Router Joinery: Dado, Rabbet, and Box Joint Jigs

The router transforms from spinning terror to precision joinery tool once you master jigs. Dadoes, rabbets, and box joints all become repeatable, accurate, and fast when the right jig guides the bit. Build these three setups and you’ll handle most carcass joinery efficiently.

Dado Jigs

A dado cut houses one board into another, shelves into case sides being the classic example. The challenge is consistent width exactly matching the shelf thickness, positioned exactly where you need it.

Build a T-square jig from 1/2-inch plywood. The long fence runs across your workpiece; the shorter crosspiece registers against the board edge. This guarantees the dado runs perpendicular to the edge every time.

Position the jig by measuring from the crosspiece to your dado location. The router rides against the fence, so offset your measurement by the distance from bit edge to router base edge. Mark this offset on the jig itself to avoid recalculating each time.

Use a straight bit matching your shelf thickness. For plywood, which is slightly undersize, you may need an undersized bit or two passes with a smaller bit. Test fit on scrap before committing to the workpiece.

Rabbet Setups

Rabbets create L-shaped recesses along board edges, perfect for back panels or box bottoms. You can cut them with a straight bit against a fence, but a rabbeting bit with bearing guide works faster.

The bearing rides along the uncut surface, automatically limiting the rabbet width. Different bearing sizes create different widths without changing bits. A set of bearings costs less than multiple bits and changes faster.

Set rabbet depth by lowering the bit. Most back panel applications want 1/4 to 3/8 inch depth. Test on scrap to ensure the back panel will sit flush or slightly recessed.

For stopped rabbets that don’t run the full length, clamp stops on your workpiece to register router starting and stopping points. Plunge at the start, rout to the stop, and lift cleanly to avoid burning.

Box Joint Jig

Box joints, sometimes called finger joints, interlock like machine-cut dovetails without the angled pins. They’re incredibly strong and satisfying to cut once your jig is tuned.

The jig rides in your router table miter slot. A key pin, exactly matching your bit diameter, positions each cut. The workpiece butts against the key, advances one kerf width after each cut, and produces perfectly spaced fingers.

Build the jig base from stable plywood or MDF. The fence must be exactly perpendicular to the miter slot. Install the key pin one bit-width from the bit, measured precisely. Any error here multiplies across every finger.

Set bit height to slightly exceed your stock thickness. The fingers should project enough for flush-trimming after assembly. Too shallow leaves weak fingers; too deep wastes material.

Making the First Cut

Start with the workpiece edge against the key. This first cut creates a notch at the corner. Lift the piece, move the new notch over the key, and cut again. Repeat until you’ve crossed the entire width.

The mating piece starts differently. Place the first cutoff from piece one over the key pin, butt piece two against it, and begin cutting. This offsets piece two by exactly one kerf, so the fingers of both pieces interlock.

Tuning the Jig

If joints are too tight, move the key slightly closer to the bit. Too loose, move it slightly away. These adjustments are tiny fractions of the bit diameter. Make test cuts in scrap until the joints slide together with firm hand pressure.

Once tuned, mark your settings or use stop blocks to recreate them. A working box joint jig is worth preserving for future projects.

Router jigs require upfront investment in setup time. But that investment pays dividends across dozens of projects. Build once, use forever, and wonder why you ever cut these joints any other way.