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Material : Material is a matter of choice. I used 3/8" Baltic Birch because I happen to have a good supply due to a friendly woodworker giving me a bunch of cutoffs. I felt Baltic Birch was a good choice here as it as a nice smooth finish, is very seasonally stable to movement, has very few voids in plys and machines well. ShopNotes has a version where they use hardboard (1/4" maybe?). I felt this thicker BB was a better choice as it gives more support on the guide rail. I used some knobs that I had lying around for the end adjustments. Any of the knobs from Rockler, Woodcraft, etc would work. In fact the way this is built a washer head bolt and wing nut would be fine. For finishing material I used some cheap Minwax poly since I still have a gallon of it sitting around. I also finished with a final coat of Johnsons paste wax.
Tools : Used tablesaw, drill, jigsaw and router. You could easily get by without the drill and jigsaw if you are pretty accurate with a plunge router. Fine grit sandpaper is needed for the finish.
The dimensions for this project are completely user dependent. Since my BB was lengthy I went for the 48" max in case I ever need to dado a whole sheet of plywood (wouldn't ya love to see that project!?!?). I cut a sheet of the 3/8" BB to approximately 10" wide and 48" long. One thing you should do here is likely make this a little wider. You need width of the guide rail plus the distance from router bit to the outside of the router base. On the Bosch 1617 it is approximately 3.5". I cut two strips of 3/8" BB to 1.75" wide and 48" long. I glued and clamped the strips to the outside edge of the 10" wide sheet. Note you do not have to get really tight tolerances here due to the way this is built. You do need to make sure that the edges of the 1.75" strip have been jointed or otherwise been made perfectly flat, as any error in the edge will transmit to your work.
While the glue is drying I made the supports for the ends. I used Ash because it is very hard and it happened to be laying in the floor next to me. Anything would work here. I'd probably stick with solid wood instead of plywood on this part. I cut this piece 2" W x 10" L. I then used the tablesaw to dado a 1" wide by 3/8" deep slot on one side of each support. This allows clearance for whatever bolt or knob you use.
Next we take the clamps off the guide board and rip it in half, parallel to the edge guides. After ripping the guide in half, I took one half and clamped it to my bench. I then took my Bosch 1617 Plunge Router with 1/2" straight bit and routed the guide board. I placed the flat side of the plunge base against the guide rail made a left to right cut. I removed that piece from the clamps and put in the other half. I then used the round side of the router base to make a cut parallel to the edge guide. In retrospect I may have been better using the fixed base which has a completely round base. I marked each half so I would know which one was made with the flat part of the base. I also wrote the router and base being used, as well as the bit being used. For future use we want to have the same style router and the same bit so the distance from the edge guide to the cutting edge of the jig is the same. Part of the easy thing about this jig is the ability to quickly set up width of dado, so using the 1/2" bit as the standard works well for me.
Next I glued and clamped the edge supports to one side of the edge guide. This is going to be the fixed side, so make sure you glue the supports to only one of the edge guides.
Next I placed the remaining edge guide on top of the supports and drilled a hole that was approximately in center of the underlying guide. I put the remaining edge guide flush against the fixed edge guide resulting in no gap. This would theoretically be the minimum dado (though in practice our dado would be a minimum of 1/2" since we are using a 1/2" bit). Drill both side of the remaining edge guide. Take the edge guide off and drill another hole about 1.5" away from the other hole. Use a jigsaw to connect these two holes and create an area for the bolt to travel.
Lay the adjustable guide back up on the support and put your bolt and know through the holes.
I took the completed jig and applied 4 coats of thinned Minwax poly using wiping method, sanding with 220 between each coat. I want this defect free on both the top, where my router base plate will be riding, and on the bottom of the jig that will be sitting on my material being dadoed. I finished with one coat of Johnsons Paste Wax.
How this works:
Take the material that will be inserted into the dado and lay it on the area that will have the accepting dado. Mark the exact width of the material along its width. Set the piece aside. Take the jig and place the fixed edge guide adjacent and parallel to the area you just marked. Clamp the fixed jig in place. Now you can set the adjustment by either placing the material being inserted into the dado in the jig and adjusting width to that material, OR you can set adjustable edge to the edge marked on the material being dadoed.
This results in a dado with that is perfectly sized to the width of the material. You will want to take the first pass going left to right and the second pass going right to left.
