How to Build a Box Joint Jig

A box joint jig is a perfect way to make tight, accurate box joints for you next project. A box joint is a good alternative to a dovetail joint, since the box joint creates a very strong joint without the complicated cuts required by a dovetail joint.

The best way to create a box joint is on a table saw or with a router. These tools are designed to create accurate, repeatable cuts and are used by professionals and amateurs alike. A box joint jig fits on the miter gauge of your saw and guides the work pieces through. The jig allows you to create the regular pattern of cuts required to make the interlocking fingers of a box joint.

Building A Box Joint Jig
A box joint jig makes it easy to create the accurate, repetitive cuts this joint requires. Your box joint jig will require a backer board to clamp your finished pieces to and a key guide which will help you line up your finished pieces for each cut. Here are the steps for creating a box joint jig:

  • Select your materials. To build your box joint jig, you'll want wood that will stand up to regular use and will resist warping. Although hardwood can be a good choice, ¾" plywood is a good, inexpensive alternative.
  • Size your joint. Typically, the fingers of a box joint are as wide and deep as the thickness of the stock you are working with. For this example, we'll be creating a ½" box joint jig.
  • Cut the key guide. You'll need to cut two strips to match the height and width of your joint fingers: one for the actual key and one for a key spacer. The key guide in this example will be ½" x ½" by 3" and the spacer will be ½" x ½" x 6".
  • Set the dado. Set the dado blade on your table saw for a ½" wide cut and set the depth slightly deeper. A deeper cut will allow the ends of the box joint fingers to extend out beyond the sides of the joint. The excess us easily sanded off to create a professional looking joint.
  • Notch the backer board. With the miter gauge set to 90-degrees, place the backer board with the long side resting on the table. Cut a notch about 3" from the end of the board with the dado blade.
  • Attach the key guide. Pre-drill a screw hole in the key guide and insert it into the notch in the backer board. Fasten the key guide to the backer board with a screw. Make sure the screw head is flush with the base of the key guide or countersunk slightly.
  • Cut a second notch. With the saw turned off, place the backer board so it almost touches the dado blade; insert the key spacer between the blade and the key guide. Move the key guide so that it sits flush against the spacer and blade. Clamp the backer board to the miter gauge and remove the key spacer. Make a second cut in the backer board so that it is exactly ½" away from the key guide.

Using Your Box Joint Jig
To use your box joint jig, clamp the joint-end of your work piece so that the edge butts up against the key guide. Make your first cut. Reposition your work piece so that the first notch fits over the key guide. Make your second cut. Repeat this process, placing the most recent cut over the key guide before making the next cut.

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