Skill Builder: A Quick And Simple Box Joint Jig

The first thing you need to do is determine the width of the fingers. A standard table saw blade is ⅛” wide, which will give you ⅛” fingers. If you’re looking for larger fingers you will need a dado stack for your table saw. In this example we’ll be using a single blade to make ⅛” fingers. It’s highly recommended to use what’s called a flat-kerf joinery blade (sometimes called a flat-bottom blade) with this approach. A rip, crosscut or combination blade will not leave a flat bottom.

Although they will work, your joints won’t have that clean, classic look. I also like to make the height of my boxes in multiples of my finger width size. For example if I’m making ⅛” fingers I want the height of the box to be divisible by ⅛” so I’m not left with partial fingers.

Go ahead and cut 4 pieces of hardwood for your box sides (in the photos I’m using ¾” mahogany). Your box can be any size you want. Just remember the taller it is, the more fingers you’ll have to make. 12″ long, 9″ wide and 2″ high is a good first target. You will also want to cut 2 extra pieces to use for test cuts when setting up the spacing. These pieces can be any size, but 2″ wide and 4″ long out of hardwood should do the trick.

Next thing you need to do is cut a kerf into a piece of ¾” thick plywood for the jig. The length and width of this piece aren’t critical — mine here is 3½”×14″. Set your blade to ½” above the table and run your plywood through the blade near the middle using your table saw sled or miter gauge with a backing board.

Now you’ll need to cut a piece of hardwood to fit into that kerf you just cut. This piece will be the pin. You’ll cut it long at first: 8″ long, ½” high and ⅛” wide. Getting that width can be kind of tricky and may take a couple of tries. It should fit into the kerf you cut in Step 2 without falling out. If you’re having trouble getting that perfect width you can always cut oversize and sand it down to fit.

Take the pin and cut it off at 1½”. The longer of the two pieces will be used as a spacer in Step 5.

Take the 1½” pin and glue it into the plywood kerf, flush with the back. Make sure it’s not sticking out the back of the plywood. You can use either wood or CA glue to secure it in place.

Next, take your board with the pin and place it up against your table saw sled or miter gauge with a backing board. Use the 6½” cutoff from the pin as a spacer between the blade and the pin. Because this spacer is the thickness of your blade, it’ll set your pin in near perfect placement for cutting the correct size fingers.

Once you get the correct spacing, clamp your pin board to your sled or miter gauge.

Use a piece of sandpaper to round over the top of the pin. This will make your workpiece board slide in and out a lot easier. You may even want to rub a bit of wax on it to reduce friction.

Now it’s time to make your first cuts! Move one of your test pieces up against the fence and against the pin. Run your board through the blade, continuing to hold it tightly. If you don’t feel comfortable holding the piece you can always clamp and unclamp the board after each cut.

After making that first cut, slip the kerf over the pin and cut another kerf. This will create an alternating series of fingers and kerfs. Keep doing this for the entire end of the board, and repeat on your second test piece. It’s OK if they don’t line up as anticipated; I’ll show you how to do that in a moment.

Test your cuts by fitting the two pieces together. They should interlock without much force. If the fit is too loose you’ll need to move the pin away from the blade; if it’s too tight you’ll need to move the pin toward the blade. This will change the size of the fingers. You will only need to move the pin over about the thickness of a business card. To do so, loosen your clamps slightly and nudge your board over. Tighten and repeat Steps 8 and 9.

Once you get that perfect fit, you can then screw your pin board to your sled or miter gauge. In this example I chose not to screw it in and just left the clamps on.

Everything look good? Great! Now it’s time to make your box.

First, mark the top of each of the four box pieces — you can line them up and make a mark all at once. This mark is key to our success and will always face toward the pin when cutting the fingers.

Set your table saw blade height to just a hair higher than the thickness of your boards. This will give us fingers that are slightly too long — they’ll extend off the side of the box, but we can sand them down after glue-up. It’s better to have them too long than too short.

We will start off with the two long pieces. Start making your fingers with the mark facing toward the pin. Cut all the fingers just as you did with the test piece, along the entire end.

Flip the board over and cut the fingers on the other end. Again, make sure the mark is facing inside, toward the pin. Once you’re done, repeat Steps 13 and 14 on the other long board.

Now it’s time to register the fingers for the short side. First, take one of the completed long sides and place it over the pin. Position the short side up against the long piece, once again making sure the marks on both pieces face inward, toward the pin.

While holding both pieces tightly up against the fence you can run your board through the blade making that first cut in the short side.

Now with that first kerf cut, you can continue to cut your fingers as in previous steps.

Once you finish cutting the fingers on the short side end, you can flip the board over and cut the fingers on the opposite.
One more time, make sure the marks on both pieces are facing inward toward the pin. Do I sound like a broken record yet? Those marks are key to a successful glue-up! After completing the fingers on the short piece, repeat Steps 15–18 on the remaining short piece.

Now it’s time to glue it all together. If the joints were cut correctly your box will self-square. Make sure your marks are all facing up and everything should go together like a puzzle. Use clamps to keep it tight while the glue sets.

Once the glue dries you can sand the fingers flush with an orbital sander or disc sander.

Sometimes you may find tiny gaps after glue-up because of blow-out or fibers chipping away during the cut. You can easily cover this up by mixing up some sawdust and wood glue and filling the gap. Once the glue dries you can sand everything flush again. This is a great trick for flawless-looking box joints.