Impossible Dovetails | Popular Woodworking Magazine
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Baffle your friends with puzzling joints
Press a dovetail plank into another plank with matching grips and you’ve created the most iconic joint in woodworking. Dovetails and sockets join the boards, so the joint cannot separate; the only way to disassemble it is to lift the dovetail board from the sockets.
So what if you can’t lift the dovetail board? How do you disassemble the joint? And how would you assemble this joint in the first place? These are the questions to ask when showing a friend the dovetail joints shown here. These disconcerting joints seem to wedge together on more than one plane – an impossible feat for traditional dovetails!
Test your skills with hand tools
The secret behind these joints, of course, is that they don’t assemble in the traditional way. The first two are elaborate sliding dovetails and the last is a complex swivel joint. There is no simple method to work these joints; they must be cut mainly by hand. And creating them will test your hand interlocking skills, due to their compound angles and large joint surfaces.
Unlike most wooden puzzles, these joints do not have to be constantly assembled and disassembled. Pieces include brittle short grain that can break easily and delicate edges that quickly show wear. It is best to glue the joints together as soon as they have been assembled satisfactorily.
You’re bound to make mistakes, so always start practicing with the joint. Be sure to use straight grain broth on all four sides – it’s hard to tip against the grain slope. Also, it’s a good idea to use hardwood for one piece and softwood for the other. This method is more forgiving, because the softwood piece will adapt to the hardwood piece when you assemble the joint. Using hardwood for both pieces requires absolute precision, because there is no forgiveness: if the pieces don’t fit snugly, the short-grained parts simply break.
Use the same steps you would follow to cut the dovetails by hand to create all three joints. Start with perfectly square cut pieces. Arrange the dovetails and grips on each piece. Clearly mark waste areas. Make sure your tools are razor sharp. Cut the cheeks first (Photo 1). The safest method is to cut out of the lines. Then, remove the waste (Photo 2). Finish by matching the line (Photos 3 and 4). It is better to engrave or cut the layout lines, so that you can accurately place the chisel for peeling; if you draw the lines, make sure they are sharp and narrow.
The secret to easy assembly
When all the cheeks and shoulders of these joints are perfectly flat, it is difficult to slide the pieces together, due to the friction resulting from the large surface of the joints. Fortunately, there is a solution. The only places where the joints need to fit snugly are the faces they show. Then, to make the pieces slide together more easily, slightly indent the surfaces of the joint that are not seen.
Create the pieces
- Arrange the dovetails and sockets. Mark the waste.
- I’ve seen dovetail cheeks in piece A and sunken cheeks in piece B. Thoroughly sawing these compound corners is tricky, so don’t be a hero – cut the lines wide, in the waste area.
- Remove waste to establish joint shoulders. Insert the jigsaw into one of the cheek cuts, turn the blade and saw on the other cheek cut.
- Respect the lines. Use a wide chisel to cut the cheeks and a narrow chisel to cut the shoulders. Chamfering the sides of the chisels makes it easier to get into sharply angled corners.
On this joint, all four faces of both pieces are shown. To make it easier to put on, hollow each dovetail cheek on piece A and the shoulder of each hollow on piece B. Always start cutting 1/16 ″ inside the outer edge to create a lip. Then draw in the center. When the pieces slide together, the 1/16 ″ lips on the outer faces will be the only parts of the joint that fit flush.
Devilish Lap Joint
In a typical lap joint, piece A just fits into piece B. Well, that can’t happen here. Nor can the two pieces separate. So what happens? A clever version of a tapered sliding dovetail, that’s what (Fig. B).
The dovetail tenon on piece A tapers at the bottom, from the shoulder to the end. At the top, its edges slope in the opposite direction, at compound angles. The mortise in piece B mirrors the tenon on piece B, angled up on the bottom and down and out on the top.
To assemble this joint, insert piece A into piece B. The dovetail tenon is thin at the end, so it fits over the bottom of the mortise. As the tenon slides into the mortise, the dovetail on the face of part A rises until it is flush with the face of part B.
Create the pieces
- Spread out the dovetail and mark the waste. Cut the bottom 1/8 “shoulder on the table saw.
- Lift and tilt the blade to cut the angled bottom face of the tenon.
- Use a handsaw to cut the square shoulders of the dovetail.
- Cut the blunt cheeks made up of the dovetail.
- Fit the cheeks and shoulders precisely to the layout lines.
- Roll out the mortise and mark the waste.
- I saw the angled shoulders of the mortise, staying away from the layout lines.
- Make a longitudinal cut in the center of the mortise to divide the scraps in half. Cut deep at the butt end and shallow at the open end, following the slope of the mortise.
- I saw the waste. Insert the jigsaw into the longitudinal cut, turn the blade and the saw to an angle. Remove the waste and then saw off the other corner.
- Respect the lines. When you shoot from the butt end, the sharp angles inside the mortise will trap the debris, so be prepared to progress slowly.
What to relieve
Fortunately, only the upper face and the outer edge of this joint spectacle; the other hidden articular surfaces can be “adjusted”. The sloping lower face of Piece A and its dovetail beveled cheeks are the easiest surfaces to reach. When indenting these surfaces, however, do not disturb the narrow wedge-shaped end of the tenon, nor the edges of its dovetail top surface.
Evil corner joint
The countersunk ends of the dovetail pins mean that this angle joint cannot be disassembled in the traditional way. And no evidence of a sliding joint appears on the back side of the joint, so it cannot go together like the double dovetail tenon in the previous joint.
Secrets are dovetails that tilt at three different angles and sockets with rounded shoulders (Fig. C and D). They allow the boards to slide together in line and then rotate 90 ° to form the angle. For the record, the slope of the dovetails on the outside face of piece A corresponds to the notches at the end of piece B, the slope of the dovetails at the end of A corresponds to the sockets on the outside face of B, and the slope of the dovetails on the inner face of A correspond to the sockets on the inner face of B. The rounded sockets in piece A provide space for the outer corner of piece B as the boards rotate.
Assemble this joint in two steps. First, with both pieces facing outward, slide piece B into piece A from the back. When the pieces are flush, the dovetails on the outer faces do not fit.
Carefully rotate the pieces to complete the joint. Bring A’s inner corners against B’s shoulders as you rotate.
Create the pieces
- Arrange the dovetails and mark the waste.
- I saw the dovetail cheeks, following the layout lines on the tip and outer face. This cut will not come close to the dovetails on the back face, because they slope more steeply. All right. Use the jigsaw to remove waste.
- Respect the lines. On the cheeks, work from each face to the center – on the back, you need to remove more material. As the front and rear slopes differ, the cheek faces will be faceted rather than flat (Fig. D). Note that the dovetails on both faces are the same width at the bottom score line.
Follow the same procedure used to cut piece B, with this exception: clear the shoulders of the socket, leaving flat labia minora (1/32 inch) on the outer face to accommodate the joint (Fig. C and D).
The cuts on the ends and outer faces of both pieces are what you see, so they need to remain accurate. To make it easier to put on, slightly widen the hollow of the cheeks and shoulders at the back of A, but don’t disturb the size at the end of the board. Gently relieves the facets. Make sure that each shoulder of the socket at A is hollowed out in a proper curve, so that the corner of B can rotate.
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