Dazzle with Double Dovetails
If a dovetail joint is considered the hallmark of true craftsmanship, a double dovetail carries twice the cachet. Also known as an inlaid dovetail, you craft it by cutting and assembling a dovetail joint, cutting and sanding the pin board flush to the tail board, and then routing and assembling a second dovetail joint. But don't be intimidated by its perceived complexity. You can craft this dazzling half-blind dovetail joint using an Incra router-table system. Here's how.
Double down on the dovetails
The key to Incra's router-table system is its incrementally positioned fence and joint templates (shown, below) for making 38 different joints, including seven for double dovetails. Essentially, Incra has done all the math for you. Begin by choosing a double-dovetail joint from those diagrammed in the owner's manual. There, it specifies which dovetail bit to use, the stock thickness, and other helpful information. Another chart helps match stock width to the templates, so you end up with a symmetrical joint. (You determine the length.)
Armed with that information, cut your stock to size. In addition to the four box sides (bird's-eye maple shown here), you'll need a piece of contrasting wood (in this case, cherry) at least 8" long for the inlaid portion of the joint. You also need three scrap test pieces (two for the sides, one for the inlay piece) for setting the bit height and centering the fence to the bit. Rip all these pieces to the same width as the box sides. Now follow these steps to cut the joint.
1. Set the bit height as specified in the owner's-manual diagram. Clamp two test pieces to the right-angle fixture. Position the fence to cover half the bit, and slide the fence's silver measuring rule to zero. Cut a half-width socket through both pieces, slide the fence 7⁄8 " to the right (away from the bit), and cut a full-width socket. Unclamp the pieces, flip one around, and test the fit. A perfect joint (below right) has no gaps and slides together with moderate hand pressure, but also stays together without being held. To tighten the fit, raise the bit; lower it to loosen it. Once you've set the bit height, don't adjust it again. You'll make all router cuts with this bit setup.
2. Now center the bit to the workpiece. Rout a dovetail groove, about centered, the length of the test piece, then flip it end for end, rout about 1", and then back it out. Check the groove: If the second cut offsets from the first, microadjust the fence half that much. Make more test cuts and adjustments until the groove is perfectly centered. Then align the joint template to the position indicated in the owner's manual for centering your stock (in this case, 6B). Don't adjust this template again.
3. Rout the tails, Round I. Position the fence to leave only 1⁄32 " of the bit proud of the fence face. Align the measuring rule's zero to the fence's cursor, making this the starting point for all cuts the rest of the way. Clamp the two tail boards to the right-angle fixture, and mark the edge that rests against the fence. (This ensures later cuts will be made using the same registration.) Rout the tails by positioning the fence at every "A" setting on the joint scale left of zero, and sliding the boards forward over the bit each time. The second pass through each socket (below right) overlaps the first pass (below left), widening the cuts. This creates wider sockets to allow for the inlay-wood pin boards.
4. Rout the inlay pin board. With the fence stop set about 4" in front of the bit, rout tail-socket grooves, up to the stop, at each "B" setting on the scale left of zero.
5. Crosscut the inlay pieces. Secure an auxiliary fence to your tablesaw's miter gauge, and make a mark past the kerf slot equal to the thickness of the tail boards. Slide the inlay pin board about 1⁄32 " past this mark and crosscut. Repeat four times. (It's good to have a spare, in case one splits during assembly.)
6. Glue your first pins and tails. Apply glue to the inlay pin pieces and slide them onto the ends of the tail boards, centering them on the tail board's thickness. Clamp securely and allow to dry.
7. Trim the new tail boards. Sand or hand-plane the inlay pieces smooth with the tail board faces. Mark a line across the width 3⁄32 " from the tips of the tails. (Some joints, noted in the owner's manual, call for a 1⁄8 " offset.) Cut away the waste, sneaking up to the line by making two or more cuts.
8. Rabbet the tails. Holding each tail board upright against the fence, rout a 7⁄32 "-deep rabbet on the inside face. Rout this in three or four incremental passes to avoid tear-out. This reduces the tail thickness to match the depth of the pin board sockets.
9. Rout the tails, Round II. Sandwich the tail boards against the fence and right-angle fixture with the rabbets to the front and rear. This eliminates tear-out on the outer faces of the tails. Using the "C" settings on the joint template left of zero, rout new tails across the boards.
10. Rout the final pins. Retrieve the pin boards and rout tail sockets in each end, using the "D" settings left of zero. Use the fence stop to limit the depth of cut—typically equal to the diameter of the dovetail bit. Sneak up on this depth by extending the stop about 1⁄32 " over the bit, then moving it away as needed.
11. Cut a bottom groove. Before assembling your double-dovetailed box, cut a shallow groove for the bottom panel in each tail board and pin board. Align this groove beneath a tail so it won't be seen after assembly. Set the saw blade low enough to avoid cutting into the tails. Cut a bottom panel to fit, then assemble your box.