There are several Instructables showing how to build Box Joint Jigs, but not much on how to actually use the jigs. This is an Instructable on how to use the jig to best advantage.
Step 1: Prepare Your Material
Cut the sides of the box 1/32 to 1/16" longer than necessary. Make sure your stock is flat and square. It is also important that the width of all sides is identical. Also make sure opposing sides are identical in length.
Mark the material to show which edge will be the reference edge on each piece. Draw an arrow on each piece to show which edge the cutting will start on. When cutting, the arrow will always point to your right.
Step 2: Setup Your Dado Blade
Setup the Dado blade to match the size of the box joint jig you plan to use. We will be using a 3/8" box joint for this project. For additional information on setting up Dado blades, see the Instructable "Setting-up a Stacked Dado Blade".
Step 3: Making a Backer to Prevent Tearout
There are two things that greatly improve the Box Joint Jig, the Backer, and the Spacer Block
One is a backer, made of thin plywood. In this case a strip of 1/4" plywood, 2-1/2 wide was used. A 3/8" wide notch was cut (sorry for the blurry photo) to clear the index block on the jig. Two pieces of double sided tape are used to hold the backer to the jig. The backer greatly reduces the chance of tearout on your workpieces.
Step 4: The Spacer Block
One pair of sides butts directly against the index block of the jig for the first cut. The other set must start exactly 3/8" (for a 3/8" joint) off the block. The easiest way to do this is to make a spacer block. It this case a small block of 1/2" MDF was used. The block must be a true rectangle with square corners because it is going to be used as a reference tool. Using the jig, cut a slot high enough to clear the height of the index block on the jig, similar to the Backer. Make sure you have the block firmly against the index block when making the cut. Set the Spacer Block aside for use with the second set of sides.
Step 5: Set the Cutting Height
Set the blade just slightly above the thickness of your workpieces. When the box is glued-up, the fingers should protrude slightly so they can be sanded off flush with the sides.
Step 6: Beginning the Cuts
The front and back will be cut in the same manner. The left and right sides will also be cut in the same way, but different from the front and back. So grab one set either front and back, or left and right, and set the other set out of harm's way.
Start the cut with the piece firmly against the index block, with the arrow pointing to the index block side.
Make the cut, then advance the piece to the next position, cut, and so on until the end is complete.
Step 7: Flip the Piece and Do the Other End
Flip the piece over. Make sure the arrow is pointing toward the index block, then cut the second end the same as the first. Do both pieces in the set the same way. Make sure to pay attention to the arrow and always keep it pointed to your right (for this type of jig).
Step 8: The Second Set of Sides
The notches cut in the second set of sides must be precisely offset from the first set. This is where the spacer block comes in.
Place the spacer block over the index block. This should place the edge of the spacer block in line with the left side of the dado cut.
Cut the second set of sides in a similar manner as you did the first set, but start the cuts with the spacer block in place. After the first cut, remove the spacer block and use the index block to setup each cut. Do one end, flip the piece over and do the other end. Be sure to keep the arrow pointed to your right.
Step 9: The Results
This is what it should look like. The joints are snug, but enough room for glue, and the edges of all sides line up without steps.
Box joints are an easy way to make strong attractive joints quickly and easily. I use them in a lot of projects, I'm sure you will as well.