If you’ve ever built a cabinet, jewelry box, toy chest or drawer, chances are you’ve made a box joint. This strong, good-looking joinery option is used primarily to fashion square, vertical joints on wide boards or panels of equal thickness for a variety of woodworking and cabinetmaking applications.
Formed with identical-size pins and slots that interlock, box joints are easier to make than other interlocking joints such as dovetail or finger joints. Several techniques exist for cutting box joints; the easiest method is with a dado-blade set and a simple table saw jig like the one shown here.
Box joints are simple to cut because all of the pins and slots are the same thickness and depth. When you design the joint, it is important to calculate pin and slot dimensions that can be divided evenly into the full width of the mating workpieces. (It’s often easiest to design workpieces that are multiples of an even fraction, such as 1/2 in.) The depth of the slots should be equal to the thickness of the stock (although you may want to cut the slots slightly shallower than the board thickness; this lets you sand the ends of the pins so they’re perfectly flush after the joint is glued).
Making the jig
The device we made to guide our box-joint cuts is so simple that it barely qualifies as a jig. It’s really just a piece of scrap plywood with a slot. But “jig” sounds more impressive than “scrap,” and you do need a square chunk of wood to use as a spacer, too. (You could describe the setup as “a two-part auxiliary miter gauge fence” if you wanted to impress someone.)
Clamp the jig stock to the miter gauge on the table saw. Set the dado-blade cutting width to equal the pin width, and set the blade height to equal the thickness of the workpiece. Make test cuts; then cut a slot in the jig.
The plywood scrap for the jig should be flat and smooth with square corners. (Ours is 3/4 x 6 x 18 in.) Before cutting the slot, make a spacer pin. The pin should be the same thickness as your stock (a cutoff piece is perfect), and the width should be equal to the planned pin width.
Adjust a dado-blade set so it will cut a slot slightly narrower than the pin width and install it in the table saw. Set the blade height to equal the stock thickness. With the workpiece clamped to the miter gauge, cut a slot in a piece of scrap (not the piece you’ll be using for the jig) and test the fit of the pin. Adjust the blade set’s cutting thickness and height, continuing to make test cuts until the fit is perfect with the spacer pin. Then cut a slot about 6 in. from one end of the jig board.
After gluing the spacer pin into the jig slot, attach the jig to the fence on the miter gauge. Adjust the jig position first so it is one pin thickness away from the dado blades.
Glue the spacer pin into the slot. With the miter gauge aligned in its slot on the saw table, hold the jig flush against the gauge. Adjust the jig until the inner face of the spacer pin is located outside of the dado blade set by exactly one pin thickness. Attach the jig to the miter gauge.
Cutting box joints
Make sure both workpieces are the same width and thickness, and then butt the end of one workpiece against the spacer pin on the box-joint jig. Clamp the workpiece to the jig and feed it through the dado blades. Shut off the saw and back out the workpiece and jig. Unclamp the workpiece and reorient it so the slot in the workpiece fits over the pin spacer. Reclamp the workpiece and feed it through the blades. Repeat this process until you have cut all of the slots in the workpiece.
Once you’ve made and positioned the jig, cutting box joints is simple. Start by butting one workpiece against the pin spacer and cutting a slot (photo 1). Then fit the slot over the spacer pin and cut another slot (photo 2).
If the first workpiece ends with a pin, not a slot, keep it clamped to the jig so the pin can serve as a spacer for starting the cuts in the mating board. (If the first workpiece ends with a slot, remove the workpiece.) Cut the pins and slots in the mating workpiece the same way you cut them in the first piece. Cut the remaining joints, always cutting the mating board in succession. Then glue and clamp the joints to form a box.
Repeat until you reach the end of the first workpiece. If the first workpiece ends with a pin, use it as a spacer to begin cutting slots in the mating workpiece.
Cut the mating parts for each corner before moving on to the next corner. When all of the cuts are complete, clamp and glue the box.
BOX JOINTS VS. FINGER JOINTS
Box joints are so similar to finger joints that the terms are often used interchangeably. However, finger joints have tapered pins and tails, mostly because they’re almost always cut with production-grade machinery. One common use for finger joints is to end-join wood stock into longer molding pieces. Both the pins and tails on box joints are square, as they typically are cut by hand with a table saw or router.