Adjustable Dado Jig

I have a dado stack for my table saw, but it's only really good at making dadoes for normal size lumber like 1/2" or 3/4" thick. I was trying to make a bench with cubbies underneath out of plywood and the plywood I was using was off by about 1/32 of an inch, so instead of looking for shims to fit into the stack, I decided just to make a jig. I looked online and found a few youtube videos showing the same basic idea: one fixed side and one sliding side that you can set the thickness on by inserting the piece that fits in the dado and setting the width.

I used the following materials:

1. some leftover 3/4" (nominal) plywood cut into the following dimensions:

• two cross pieces @ 3.5" x 13"
• one fixed side @ 2" x 32"
• one adjustable side @ 4.25" x 32"

1. Two pieces of leftover 1/4" hardboard cut @ 4" x 32
2. Two 1/4-20 x 3" carriage bolts
3. Two 1/4" washers
4. Two 1/4-20 wingnuts
5. Drill press, table saw, jigsaw, sanding drum, router

I started with some scrap 3/4" plywood. The wide one was ripped down to 4.25" wide and the narrow one was ripped down to 2" wide. Both of these pieces were then cut to 32" in length. After that was done, I ripped another piece that was 3.5" wide and cut it into two pieces that were both 13" long for the cross pieces.

Next I cut a 1" rabbet down both of the long pieces. I made sure to put the rabbet on the factory edge side so I knew for sure it was square.

I wanted the jig to be able to sit on top of the workpiece without being supported from underneath and didn't want the jig to be thicker than 3/4", which is why I decided to rabbet a groove to sit the MDF in instead of just gluing a large piece of MDF to the bottom of the plywood. I have a rabbeting bit, but not one that was wide enough to cut a 1" deep rabbet, so I marked it, cut the rabbet, and then removed the remaining 1/4" with a chisel. Yea it's ugly but it worked.

I also took this time to cut some relief in the two side pieces so when I got to the edge of my workpiece I didn't cut into the jig. I thought about leaving it as is to prevent tearout, but this way it makes it easier to see the end of the workpiece when there's sawdust everywhere. I also cut a section out of the thicker movable side to make attaching a clamp easier.

You can put the MDF into the rabbets with glue and pin nails if you want. I didn't have any pins shorter than 3/4" so I just put the glue and clamped it all up.

So this is the really important part if you want the jig to be square. I made the mistake of referencing the MDF when squaring up the side when I should have referenced the plywood that was underneath it (which I made sure was the factory edge. If the sides are square to that, then when the router base rides along the plywood end it will also be square to the side. I messed up and had to take one screw out, realign it, and drill a new hole. Oh well, it works now.

When you do this step, it will set the jig up for the bit you are using, so be sure to choose the bit you want and not just any random straight bit.

I chose a 1/2" bit so that I can cut dadoes at any width from 1/2" up to 1". I don't really work with anything thicker than that.

Set the jig up on your work table, separate the two sides by about 1/2", and clamp it down. Put your router bit in, and make sure the depth is at least enough to go all the way through the MDF but not enough to cut your table. Keeping the edge of the router base pressed firmly against the side, turn the router on and run it down one side and then back along the opposite side. I marked with pencil where the side pieces were so I could stop the router before it got that far and it would leave me with a visual as to where the end is when I use the jig. Once you do this, the MDF will show exactly where the bit will cut so you can use it to align.

My router base has a flat side and the rest is round. I referenced from the round side because I find its easy to push at an angle when using the flat side and have the router pivot up off the line.

I made the MDF a little big for my base, so I had to cut some of the excess off the end of the adjustable side to get the sides to come down to 1/2". I didn't want to make it too small and have to scrap it at the end.

To use the jig:

1. First you mark where you want your dado to be.
2. Loosen the wing nuts on the jig, open it up a bit, and put the piece that will fit inside the dado into the jig. Close down the adjustable side of the jig until it traps the piece inside. I have found that this jig gets the dadoes so tight that it's helpful to use a sheet of paper or something really thin (feeler gauges work well too) to get a little extra clearance.
3. Once you tighten the jig down, you can take it to your workpiece and align it with the line you drew. Butt the square side of the jig against the workpiece and clamp the whole thing down.
4. Set your router to the depth you want + 1/4" (to account for the thickness of the faces of the jig).
5. Starting at one end, run your router down one side and back along the other and you should end up with the whole thing cleared. Don't be an idiot like me and accidentally turn the flat side against the rail, I have a little hole in one side of my jig because of that. It doesn't affect anything since the plywood rail is what the router rides against, but it looks stupid now and bothers me.

This thing makes dadoes so much easier if I'm not wanting to hassle with setting up a dado stack. I can have it measured and cut in two minutes instead of fiddling with changing blades and trying to add up shims to add to the stack to get the right thickness. This thing gets the thickness right every time.