Dovetailing Wide Panels
So your through-dovetail jig only accepts boards up to 12" wide. That doesn't mean you can't use it to cut joints in panels as wide as you care to make. Simply free the template from the restrictions of the jig base and take it to the panels. Here's how.
You'll need two routers: one for the dovetail bit and one for the straight bit that cuts the pins. This eliminates switching bits in and out of one router and resetting bit heights. Also prepare two test boards the same width and thickness as your project parts. You'll cut dovetails on one (the tail board) and pins on the other (the pin board).
Remove the through-dovetail template from the jig and remove any stops or brackets. Make two spacer blocks about as long as the template, 3" wide, and thick enough to extend 1⁄2 " into the slots on both edges of the template. Screw one block to the underside of the template and set the other aside for now.
Mark a baseline on the inside face of the tail board by standing the pin board on end, flush with the end and edges of the tail board. Scribe the pin board's thickness on the tail board.
Clamp the test boards to opposite sides of the spacer block, with their outside faces out, and tighten to the underside of the template, photo above. Turn the assembly template side up, place the other spacer between the panels, then clamp this assembly in a vise or to the front of your bench with the tail board facing you, photo below.
Now mark with an "X" the material you intend to rout away. For variably spaced dovetails, don't rout in each dovetail slot. For the blanket chest's 153⁄4 "-wide panels, for example, we marked below the first two slots, then skipped one and marked below the fourth slot, photo below, skipped two more and marked below the seventh, then skipped three to mark below the eleventh slot.
Finally, set the bit in each router to cut 1⁄32 " below the baseline on the tail board, photo below.
Cutting the joint
Rout the tails first, photo below. Then switch routers and cut into every slot on the pin side of the template. Especially when routing the pins, keep the router flat on the template. Tipping creates an uneven edge between the pins.
Loosen the upper clamps and slide the template down to center the newly cut pins in the slots, photo below. (You may need to tap the spacer block with a mallet to slide the template.) Mark the pins across from a tail. Clamp the panels and spacer again, making sure the boards are snug against the template and that their edges are flush; then rout away the marked pins. Depending on the jig, this may leave a small sliver you can snap off, or you can slide the template again for further cleanup with the router.
With the pins cleaned up, loosen the upper clamps again and slide the template so the unrouted edge of the tail board splits a slot in the template, photo below. Make marks to complete your tail layout. (For our panels, that was just one slot.) Then repeat the routing process to complete the joint.
Fine-tuning the fit
Assemble the joint if you can. To dial in a perfect fit, move the template forward or backward on the spacer. For a too-tight joint, shift the template toward the tail side of the spacer. For a too-loose joint, move the template toward the pin side. Cut the ends off your test boards and try again until you get a snug fit.
After the test joint fits properly, rout joints on your project parts. Remember that you rout opposite corner joints on each end of the jig: With the tail side of the jig facing you, the front left and back right joints start on the left end of the jig. The front right and back left start on the right end. (This puts the dovetails on the front and back panels.) Before routing your project pieces, clamp them together and label each piece and corner so you keep them straight.