This dado jig attachment will turn your crosscut sled into a dado sled, allowing you to safely cut dados and rabbets across longer boards like those on bookshelf sides with your table saw.
I first came across this idea from an old Shopnotes magazine article (#99) from a few years ago. Their version has the fence of the crosscut sled extending past the blade to the right, with the dado attachment sliding under the fence. My version has it's own fence, but otherwise works the same.
The short video on the left shows the dado jig attachment in action cutting a rabbet and a couple of 3/4" dados.
Because the jig can be adjusted so that it's right up to the outside of the blade, a zero clearance effect is achieved, with support on both sides of the blade. The results are some very nice dados.
It can also be used with your crosscut blade, although the height of cut you can make is dependent on the height of the slotted adjustment brackets at the front and rear of the attachment. These brackets can be made higher where the blade passes through so that enough material is left to support the jig attachment while giving you more cutting height.
The base for the jig attachment is 10" wide by 21 3/4" long by 1/2" thick baltic birch, the same stuff I used for the crosscut sled. This is sized to fit my crosscut sled, so if your dimensions are different, just add 3/4" to the length of your sled to get your base dimension. This 3/4" is added to accommodate the 3/4" front fence bracket.
The fence mounting holes are drilled and countersunk for 1 1/2" long wood screws.
Three holes on each end of the base are drilled and countersunk for 1 1/2" long wood screws to mount the front and rear fences for the dado attachment. The holes on both ends are located 3/4" in from each side with one in the middle.
The rear fence holes are centered 3/4" in from the end for the 1 1/2" thick fence, while the front are located 3/8" in from the end for the 3/4" thick fence.
About the only problem I've had with my crosscut sled is having to clean the sawdust away from the fence after every cut so that the next piece would sit flat against it. This was a pain in the butt when cutting many pieces one after the other.
A 3/16" by 3/16" rabbet was cut in the bottom of the rear fence.
So before I got started with the dado attachment, I removed the fence and cut a 3/16" square rabbet along the bottom front edge. The recess not only gives the sawdust some place to go, but it also lets the workpiece sit flat against the fence if it's bottom edge isn't quite perfect.
I cut a matching rabbet in the rear fence for the dado attachment at the same time. After the fence for the crosscut sled was squared away again, I trimmed the fence for the dado attachment to 10" long, the same as the width of the base.
The rear fence for the dado jig attachment must be the same thickness as the fence on the crosscut sled or it won't align properly when the slotted bracket is attached. If you haven't built your crosscut sled yet, make sure you mill a piece long enough to get both fences out of it.
A thick, square board is used to align the fences with one another.
With the blade lowered below the table, set the jig attachment base in position flat against the right edge of the sled and even with the sled at the back edge. The holes spaced 3/4" from the end should be at the rear with the countersinks facing the table.
Using a fairly long straight edge, I clamped the fences together so that the front faces were even with each other and butted together at the ends. I jointed a true 90 degree corner on a thick board for this purpose, but any decent straight piece will do. I used a c-clamp to hold the dado jig's base to it's fence and drilled pilot holes before screwing the base to the fence from underneath.
To allow the jig attachment to slide back and forth and allow for different width dados, a slotted bracket is screwed to the dado jig's fence.
The bracket is 10" long x 2 1/2" high x 3/4" thick. I made mine from birch to match the rest of the sled. Cutting the slot on the router table is easiest if you have one.
The illustration on the right has all the critical dimensions for the bracket, but if your sled is different (higher fence etc.), you'll have to modify them to fit.
The bracket is clamped in position and screwed to the fence with four wood screws.
Attaching the bracket to the fence is pretty straight forward, with the center of the bracket being aligned even to where the two fences meet.
I removed one clamp and reclamped it with the bracket in position, leaving enough room around the holes to drill pilot holes and drive in the screws.
With the bracket secured tightly to the dado attachment's fence, a 1/4" hole can now be drilled through the fence on the crosscut sled. This hole will accept the machine screw, washer and knob that will be used to hold this end of the dado jig attachment in position after adjusting it for blade width.
The hole through the rear fence is started from the far right end of the slot.
I used a c-clamp to hold the end of the bracket against the crosscut sled's fence to keep things in position while drilling the hole. This hole needs to be drilled at the far right end of the slot, completely through the fence.
The hole is then countersunk on the front side of the fence for a 1/4" x 3" tapered head machine screw (stove bolt). Make sure the head of the screw is sitting below the fence face so your stock will lie flat against the fence.
The front fence assembly consists of two 3" tall by 3/4" thick birch boards. The short board that attaches to the front end of the crosscut sled is 8" long and has one corner mitered at 45 degrees. The board that attaches to the dado jig attachment is 16" long and also 3/4" thick.
The short stub fence is attached even with the right front corner of the crosscut sled from underneath with three wood screws.
The short stub fence for the crosscut sled is mounted from underneath with wood screws, so three holes need to be drilled and countersunk 3/8" from the front edge. Place one 3/4" in from each end and one in the middle.
Because my motor was in the way, I had to flip the crosscut sled over to drill the holes. With the stub fence clamped even with the right corner, the screws were driven in and the sled flipped back in position.
Before the fence for the dado jig attachment can be mounted to the jig's base, a slot must be cut to allow this end of the jig to be clamped with a screw and threaded knob like the back fence.
A 1/4" starter hole is drilled 11 1/2" from one end (down the centerline of the board) and then the slot can be routed to a length of 2" on the router table.
After the slot is made, miter both top corners at 45 degrees 3/4" from the ends to complete the fence bracket.
With the dado jig sitting against the crosscut sled and the back fence clamped tight with it's screw and knob, clamp the front fence bracket to the short stub fence and to the base of the jig. At this point, it should line up nicely with the fence on the crosscut sled and the holes in the bottom of the jig's base.
Drilling the 1/4" hole through the front fence of the crosscut sled.
Pilot holes can now be drilled in the fence bracket and the fence screwed to the jig's base.
A 1/4" hole can now be drilled through the front fence on the crosscut sled, but this time, drill the hole at the far left of the slot as in the photo. Countersink the hole on the opposite side and install a 1/4" x 2 1/2" stove bolt, 1/4" fender washer and threaded knob.
I set up my freud dado blade to 3/4" wide to cut a starter slot in the fence brackets. With the blade set at around 3/4" above the plywood base, the dado jig attachment is adjusted so that it's as close as it can get to the blade, without actually touching it.
Cutting a 3/4" starter slot in the fence brackets. A piece of scrap prevents tearout.
After clamping a piece of scrap material on the back side of the brackets to prevent tearout, I lifted the front of the sled over the blade and then ran it through to cut the slots.
After a little clean-up with sandpaper, I waxed the bottom of both the sled and dado jig attachment with paste wax to help it slide smoothly across my table.
Some people go crazy with stain and other finishes on their shop jigs... I'm not one of them. I don't really care what they look like, as long as they work properly. If you live in a very humid climate though, a coating of some kind of wood sealer probably wouldn't hurt.
I appreciate your feedback. If you have any questions or comments about this article, please don't hesitate to contact me and let me know your thoughts.