Introduction: Box (finger) Joint Jig

This instructable is for a box joint jig, which is necessary for this particular carpentry technique. I made it at TechShop in Menlo Park ( I needed this tool so I could create a box joint (also sometimes called a finger joint) using the dado stack bit on the sawstop table saw.

A jig is needed to easily cut 1/2" "fingers" and equally sized spaces in the end of a board. Using this tool will make those cuts clean and efficient.

The box joint is an alternative to the more popular dovetail joint. An interesting point that the box joint is stronger than the dovetail joint, and does not require the custom dovetail jig. Instead, you can create this one with scraps and belt out box joints all day long. Strength test was done at

Step 1: Parts and Tools List

To create this instructable, you'll need the following parts:
- Wood scraps - ideally some 1x12 and 2x4 scraps - length will depend on your table saw, but two feet long should be plenty.
- Wood screws - 1 1/4" (at least 14)
- Wood glue

You will also need the following tools (all of which are available at Tech Shop!):
- Table saw with standard blade and dado stack bit set
- Mitre saw
- Power sander
- Power drill
- 1/8" wood drill bit
- Phillips or flat head bit, as required for your wood screws
- Carpenter's combination square
- Pencil

If you're building a jig, it's assumed that you already familiar with the above tools and parts. That said, this is an instructable, so I'll go into some detail as necessary. Please comment your corrections.

Step 2: Make the Rails

The sizing of your rails may vary with your table saw. The grooves in the SawStop table saw at the MPK Tech Shop are slightly narrower than 3/4", and about 1/4" deep, and somewhere between a third to a half of the overall length of your table. Measure your grooves and rip cut two strips of wood off the end of your 1x12" board that are thick enough to sit just below flush to the surface. If your rails are too high, your jig will rub on the bottom of the groove rather than sit flat on the table. 

I ripped the strips off using the table saw with the standard blade and tuned them to the right width and height on the sander.

Ideally, your guide rails should be at least about 1/3 to 1/2 as long as your table. Too long and you'll have too much resistance, and you could potentially hit something on the other side of your table. Too short and the guide rails won't do their job. However, the length is not as important as the height and the ability to flow smoothly through the channels.

Before moving on, take a minute to slide your guard rails back and forth and side to side. They should glide easily along the groove (sanding will assist with that too - the guide rails will be smoother)

Step 3: Rip Cut a Back Plane

Rip cut a 4" board off of your 1x12 scrap. (I missed this photo when assembling the instructable. I'm sure you'll figure it out, you're smart)

For your entertainment, here's a photo of a pegasus flying from a pizza hut floating island to an astronaut. I didn't make this, I found it here:

Step 4: Make the 1/2" Spacer

This box joint will be set precisely to do half-inch cuts. You could make it do wider or narrower cuts, but it will be fixed to whatever width you build it as. Any table saw that works with a dado bit will almost certainly support a half-inch wide groove. If you decide to build your jig to a different gauge, then make replace all the measurements here with the desired width.

Set the rip fence at 1/2" and the blade height at a hair above 1/2". Take your remaining 1x12, hold it vertically against the rip fence, and pass it over the blade. This will set one width of your spacer.

Next, set the height of your blade appropriate for the full thickness of board you're working with, and leave the rip fence at 1/2". Cut out the rest of your spacer. 

Note: this will be tough at the end, because your smallest piece will be between the blade and the rip fence. Use a push stick, another scrap. or even spin the board around to the other side and re-set your blade. I just found that doing leaving the rip fence at 1/2" for both cuts gave me a perfect square without any thought, but it can get tricky handling such a small stick on a table saw. Be careful as always!

When you're done, you'll have a stick that is 1/2" by 1/2". Use the mitre saw to cut off two lengths of this stick, about four inches long

Step 5: Attach the Guide Rails to the Base of the Jig

Now that you've done all the rip cuts, it's time to start attaching the rails. For this step, you'll need the rails, glue, and the widest remaining piece of your 1x12 (should be about 6-7" wide by now - I was lucky enough to have a fresh piece, so my base is a full 12" wide, but it doesn't have to be that wide).

Square up at least one end of this board on the mitre saw (that is, make sure it's perpendicular to the sides). This is important for the next step.

Measure the length of your remaining widest piece of 1x12, and the width between your guide rails. Subtract the guide rail distance from the length of your board, divide that number in half, and measure that distance from the end.

For example, assume I have 24" of board, and the gap between guide rails is 10". The difference is 14", and half of that is 7". So, I come 7" in from one end of the board and mark it with the carpenter square (for now, just do one end). Glue one of your rails down to the side of the line closest to the end of the board. Use your square to straighten it up. (In the first picture, I obviously did not use a carpenter's combination square. I suggest always using a combination square, as my jig did come out a bit off angle).

If you're too impatient to let the glue dry, predrill a few holes and screw it in place. This doesn't have to be pretty, so I screwed them in tightly so as to countersink them below the surface of the rail. This will prevent the screw head from rubbing on the table. This is what the 3/4" screws are for.

Since you know your width between your rails, measure out the width from the edge of the fixed rail closest to the center, mark your next line, and set the next rail in the same way. While the glue is wet, take a second to fit the jig in your table saw to make sure you got the width correct. It's important that the guard rails line up to the table saw grooves, moreso than lining up to the ends. Once you know the width is correct, fix the rail in place with 3/4" screws, being sure to countersink the screw heads again.

To make sure your guide rails work, test for squareness of the rails, and to get clear on where the middle of the board is, cut through your base about halfway and check it with the square.

Step 6: Affix the Backplane

Go find that 4" rip cut you made in step 3. Go ahead, I'll wait.

OK Got it? Good. Use your square to draw out a line perpendicular to the end of the board you squared up in the previous step about two inches from the back of your base. Double-check that line is perpendicular to the saw cut in the middle. It's more important that you are perpendicular to the saw cut than to the end of the board anyway, so having both there is a good way to make sure you're square all around.

Set your backplane in with glue and screws. Put two screws on each side from underneath. Use the 1 1/4" screws this time, but make sure again that they are countersunk below the surface so they won't scratch your saw table. Also, make sure to give several inches of clearance from the center cut.

Support this board with the 2x4 scrap. Put it in place behind the backplane and screw it down from the bottom and through the backplane.

Step 7: Set the First Dado Cut Groove

OK, time to get interesting... Mount up your dado bit at 1/2" (or whatever width you choose - smaller than 1/2" is probably not worth it).  Move the rip fence out of the way. Set the height appropriate for the board you used for your base. Pass the jig through to cut a 1/2" (or whatever) wide channel in your jig.

Step 8: Mount the Spacer - and You're Done!

Bring over the two 1/2" square sticks  you made in step 4. Set one of the sticks down flush to one side of the groove. Glue one side of the other stick and butt it up against the other piece and the backplane. Remove the first piece without moving the glued-on piece. Let this piece dry. Don't bother trying to screw this one in place, it's too small. 

Your jig is finished!  The small piece is still necessary, so don't get rid of it.

Step 9: A Quick Demo

I didn't have a companion with me to video record the first use of the jig, but I took plenty of photos here.

 Set your dado bit above the jig base high enough to cut a groove as wide as the alternate board for your corner joint (e.g., if your boards are 3/4" thick, then set the dado bit 3/4" above the base of the jig).

Start with one board, butting it up to the spacer in front of the backplane. The free spacer is there to offset the boards so the grooves actually mate each other rather than meet each other. The mate for the joint will be started without the spacer, and your boards will line up perfectly.

Pass the dado bit through your material. Remove the loose spacer and set aside. Butt your material up to the fixed spacer and make your next cut. Now, set the groove you just cut over the spacer and make the next pass. Repeat until you've made cuts through to the end.

For the partner to this joint, start your first pass without the loose spacer and repeat the same above steps to cut grooves through to the end.

Once you have two boards cut to mate each other for a joint, you can fit them together and the tops andbottoms should align.

Congrats! You've got a jig! Now go make something awesome with it! Or, come to the MPK Tech Shop and use mine!