Build a Box Joint Jig for Your Table Saw or Router Table
Box joints (also called finger joints) are a classic corner joint for boxes and drawers. The interlocking fingers are glued on all four long-grain faces, producing a joint that’s both very strong and visually interesting. The key to making good box joints is consistency – every finger must be exactly the same width and spacing. That consistency comes from a jig, not from layout and individual cuts. A well-built box joint jig turns a potentially tedious process into a fast, repeatable operation where every cut in every piece is identical.
How the Jig Works
A box joint jig uses a registration pin that locates the workpiece for each cut. The pin is the same width as the box joint fingers. The first cut is made against the fence without the jig (or with a setup guide). The workpiece then drops over the pin – which sits exactly one finger-width from the blade – for the second cut. Each subsequent cut uses the previous kerf to locate the next. Every finger on every piece is cut at the same spacing by the same mechanism, so pieces fit together automatically.
Building the Jig for the Table Saw
The jig body rides in the miter slot. Cut a piece of 3/4-inch plywood slightly wider than the slot and sand it to fit snugly without play. Glue a hardwood runner cut to the exact dimensions of the slot to the underside of the jig. The fence is a piece of 3/4-inch plywood attached perpendicular to the sled, with the registration pin protruding from its face. The pin is a piece of hardwood precisely sized to your chosen finger width – 3/8 inch or 1/2 inch are common starting points. The pin sits at a distance from the blade exactly equal to the finger width.
Setting the Pin Distance
Make a test cut in scrap wood at the blade. Move the sled so the right edge of that kerf is exactly one finger-width from the blade. Lock the fence position. Glue the registration pin into a hole drilled in the fence at that exact position. Test by cutting a set of test fingers and assembling them dry – the fit should be snug but hand-pressable. Adjust pin position if needed.
Cutting Box Joints
Mark the mating pieces of each corner. Cut the first piece starting with a finger at the edge. Cut the mating piece starting with a space at the edge. Use the registration pin consistently to index every cut. When the pieces are assembled, the fingers of one piece fill the spaces of the other. Mark the top and outside face of every piece before starting to avoid cutting the wrong edge or face.
Pro Tips
- A backing board on the jig behind the workpiece prevents tearout on the back face of the cut. Re-cut a fresh backing board when it becomes riddled with kerfs.
- The tighter your finger fit, the stronger the glued joint – but a fit that requires a mallet to assemble makes glue-up very difficult. Aim for a joint that closes with firm hand pressure.
- For a router table version, use an upcut spiral bit of the correct diameter and build the same registration pin system. The router version produces cleaner cuts than a standard flat-top table saw blade.
FAQ
What size fingers should I use?
Match the finger width to the material thickness. For 1/2-inch drawer sides, 1/2-inch fingers look proportional. For 3/4-inch box panels, 3/4-inch fingers. For very thin material (3/8 inch), 3/8-inch fingers. Proportional fingers look more refined than oversized fingers in thin material.
Can I use a dado stack for box joints?
Yes – a dado stack set to the exact finger width cuts each space in one pass rather than multiple saw blade passes. This is faster and produces a flat-bottomed slot that mates more consistently than multiple narrower cuts.