Here is how to easily modify your existing steel rip fence to make easier rabbet cuts with your dado blades. The dado blade sits underneath the slot in the wooden fence, allowing you to make easier rabbet cuts.
You will need:
A 2x4 or there around piece of lumber
3 or 4 bolts, I used three 1/4" wide, 2" long bolts with 3 washers, nuts and lock washers
A table saw
A small, pointed object
Drill press or hand drill with appropriate bits, I used a 1/4" inch bit and a 3/4" bit
Step 1: Examine Your Rip Fence
My rip fence came with 3 pre-drilled 1/4" holes in it. If your rip fence does not have pre-drilled holes in it, you can safely drill 3 or 4 1/4" holes in it without compensating its integrity. To do this, measure out 3 or 4 holes on your rip fence. If your saw blade is in the middle or near the middle of the rip fence, measure out 4 holes. If the blade is closer to the back of the rip fence, 3 holes is enough. MAKE SURE that you do not put the holes in the area where the blade cuts, keep the holes at least an inch or two from the blade area.
Step 2: Find the Perfect Piece of Wood
Once the holes are drilled, you must find a perfectly straight board. If you have a planer, plane a piece of 2x4 until straight, but not less than 3/4" wide. If you do not have a planer, find a piece of wood that has the slightest bow, this will work also. MAKE SURE your board is the same thickness throughout, this is essential to making the fence true. If you have a planer, use it. The wood should be around the same height and length as the rip fence, but a couple inches difference in length won't hurt.
I used an old piece of decking that was the perfect width, cut it down to size if necessary.
Step 3: Mark and Drill Holes
Attach the piece of wood to the rip fence with a couple of C-clamps, making sure the wood hovers a couple millimeters above the table. IF your wood is bowed, put the curve to the outside so that the ends of the board touch the rip fence. This will make the bow disappear when you tighten the middle screw. Next you need to mark the holes somehow. You could measure out the same lengths on the wood, but its easier and more accurate to mark the holes. I used a sharp pointed Philips screwdriver and poked it into the wood to score it.
After the wood is scored, you need to drill the holes. Whatever the size of the holes on your rip fence, make them the same size on the wood. I made three 1/4" hole through the wood where I marked the holes. Then I used the 3/4" bit to counter sink the hole about halfway into the board. Make the counter sink deep enough to hide the bolt head and washer so it is at least flush.
Step 4: Assemble the Rip Fence
Now, put the washer into the counter sunk hole. Insert the bolt into the hole, it should fit snugly. Stick the board onto the rip fence. If you did it right, the holes should line up nicely and the bolts should slip nicely into the rip fence. Put on another washer on the outside if you need it, followed by a lock washer and the nut. This does not need to be tight, just enough to remove any bowing and to keep the fence in place. Over tightening could bow the wood more and make the fence not as true.
Step 5: Carve Out Blade Section
Finally, you must carve out the area where the dado blade will fit in. This is where the position of the bolts is important. If the bolts are too close to the blade, they will need to be moved or the blade lowered.
Raise the blade to about half way up the fence, or about an inch, no more. If you think you will be making rabbets deeper than 1", make it higher, but not as high as to weaken the fence.
Trace the outline of the blade onto the fence. Now, you can either cut out the curve with a band saw or jig saw, which is much safer, or you can cut it out with the table saw which is not as safe.
To cut it out with the table saw, raise the blade so that the teeth are just on the bottom edge of the wood. You will slowly need to nudge the wooden fence over the blade to carve it out. Be careful not to go all the way to the metal fence as this will dull the blade quickly. Do this repeatedly until it is at the desired height.
And there you have it! You now have a perfect little cutout for the dado blades to sit so you can finely adjust the distance of them in the fence while having a straight fence to cut on.