Zero clearance inserts are plastic or wooden spacers that leave no room around the cutting action of a blade or bit. They are not unique to table saws. Zero clearance jigs are useful even on handheld saber saws, to prevent splintering cuts in a workpiece, and can be as simple as a 1/8" piece of hardboard, carpet-taped to the shoe plate of a handheld power saw with a pre-cut kerf for the blade to fit through.

On a router table, the zero clearance jig usually applies to the particular shape of a router bit as it is entrapped in a sacrificial fence to reduce the open space around the bit. This allows a much cleaner cut, as the wood being shaped is fully supported when it approaches the cutting action. On a router table fence, you can often countersink and screw a piece of 1/4" plywood on to the main fence, then very slowly move the fence into and through the spinning bit to form a perfectly shaped opening. When the fence is retracted to a measurement that exposes the desired cut of the bit, passing a workpiece smoothly along the sacrificial fence assures a clean, precise pattern. It is a good idea to also cut out a small opening above the bit for air-flow, and leave sufficient room in your table opening for sawdust to escape the cut. Sawdust buildup can prevent a smooth action in passing the workpiece along the fence, and can cause heat to build up, which is damaging to router bits and machinery in general.

On a table saw, a zero clearance insert prevents small pieces and cutoffs from falling in beside the saw blade or jamming against open edges, which may interrupt the cut and cause serious trouble. Zero clearance inserts also allow a cleaner cut, helping prevent tearout. Many woodworkers make these inserts in batches of a dozen and think of them as expendables, to be replaced often. They might also make them for unique purposes, such as a 3/8" dado blade setting, and mark each one for its intended use.

Table saws vary in how the factory inserts are attached, but they will all come out of the tabletop for you to examine your particular one. (Unplug your table saw before custom fitting a new zero clearance insert.) You can use the factory insert as a guide to trace the needed size onto wood. Good quality, half-inch plywood will work for this application, as will thick hardboard or a wide variety of plastics.

Rough cut the insert to size and use a flush trim bit in the router to quickly fit the insert to the tabletop opening. If you have no router, a decent job can be done with a scroll saw or saber saw, if you are careful keeping to the line. Then rabbet the edges on the underside to the depth necessary for it to rest absolutely level with the tabletop. The insert must rest flush with the saw's table, so a workpiece will not catch or dip down when passed over it.
A wood button tab attached under the back, or an extended screw with a few small washers and a bent metal tab to fit under the lip of the table saw opening, will secure the back edge of the insert against the upward rotation of the saw blade. Whatever arrangement you use, be sure it does not interfere with the saw blade's action when it is raised to its highest level.

Many table saws can lower the blade height well below the tabletop, which makes it simple to cut the kerf for the saw blade to rise through. Lay your new insert in place with whatever attachment setup is common to your saw. Bring the saw's fence to the edge of the new insert and tighten it down to effectively clamp the insert in place, being careful not to extend the fence so far over the insert that the rising blade might hit the fence. If you are uncertain where the blade will protrude, clamp a waste board on the inside edge of your fence to be safe. Then clamp another board over the other long edge of the insert, using wood clamps at opposite sides of the table, and leaving the center of the insert openly exposed where the saw blade will come through it. Plug in and turn on the saw to very slowly raise the saw blade to its full height. You will now have a zero-clearance insert for use with that saw blade. Thin kerf blades will naturally require a unique insert, as opposed to that for the regular 1/8" kerf saw blades.

Many smaller table saws do not allow a range of movement on the saw blade that will retract it far enough below the table to bring up through a new insert. In this case, there are two solutions: either install a smaller, seven-inch blade to raise up through the insert, followed by reinstalling the regular 10" blade to finish the cut, or cut the kerf by hand with the insert off the saw. This latter method presents the problem of placing the cut exactly where the blade should emerge, which is often just off center, and rarely results in a true "zero" clearance, but it can be done if one is careful.

Zero clearance inserts are available commercially for almost all table saw models. As an expendable item, however, they are expensive to purchase for various blades and uses. After you've done them yourself once, you will realize they are quite simple to make, and after using one for a few projects, you will probably realize you don't want to be without one.