on the correct side of the start mark (left),
and plunge the spinning bit into the work. Advance the
router through the cut, halting as the bit approaches the stop mark, and
retract the bit.
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T-Square
Make yourself at least one T-square. Simply glue and screw two straight-and-true scraps together in a T-shape. One piece, called the crossbar or the head, butts against the edge of the workpiece, and the other, called the fence or the guide or the blade, extends at a right angle across the workpiece surface.
The big advantage of the T-square is that it saves setup time. Instead of having to mark the full length of the dado, usually a single tick-mark is sufficient. So long as the fence is perpendicular to the crossbar, you can be assured that the dado will be square to the edge.
Dual-Fence Jig
The dual-fence jig enables you to rout custom-width dadoes (within limits). It is ideal for cutting dadoes perfectly sized for the particular plywood you are using. Since routers vary in base size from brand-to-brand and model-to-model, you build the jig to work with a particular router and a particular bit.
When the fences are collapsed, the router's base just fits between them, and the cut width equals the bit's diameter. As the fences are spread, the router's path widens, and so does the cut.
Construction is straightforward. Cut the parts to the dimensions on the drawing. The width of the fence base is the sum of the fence width plus the router base's radius. Glue the fences to the bases as shown.
Attached the fixed fence to the crossbars, making sure it's at right angles to both crossbars.
After routing the slots in the movable fence, butt its base against that of the fixed fence and trace the slots onto the crossbars. Mark the centerpoint for the fence’s machine screw “stud” at the end of the slot that’s farthest from the fixed fence. Then drill a 3/4"-dia. counterbore about 5/16" deep. Bore a 1/4" hole in the center of the counterbore, and countersink the hole on the underside of the crossbar. Install the screws, then the movable fence.
Finally, trim the bases using the bit you’ll always use with the router. I recommend a 3/8" bit, since that’ll allow you to rout dadoes for 1/2" plywood and thicker stock as well. Jam the movable fence tight against the fixed one, so the bases contact edge-to-edge. Adjust the router and bit for a cut just a hair deeper than the thickness of the bases. Clamp the jig onto scrap stock to support the cut. Set the router between the fences and cut from end to end.