A zero clearance insert is a replacement for the throat plate that was included with your table saw. If you use a stacked dado blade set for cutting dadoes or rabbets, your stock throat plate would need to be removed, as the slot for the saw blade isn't wide enough to accommodate the width of the dado blade. Using the table saw without a throat plate in the wide-open throat insert of the top of the saw table would be unsafe. The solution is to use a zero-clearance insert in the throat, into which a slot can be cut to accommodate the dado blade.
Zero Clearance Plate
A zero clearance plate is simply a piece of material that is cut to the same size, shape, and thickness of the standard throat plate. It sits comfortably into the throat on the tabs, just the same as the standard throat plate. The difference is that a zero clearance plate has no pre-cut slot in the plate.
The plate is made from wood or plastic that is easily cut by the saw blade, so after installing the dado blade followed by the zero clearance insert, you merely turn on the saw and raise the blade. As you raise the blade, the necessary slot in the plate is cut by the blade, leaving no gap around the blade.
Buy or Build
There are a number of manufacturers who make zero clearance plates for almost any model table saw. Should you choose to buy zero clearance plates, remember to buy them designed for your exact model table saw, or else they may not fit. If you need a plate for every conceivable width that your dado blade can cut, you might end up buying quite a few plates.
A more practical solution might be to build your own zero clearance plates. All you need is some plywood or MDF of the same thickness as your standard plate, and a band saw to cut out the zero clearance plate.
How to Build a Zero Clearance Plate
To make a zero clearance plate, begin by measuring the distance from the top of the tabs in the throat of your table saw to the top of the saw table. In most cases, this will be 1/2-inch or less. Then, find a piece of plywood or MDF of the same thickness (or slightly thinner - if the plate is thicker than the measured distance, the plate will stick out above the table top).
Next, simply place the standard throat plate upside-down onto the plywood or MDF, and trace the outline of the plate. Then, cut out the outline with your band saw.
Using the Zero Clearance Plate
If you place the zero clearance plate into the throat at this point, you may have trouble getting it out. Drill a 3/4-inch hole near the front edge of the plate (to the side of where the saw blade will cut into the plate) so you have a finger hole to remove the plate easily.
Then, with the dado blade set installed and lowered beneath the top of the table, place the zero clearance plate into the throat. It should fit snugly but not so tight that you have trouble removing it. If you need to modify the plate so that it fits properly, make those changes now.
Once the plate fits properly, turn on the table saw, and slowly raise the dado blade. It should cut into the zero clearance plate as you raise the saw trunnion. Raise the blade until it cuts through the zero clearance plate. Then, adjust the height of the dado blade until it is at the height necessary to make your dadoes or rabbets.
Keep in mind that you'll likely need quite a few different zero clearance plates (to accommodate the different widths of dadoes from your set), so it might be a good idea to create a few extra plates and keep them in a drawer for when you need them in the future.