Making a box joint jig is a fairly simple operation, just be careful to pay attention to all of your measurements and will do just fine!
The jig consists of two parts,the first part is a miter guage extention which supports the work piece, and keeps your fingers away from the blade. This part is attached to your table saws miter gauge ( that half round thingy that slides in the notch in the table ). The second part is the key/tenon, which sets the spacing between dados, and the width of the box joint tenons.
Let's make the jig!
1. Cut a straight piece of 2x6 30" long. The 2x6 is the miter gauge extension.
2. Measure from one end, along the miter gauge extension 13", and make a mark.
3. Install a dado blade on your table saw and set it to the width you want for the box joint tenons. As a general rule, the tenons are the same width as the thickness of the stock you are joining. ie. If you are using 3/4" boards, set your dado blade to 3/4" width.
Note: Tenons are the spaces between the "teeth" of your box joint.
4. Set the dado blade height. Set the blade 1/32" to 3/32" higher than the stock thickness. For a 3/4" board that would be 13/32" to 15/32".
THIS STEP ASSUMES THAT YOU ARE CUTTING YOUR BOARDS A "SMIDGEN" OVER THEIR PROSCRIBED LENGTH AS DIRECTED IN THE BEESOURCE.COM PLANS. Remember when you set the dado height up by 1/32" this makes the inside dimention of the box closer by 1/16" ( a 32nd on each side ). I cut my boards to 19 15/16" instead of 19 7/8" and set my dado blade up 1/32" higher that the 3/4" height. This made my inside dimension come out to 18 3/8" just where it should be!
The reason for giving yourself the extra length is to allow yourself some wiggle room. Remember, it's always easier to cut more off, than it is to add it back on!
5. Align the mark on the miter gauge extension with the center of the dado blade and cut. The dado you make with this cut holds the key in place.
6. Align the miter gauge extension with the blade for a second cut. Set up the second cut so the distance between each dado is exactly the same as the width of the dados. ie. For a 3/4" board make the distance between dados 3/4". You have to be pretty exact here because the space you leave here is going to create the "teeth" in the box joint. And if the space is wrong, and the "teeth" are too big ( wide ) then they won't fit into the corresponding dado ( space ).
7. Cut a block of wood 6" long and the same height and width as the dados ( 3/4" x 3/4" ). The block is the key in your jig. Secure the key into the first dado with a countersunk screw ( one set below the surface of the wood ). Don't glue the key in place, or you won't be able to replace it if it becomes worn or broken. I like to use hardwood here because it it doesn't get worn or dented as easily as pine. Mine in the picture is oak.
8. With the saw unplugged, align the second dado with the dado blade and secure the miter gauge extention to the saw's miter gauge. I just ran two drywall screws through the two holes in the miter gauge, and into the back of the extention.
There now, that wasn't so hard was it? Once the jig is made, the most difficult part is behind you. However, you should practice making joints in scrap before you begin on your project pieces. Always make a test joint each time you reassemble the extension to the miter gauge. Remember, if you aren't testing on scrap, you're testing on your project!
Let's make a box joint!
1. Stand the work piece on end, flat against the extension, with one edge butted against the key.
2. Hold the work piece firmly against the extension and make the first dado in the work piece. Make a small X on the tenon created by this dado.
3. Reset the jig ( pull it back toward you over the blade ) and place the dado over the key. Make a second cut.
Continue cutting dados untill you run out of board.
4. Cut dados in the second work piece. The second work piece should mate to the first, so the dados must be offset from those on the first work piece.
Place the first work piece over the key with one full tenon ( the one marked with an X from step 2 ) between the key and the blade. Butt the second work piece against the first one and make the first dado in the second work piece.
5. Remove the first work piece from the jig and continue making dados until you run out of board.
6. Glue and clamp the pieces together to make the box joint, if you want, finish with nails.
If you use nails, predrill the holes. If you use polyurethane glue, like GORILLA glue, you probably don't need nails. Don't forget to cut your rabbets for the frames, in the ends first though. The rabbets are much easier to cut BEFORE assembly.
7. Since the dado blade height was set slightly deeper than the width of the stock, you should sand the raised end of the joint flush with the face of the board to which it is joined. Or, if you don't want to, forget it. The bees won't mind.
If you have any questions about this proceedure, don't hesitate to PM me. I'm only too willing to explain myself!