Router T- Square
This jig makes it easy to cut dados, just line up the mark on the material with the inside edge of the dado in the jig, clamp it in position and make the cut. You will have to make a separate guide for each width of dado that you wish to cut.
Any material, plywood or hardboard will work for the guide, select a piece aprox. 6" longer than the material to be cut and about 10" wide. You can make a diagonal cut on the left hand side if you wish, it works just as well with out it but I think it looks better. Fasten the guide to the 1 X 4 with a 3/4" long screw at the left hand side. With a carpenters square position guide on 1 X 4 so the right hand edge is at right a right angle to front of 1 X 4, hold it in this position with a clamp.
Select a scrap piece of 1 X 8 or 1 X 10 and placing guide on it draw a line with a sharp pencil across the board. Move the guide to the opposite edge of the board and check if the guide lines up with the line. If not loosen clamp and adjust until it is square, then add second screw at right hand edge.
Clamp the finished guide to the scrap material, set the router to the desired depth and make the indicator cut through the 1 X 4.
If you are making several of these for different widths of dados it is a good idea to mark the width of the dado on each of them.
Use the indicator cut in the guide to set the depth of the router bit the next time you are setting up.
Don't try to cut too much at once, especially working with hardwoods using larger bits and smaller routers . Listen to the sound of the router, if it is slowing down it is working too hard.
When building shelves dados are cut at a depth equal to 1/3 the width of the material used for the sides, for example 3/4" thick material would have a 1/4" deep dado.