In simple terms, the zero clearance insert is one of the most important accessories you can buy for your tablesaw. It closes the gap from the metal insert that came with your saw which reduces the possibility of tear-out on your cuts. Most importantly, the zero clearance insert prevents small stock pieces from falling down into the blade well. This helps prevent kick-back which can be dangerous. 

Fits: Shopsmith Models: 505, 510, 520, Pro Fence