You don't need an expensive dado blade set for your table saw to make accurate dadoes. As I've mentioned before in other Instructables, I have a small one (not something a DIYer brags about), and my small table saw will not take dado blades.
Step 1: WHAT IS a DADO?
A dado (pronounced DAY DOE) is a grove cut across the grain to receive the butt end of another piece. These pictures really explain it best.
That's Smoochy who insisted on assisting with research for this Instructable.
Everybody sing: Dado, day day day doe, dado come and we wan' go home.....
Step 2: PARTS OF MY JIG
I made this jig for my small router. It was all made from scraps from my lumber cart. I bought the wing nuts and bolts.
Hmmmm..small table saw, small router.... 'scuse me while I check my hands -- nope, they're OK. Whew!
Step 3: DADO PARTS
These pictures show all the parts to create this jig. I made this for the Shadow Box Bookcase I built. This jig was made for a specific router and for routing into boards up to 12" wide and determine the final dimensions shown in the last picture. Yours will probably be different if you make one.
Step 4: MAKE TWO
The first step is to secure the hardboard to the 1 x 4. Do this twice. Place the router against the 1 x 4 and rout the hardboard. This cut in now EXACTLY where the blade cuts. Make two of these. Use a dado router bit the exact width or NARROWER than the thickness of the boards you will be using to go into the dado.
Step 5: ADD RIGHT ANGLE GUIDES
I cannot stress the importance of this step. These two guides which are further apart than the width of the board you are dadoing must be secured at right angles. If not, every dado you make will be off. Secure two bolts and epoxy them in place.
Step 6: ADD ADJUSTMENT SLOTS
Line up the two bolts you epoxied and mark them on sliding part of the jig. These slots are made by drilling two holes and connecting the holes using a jig saw. Make them as far apart as you'd like to adjust the gap. Mine went from zero to 1 1/2+.
Step 7: THE GAP
Slide the adjustment guides apart and put in a piece of wood you'll be making a dado for and close the adjustment slide tight to the wood. Secure the wing nuts and this gap now matches the exact thickness of the wood you are using.
NOTE: Obviously, the round over bit shown in the router is not the one used to make dados. See step four for the bits to use. I was just too lazy to change bits.
Step 8: TEST THE FIT
The fit should be snug. A Goldilocks type of fit. Not too tight, not too lose--just right.
CLAMP THE JIG TO THE WORK PIECE. Because the slightest slip of the jig and your project is ruined.
NOTE: Route up the left side of the jig then down the right to get the full width of the dado.
Step 9: DEPTH OF CUT
The only other thing you must take into consideration is the depth of the dado. You should add this depth times two (one for each side) to the length of the shelf you are installing. If you look at the intricacies of this shadow box bookcase I built you can see how an error can magnify itself by the time you're done.
Hope this Instructable helps you. Let me know if you have any questions.