Exact-Width Dado Jig
Cutting clean dados for case joinery is a fundamental skill for furniture makers. Dado joints are strong, self-squaring, and attractive. Some woodworkers don’t have a dado set, so this jig allows you to use a standard-width blade to create perfectly fitting dados. Best of all, with the use of this jig, they are easy and quick to cut.
To begin, you’ll need a piece of hardwood about 1-1/2″ wide, 3/4″ thick, and 15″ long.
Small parts can be dangerous to mill, so ensure that you use push pads and other shop aids that will ensure your hands are nowhere near machine blades. Whenever you can, keep the workpiece as large as possible during machining, then cut it to final dimension afterwards. Prepare the workpiece 1-1/4″ wide, 3/8″ thick and about 15″ long.
In order to locate the 1/4" holes to help start the routing operation, Nicholson marks the hole location with an awl before drilling the hole with his drill press.
Keep it Long
Working with a long workpiece while doing operations like routing a grove is safer and easier. Notice a featherboard for inward pressure, and a clamped stop block to safely and consistently stop the groove during each pass.
Ready for Action
Once the groove is created, the workpiece can be broken down into several different parts.
In order to keep the head of the metal hardware out of the way, Nicholson used his drill press to create a hole to accept it. This piece of hardware will slide in the groove in the other piece.
Nicholson uses a riser block to raise the nearly completed jig during the cut. This riser takes into account the knob that’s in place.
Lay out your blank
From one end of the blank, make your first mark at 1-1/4″. Then, from the same end, make a mark at 3-1/4″. Square off these marks, and measure up 5/8″ from the edge to mark the center points. Use an awl to punch the center points, and then a drill press to bore out both holes with a 1/4″ drill bit (ensuring to back up the cut).
Rout a slot
The next step is to rout the 1/4″-wide slot in the center of what will eventually be the moveable portion of the jig. Install a 1/4″ spiral up-cut bit in your router table, and raise the bit to about 1/8″. Using the router table fence and push sticks/pads, place the left hole over the immobile router bit. Hold the oversized piece firmly with a push pad (and well away from the bit), turn on your router, and move the workpiece to cut the slot (stopping at the right hole). Turn off the router, remove the workpiece, raise the bit another 1/8″, and then repeat the cut. Continue doing this until the slot is fully routed out (three rounds should be plenty). You could also use a second stop block to position the workpiece at the start of the cut. With this approach you could turn your router on, butt one end of the workpiece against the stop block and then slowly lower the far end of the workpiece onto the rotating bit. The oversized workpiece allows you full control without getting near the spinning bit. Setting up a stop block makes positioning the cut foolproof.
Create a stop
Now that the slot is cut, you can break down the jig into its parts. First, cut 1″ off the blank to act as your stop. The slotted section should be 5″ long, and the base 6″ long. Glue the stop onto one end of the base. After ensuring that it is sitting square, place a clamp on it to dry.
Next, bring the base to the drill press. Layout a mark for a centered 1/4″ hole, 1-1/4″ away from the end without the stop. Use a 1/2″ Forstner bit to counterbore the exit hole, so the head of your T-bolt will sit below the surface of the base. Use some 5-minute epoxy to secure the T-nut in the jig, but make sure no epoxy protrudes beyond the face of the jig.
Assemble the jig
Assemble the jig by placing the arm over the exposed T-nut in the base. Use a 1/4-20 knob and washer to tighten down the arm, and align the edges of the jig.
Finally, it’s time to cut the kerf in the jig. Take your time doing this, as the results of this cut will determine how loose or tight your next 1000 dados will be. After installing the blade that will be used to cut your dados, screw a 1″-tall × 3″-long block to your mitre gauge fence to act as a “riser”. The riser will compensate for the knob’s height, which would otherwise be in your way for the cut. Place the jig face down on the riser and align the kerf of the blade so that the width of the blade cuts though the stop and the arm of the jig, but not though the base. After clamping the jig securely to the mitre gauge’s sacrificial fence, you can make the cut with your hands well out of the blade’s path.
Close the jig fully, trim the ends flush, sand, and apply a light coat of a finish of your choice (I used water-based polyurethane).
To use the jig, place the material directly in the jig against the stop, slide the arm over, and tighten the knob. Remove the material, and your jig is set. Place the jig against a 3/8″-thick stop on your miter gauge fence, make a cut, then flip the jig end-for-end, and make your second cut. After removing the waste between the two kerfs, the original piece will fit exactly in the dado – perfect!