This is a adjustable router jig I made from scrap wood and knobs from a old lawnmower.
Step 1: Building the Jig
The purpose for this jig was for me to be able to make mirror image dados for built in bookcases. I built mine to span two 11 7/8 inch side panels butted together but you can make it any size you need. The jig is made with six pieces of scrap wood i had left over from other projects. I chose MDF for the base of the guides because it is very smooth and would let the router glide on top. I took two 30 inch pieces of 1/4 x 8 inch MDF and glued them to two 30 inch pieces of 1/2 x 4 inch plywood. When the glue has set up choose which bit you will use for this jig and adjust the router. Use the router to cut the MDF to the exact width of the edge of your router to the bit. On one piece I drilled 1/4 inch holes on each side along a straight 2 inch line on my drill press. I then used a file to square up the sides of this groove in order to make a slot for the adjustment knobs. The side rails are made from 1 x 4 x 24 inch poplar. A machine screw is counter sunk from the bottom on one side of each rail for the the adjustment knobs. The other side of the rails are glued and screwed to the top guide so you end up with a permanent side and a adjustable side.
Step 2: Any Size Dado
With the jig assembled you can now use whatever stock you need a dado for to set the width of the jig. Here I used a 2x4 to set the width followed by a 3/4 inch piece of plywood
Step 3: Perfect Fit
As you can see from the pictures, since the piece of wood dictated how thick its own dado will be, the fit is always perfect.
Step 4: Measure Once Cut Both.
As i said at the beginning, this jig is used to cut both sides of the bookcase at one time, Just set the hight of each dado and clamp the edge of the stationary side to the line. Adjust the gap using the shelf itself and cut the dado.