I have a project where I need to cut some furniture grade baltic birch plywood. This isn't the cheap stuff you get at the home improvement center. This stuff is higher quality; more plys, thicker veneer, no voids, etc. Expensive and hard to find. I didn't want to ruin the edges with tearout from poor cuts. I started by purchasing a better blade for my DeWalt DW745 10" table saw; a Diablo 80 tooth plywood blade. But to get the best possible cut, I wanted a zero clearance insert to minimize the chance of any tearout. You can buy one, but why when you can make one in about an hour? I used 1/2" plywood for mine but you could use just about anything, MDF, UHMW sheet, lexan, etc. The 1/2" thickness was perfect to fit even with the surface of the table.
Step 1: Rough Out Your Insert
Start by tracing your insert on the plywood. Mark the location of the blade slot and finger hole as well. Then cut it out just a little proud of your trace lines. We'll sand it down later for a perfect fit. I cut it on the table saw.
Step 2: Make It Fit
I cut the sides almost exactly on the lines but the ends needed some attention. I sanded down to the lines on my belt sander. When you are close, check your fit and sand more if needed. It should be a nice snug fit.
Step 3: Add the Details
Use a 3/4" forstner bit to drill the finger hole. This is optional but it will make it easier to remove the insert. Then mill a 1/2" round slot just over half way through the plywood where the blade is going to go. This reduces the amount of wood causing friction and heat on the blade. It also makes it easier to fit the insert in later steps. If you can't mill the slot, use your table saw or router to cut a 90 degree groove instead.
Step 4: Secure It to the Table
Now we need to affix it to the table. I made some screw on aluminum clamps that go below the insert, but you could use wood instead. My clamps are 1" wide and stick out about 1" on either side of the insert. I drilled and tapped for a 10-24 screw in the middle of the clamps. With the insert in place, look beneath the table and find empty space away from the blade where the clamps can go. Mark the spot with a pencil. With the insert removed, drill a countersunk hole in the center of the insert at each end. Place the insert back in the table saw and screw on your clamps. Check the insert from below to make sure your clamps will not hit the blade. If the top of the insert is pulled below the table surface, use some shims below the insert to even it up.
Step 5: Cut the Slot
Now it's time to cut the zero clearance slot. With the insert securely clamped in place, start your saw. Then slowly raise the blade through the insert as high as it will go. Preserve the wood with your favorite finish and you're done! Go and cut your wood without fear of tearout.