The dado joint, a channel cut in one piece of wood that holds another piece of wood, is one of the bread and butter joints in woodworking. It isn’t as charming as a dovetail joint, or as manly as a mortise and tenon, but it does the job of making a strong corner. Its simplicity is its main advantage. In theory it is quick to make, but as with most simple things in woodworking, the best way to do it involves choosing between a complicated bunch of solutions. For the joint to work, the width of the groove (if it goes along the grain) or the dado (if it goes cross grain) has to match the thickness of the part that gets inserted.
One might think it is as simple as measuring the thickness and then selecting a matching router bit, or setting up the correct cutters of a stack dado head. That works if you mill your own material, measure precisely, and can hit the numbers exactly. If you’re not that precise, or working with plywood, the inconsistent and unpredictable nature of the material leads to a drawer full of slightly undersized router bits, or lots of time spent at trial and error testing on the table saw and a lighter wallet without any guarantee of a good fit.
Somewhere along the line I realized that building a simple jig around the parts would expedite the process and guarantee that Tab A would fit in Slot B with minimal amounts of fuss and fiddling. I start with two pieces of allegedly 3/4″ thick plywood. In the example shown, the parts are about 3″ wide and 12″ long, with clean straight edges. The exact dimensions don’t matter, as long as there is a wide enough surface to the router to ride on, and there is room to clamp the jig to the work without interfering with the router.
The first two parts are glued and screwed together at a right angle. This is a simple and familiar approach to routing a slot and if used this way, the groove would match the diameter of the cutter. To match the thickness of the piece, we need to add a couple more pieces.
Stand the piece that will go in the slot on end up against the guide bar of the simple jig. Clamp a matching guide bar on the opposite side of the work, and screw it down to the piece that butts against the edge of the board. Add a piece to the other end to keep the guide bars from spreading and you’re almost ready to go.
To make this work, you need a router bit with a bearing above the cutter and a diameter smaller than the slot to be cut. This bit is a good choice. It is 5/8″ in diameter with a 1″ cutting length. The bearing rides up one side of the assembled jig and down the other, making a slot with a width that matches the distance between the two guide bars.
To set the depth, you need to factor in the thickness of the guide bar. Because my guide bar is a piece of nominal 3/4″ plywood I can’t rely on the nominal dimension. Rather than measure and add (quick, what’s 45/64″ plus 3/16″) I set my adjustable square on one edge of the plywood and slide the blade to the intended depth (look at the far side of the rule). I don’t have a clue what the actual distance is, but I don’t need to know; the distance between the stock of the square and the end of the rule is exactly what I need to set the depth of cut on my router.
With the bit chucked in the router, put the router upside down on the bench. Hold the end of the blade of the square next to the bit on the router base and adjust the height until the point of the cutter just touches the stock of the square. Get your eye inline with the end of the bit to get a good look. If there is light on the other side of the bit you’ll be able to see teeny-tiny gaps.
Don’t be surprised if one cutter is a different height than the other, just set the bit to the tallest cutting edge with the end of the square held firmly to the router base plate. Precision isn’t about measurements and numbers, it’s about ensuring that this matches that. The whole process of making this jig and setting up to use it doesn’t take long, mainly because we steered clear of measuring and mathematics.
Because the dado in this piece stops before it reaches the front edge of the cabinet side I added a stop block to the top of the guide bars. With the stopped dado marked on the work, I put the router in position and marked on the guide bars the position of the leading edge of the router base when the cutter was close to the stopping point. A couple of screws hold the stop in place.
Clamp the jig securely to the work piece. I use one clamp across the work and a second (just to the left of the router in the photo) to hold the jig down. The inside of the guide bars align with pencil lines on the work piece. It’s a good idea to also clamp the work down to the bench.
The first cut will make a slot in the back piece of the jig. If you’re nervous about having the depth set right, make a short cut into the jig and measure the depth. If you set the depth as outlined above, pat yourself on the back and make the cuts; up one side until the router hits the stop, over to the other side and back towards you. If you hold the router against the guides as you cut, you won’t need to make a second pass to “make sure”.
Remove the jig and test the fit. It should take a little effort to get the workpiece in the dado, but you shouldn’t have to beat on it. When the shelf is in place, you should be able to lift both pieces off the surface of the bench by lifting only the piece in the dado. There will be a minimal amount of chisel work to square off the rounded corners at the end of the dado. Don’t be scared, it’s easy to do.
It might occur to you that you could make a super-duper-micro-adjustable universal dado jig based on this idea that would work in any situation. I advise you to ignore that thought and get on with making the cabinet. In the time it would take you to make the super jig, you could cobble together dozens of these simple jigs. The other advantage of the simple jig is you don’t need to store it, remember where you keep it or rebuild it when you come across a situation you didn’t anticipate. It is a means to an end, not an heirloom so you won’t need to feel guilty if you trash it or use the parts somewhere else..