Making box joints — among the strongest joinery methods for square corners — is quick and accurate on the table saw with an easy-to-make jig.
Box joints are really a type of dovetail joint (historically, “dovetail” has been used to describe both joints), but with straight sides instead of angled. While they lack the locking characteristic of their angled cousins, their uniformity means box joints can easily be created and repeated quickly using mechanical methods. With an extremely simple jig — just two pieces that go together in a flash — your table saw is the perfect way to add these strong, versatile joints to your repertoire.
STEP 1: To keep things easy for this example, let’s create 1/2″ box joints in 1/2″ stock. Begin by installing a 1/2″ dado set into your table saw (Photo 1). You’ll also need to replace your saw’s regular throat plate with either a dedicated dado insert or a shop-made zero-clearance insert, which is what I’ve done here. As you’ll see in subsequent photos, a zero-clearance dado insert that exactly matches your dado set makes for tear-out free cuts and maximizes your saw’s dust collection.
STEP 2: Use a piece of the same 1/2″ stock you’ll be cutting to set the blade height, as in Photo 2. When cutting box joints, I like to set the blade just a hair over the actual joint depth of 1/2″ so the joint fingers stand just the tiniest bit proud of the finished joint. (A quick sanding after gluing and clamping yields a perfectly smooth joint.) Do this by setting the blade to exactly match the stock thickness, and then raise the blade enough that a straightedge just begins to “catch” the tip of a tooth.
STEP 3: Mount a sacrificial fence on your miter gauge. If your miter gauge has mounting holes (many do) you can attach this fence from the back with screws; if not, use clamps as I’ve done in Photo 3. Position the fence so that cutting a 1/2″ x 1/2″ slot in the bottom edge leaves sufficient room on the outboard side to fully support the stock you’ll be milling. In Photo 3 I’ve allowed about 3″, which will accommodate my 2-1/2″-wide workpieces just fine.
STEP 4: Glue and clamp a short length of 1/2″ x 1/2″ stock into the slot you’ve just created to finish the fence jig (Photo 4). This small piece is the exact size of your joint slots, and will act as an index key on which to register your workpiece as you make the cuts.
STEP 5: Loosen or remove the jig from your miter gauge so you can slide it to the side to offset the key from the blade by exactly 1/2″. In Photo 5, I’m using a small offcut from my actual workpiece so the distance between the key and blade is perfect. With the jig correctly placed, reattach it to your miter gauge.
STEP 6: The fingers and slots of box joints are offset from each other, meaning that opposite sides of the box will be in matching pairs. Clearly label one opposing pair of sides as “A” and the other “B”. Begin making cuts by butting what will be the top edge of an “A” side against the key on the jig. Hold or clamp the workpiece firmly against the fence and slide everything through the dado blade (Photo 6).
STEP 7: Once through the blade, turn off the saw and return the miter gauge/jig assembly to the front of the blade. Now, reposition the workpiece by sliding it sideways so the slot you just cut slips over the index key. Hold or clamp the workpiece securely to the jig, power up the saw, and make your next cut by sliding everything through the blade again as I’m doing in Photo 7. Since my workpiece is only 2-1/2″ wide it only needs two cuts to complete the joint’s three fingers, but for wider workpieces just repeat the cut-reposition-cut sequence until you’ve finished all the fingers required.
STEP 8: The “B” workpiece receives the same cuts, but to match the one you just did they have to be offset from the fingers of the first piece. To achieve this offset, position the top slot of your “A” piece over the jig key to create a spacer, then butt the top edge of the “B” piece up against it. As before, hold or clamp everything securely to the jig, and slide everything through the dado blade (Photo 8).
STEP 9: When you’ve completed this cut, you can remove the “A” piece and put it aside. Continue cutting and repositioning the “B” piece, slipping each newly cut slot over the index key, until all your slots are cut. When complete, the two workpiece ends should slip together smoothly as in Photo 9.
A note about fingers and slots: Your box sides are typically sized in increments of the slots you’re cutting. In our example here we’re cutting 1/2″ joints, so the sides must be in increments of 1/2″ or you’ll end up with partial slots/fingers on one edge or the other. If your box sides have an odd number of increments as in my example here of 2-1/2″ sides, there’s no real need to keep track of top and bottom edges unless you’re grain-matching — the slotted ends will be symmetrical. However, it’s still a good habit to get into: If your box sides have an even number of increments the slotted ends won’t be symmetrical, and for this reason you must keep careful track of top and bottom edges as you make your cuts.