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The biggest problem in making box joints has always been getting a precise fit, because the line between success and failure is only a few thousandths of an inch thick. Fortunately, the solution simply requires that your jig be easy to adjust, not difficult to make. I’ve added a micro-adjust system to my jig that is incredibly precise but takes only a minute to put together.
This jig is specifically designed for the jewelry box on page 64. You can certainly use it for other projects, but there are some limitations. It’s dedicated to only one size of router bit. To make wider or narrower box joints, you must build another jig. For box joints wider than 1/2 in., you’re better off using a tablesaw and a different kind of jig. If your project requires box joints that are more than 5 in. wide, widen the jig accordingly.
Setting up this jig does require some test cuts. Plan ahead by milling some extra parts from the same wood or wood of equal hardness. In addition, make all the pieces extra wide by 1/4 in. or so. It’s much better to rip your pieces to final width after all the box joints are cut. Then the last finger or notch will be exactly the same size as all the others.
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Photo 1: Rout a groove down the length of a piece of plywood to begin making the jig’s base (Fig. A, below). Make the base the same length as your router table.
Use the same size bit that you’ll use for the box joints. Here, it’s 3/8 in. A spiral bit makes the cleanest joints (see Source, below), but a straight bit works fine.
Photo 2: Fasten a runner to the jig’s sled section (Fig. B, below). The runner’s fit in the base is crucial, so begin slightly oversize. Rip the runner on the tablesaw so it barely slides in the base’s groove. Then sand one edge with a block until it slides smoothly.
Photo 3: Clamp both ends of the base to the router table so the bit is approximately centered in the hole. The base’s groove goes in front of the router bit as you face the router table. Raise the bit so it’s exactly as high as your workpiece is thick (see inset).
Photo 4: Position the base so the runner is exactly 3/8 in. away from the bit. Use a drill bit as a measuring device. To adjust the base, withdraw the drill bit, loosen one of the clamps and gently tap the base’s edge with a hammer. Recheck the spacing with the drill bit and tighten both clamps.
Photo 5: Rout the first notch in test piece A. Mark one edge as the bottom. Butt the workpiece up to the runner, and slide the sled back and forth 1/2 in. or so to cut the notch all the way through. Press down on the sled so it doesn’t tip forward.
Photo 6: Continue routing notches all the way across test piece A. Lift the workpiece and place each notch onto the runner as you go. This automatically produces a series of fingers that are the same size as the notches.
Photo 7: Rout the first notch in test piece B. This time, one side of the notch lines up with B’s bottom edge. To set this up, turn piece A around and clamp it to the sled. Its bottom finger gives you a perfect 3/8-in. spacing. When you’re done with the first cut, set aside test piece A.
Photo 8: Butt the end notch against the runner and rout again. Continue cutting notches across test piece B, just as you did on piece A.
Photo 9: Assemble the two test pieces. Ideally, they should slide together without any effort. To make gluing easier, the ends of the fingers should be even with or slightly below the surface of the mating piece. Chances are you’ll be close on both counts but will still have to tweak the setup. These joints are too loose by a paper’s thickness.
Photo 10: Add a micro-adjust block to fine-tune the joint’s fit. It’s simply a stop block, one playing-card shim and one paper shim clamped next to the jig’s base (Fig. C, below). To tighten your box joints, loosen the clamp to the left of the stop block and remove either shim. Then, pivot the base to the right and reclamp it tightly against the block. This slightly increases the distance between the router bit and the sled’s runner and widens each finger. To make the joints looser, add another shim.
Photo 11: Fasten a new face to the jig once you’ve made a pair of test pieces that fit just right. This zero-clearance face prevents your pieces from splintering out. (Half the pieces you rout will have their good sides facing in.) Mark the bottom edge of all your pieces and always begin routing from there.
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