Zero clearance table saw inserts help tame tear-out on the table saw. Unlike most stock inserts the zero clearance insert supports the work piece right at the blade. This eliminates or greatly reduces tear-out on the bottom of the sheet. With a good blade and a zero clearance insert it is possible to cut plywood without tear-out on both sides.
You can buy inserts for almost all table saws. A shop made insert will be a lot cheaper and of equal quality.
So, how do I make zero clearance inserts in my shop?
1) Start with a scrap of MDF a little bigger than the stock insert. 1/2" or 3/4" material works well. The only requirement is that the MDF be slightly thinner than the stock insert. Otherwise the insert will protrude above the table's surface. Leveling screws take care of thinner inserts.
Attach the stock insert to the MDF using double sided tape.
2) Trim and excessive overhang on the table saw or band saw. Be careful not to cut into the insert.
3) Mount a flush trim bit in your router. Set the depth of the bit such that the collar rides on the stock insert. Trim the MDF flush with the insert.
This task came be a bit challenging when the stock insert has holes, mounting clips or other irregularities. Rout around these as close as you can. Finish up with sandpaper and/or other tools. You only have to mind the irregularities once. In the future use the zero clearance insert you just made as the template instead of the stock insert. Your insert is smooth.
3) Cut a hole for your finger such that the insert can be removed. Make sure the hole is out of the way of the blade.
4) Drive very short screws into the bottom of the insert in such a way that the heads of the screws ride on the lip around the insert opening on your saw. The screws act as leveling screws. Sink them into the insert until the insert is flush with the table when installed.
5) Install the insert and the blade you want use with the insert. Lower the blade all the way down. Hold the insert down with a piece of wood you don't mind losing. Turn the saw on and slowly raise the blade until it cuts through the insert and stars to emerge. Keep your fingers out of the the way of the blade. Once the blade is raised all the way, the insert will stay in place on its own.
Zero clearance inserts are made for each blade. If you change the blade, you must also change the insert. The same goes for the angle of the blade. Each angle requires a new insert.