AW Extra 4/12/12 – Tapered Sliding Dovetails
Tapered Sliding Dovetails
Create perfectly-fitting joints that don't need to be clamped.
By Tim Johnson
How would you like to assemble a rocksolid cabinet without using clamps or fasteners? That’s the promise offered by tapered sliding dovetail joints. The joint consists of a tapered socket cut into the face of one piece and a tapered dovetail cut on the end of the other. The pieces simply slide together (photos at right). Like a dado joints with attitude, tapered sliding dovetails lock mechanically to form rigid 90- degree joints.
Before reliable glues or economical fasteners were available, cabinetmakers relied on these sturdy joints to connect cabinet components. The tapered parts must fit precisely to create a wobble-free joint, so cutting these joints by hand is a real woodworking tour de force.
Fortunately, a dovetail bit, a router table and a simple shopmade jig make tapered sliding dovetails much easier to master. You use the jig (Fig. A, below) to make the sockets and the router table to make the dovetails. Shims make it easy to create the tapers.
The dovetails and sockets increase in width at the rate of 1/16-in. every 12 inches. (Both sides of each dovetail and socket are tapered, so each side increases by 1/32-in.) Shims milled to 1/32- in.-thickness create perfect tapers on the 12-in.-wide workpieces shown here. To maintain the taper angle on assemblies wider or narrower than 12-in., simply adjust the shims’ thickness.
Make the jig and rout the sockets
Use a sled to make the jig’s tapered guide boards (Photo 1). The sled and both guide board blanks must be squarely cut. Mark the taper’s 12-in. run on the sled. Position the sled flush against a block and a stop. Tape a shim on the block, above the mark you’ve just made on the sled. The shim’s 1/32-in. thickness constitutes the taper’s rise. Butt the guide board blank against the stop and the shim and nail it to the sled. Cut the taper (Photo 2). Mark the tapered edge and the direction of its slope.
Assemble the jig (Photo 3). It should fit snugly over the cabinet sides. Make sure the tapered guide boards angle outward from front to back. The distance between the guide boards at the front of the jig determines the narrow width of the dovetail socket. For example, to make a 5/8-in.-wide socket using a 1/2-in. dovetail bit, the distance between the faces would measure the diameter of your router’s base plus 1/8-in. This socket would swell to 11/16-in. at the back of a 12-in.- wide workpiece. Tapered sockets (and dovetails) of this width are perfect for the 3/4-in.-thick stock shown here.
Rout the sockets (Photo 4). The sockets’ depth can vary. In 3/4-in.- thick stock, 5/16-in.-deep sockets are ideal.
Rout the dovetails and fit the joints
Install the dovetail bit in your router table. Then attach shims to an extra long shelf (Photo 5). Use the shelf’s extra length for test cuts while you adjust the joint’s fit. The shims hold the back end of the shelf 1/32- in. away from the fence when you rout (Photo 6).
Test the dovetail’s fit in a socket (Photo 7). If the dovetail is too wide, the joint won’t go together. If it’s too slender, the shelf will slide past the cabinet side’s front. I won’t lie. These joints are finicky. To dial in a perfect fit, you’ll have to be able to make paper-thin adjustments. So when you get close, outfit your router table with a simple micro-adjust system that’s up to the challenge (Photo 8).
Sliding dovetail joints don’t have to be glued: They’re the predecessors of knock-down hardware. But gluing makes them stronger for the long haul. Apply glue to the beveled sides of the sockets. Slide in the dovetails and tap them home (Photo 9). Give your clamps a rest.
Fig. A: Routing Jig
This story originally appeared in American Woodworker October 2007, issue #131.
Click any image to view a larger version.
In a tapered sliding dovetail joint, the dovetail and socket both gradually taper from back to front. The parts fit loosely at first, because the dovetail’s narrow front end enters at the socket’s wide back.
As the dovetail slides forward in the socket, the fit gradually tightens. The result is a snug clamp-free joint.
1. Fasten the jig’s guide boards to a sled for tapering, using a block, a stop and a shim for positioning. Butt the sled to the stop and the block. Then tape on the shim. It’s thickness and location determine the taper’s slope. Butt the guide board to the stop and the shim. Then nail it to the sled.
2. Taper the skewed edge of each guide board. Holding the sled against the rip fence skews the guide board’s back end toward the blade. The taper is very slight, so indicate the tapered edge and the taper’s direction.
3. Assemble the jig around the cabinet side. Butt everything against a block to guarantee the jig goes together squarely and the guide board tapers run true. Spacers elevate the rails for fastening the guide boards.
4. Rout the tapered sockets. Orient the jig’s front with the cabinet side’s front, so the sockets grow wider from front to back.
5. To create the tapered dovetails, attach shims at the back edge of each shelf. These shims must be the same thickness as the shim used to skew the guide boards.
6. Use a tall fence to rout the tapered dovetails. The dovetails gradually decrease in width from back to front, because the shims hold the back end of the shelf away from the fence.
7. A stop and paper shims installed behind the fence allow microadjusting the fence to dial in the perfect fit.
8. The shelf fits perfectly when it can be pushed to within 1-inch of the end by hand. Tap it home with a mallet.
9. Gluing tapered dovetails is easy. The glue doesn’t get forced out because the joints stay loose until the last inch. Once you tap them home, they’re rock-solid.