I can't picture how a miter saw insert prevents tearout, as such an insert does on a table saw. On a table saw, the direction of spin has the teeth closest to you going down into the as-yet uncut part of the board as you push it into the blade, cutting the edge of the board farthest away from you, so that when the teeth emerge on the bottom of the board, a zero clearance insert supports the wood fibers on each side of the cut. However, on a miter saw, the blade's direction of spin also has the teeth closest to you going down, but now the cut is starting on the edge of the board closest to you, with the teeth on the bottom and furthest away from you doing the cutting, scooping away from you and up through the wood. A zero clearance insert would prevent tear-out only for the teeth closest to you cutting down through the wood and emerging on the bottom of the board, which doesn't happen in a normal cut (i.e., nearly none of them). The only scenario where the teeth closest to you would be cutting fresh wood and emerging on the bottom is where the board is wider than the diameter of the blade, and where you start the cut with the saw pushed all the way away from you, plunge it down through the board, then pull it toward you, cutting the closest few inches of this extra wide board essentially backward from how one normally uses this saw. The normal way, pulling the saw all the way toward you, pushing down, then away from you, has the teeth closest to you going down not into fresh wood, but into the kerf already cut by the blade. The cutting is done by the teeth on the bottom, and farthest away from you, which scoop away from you and up, emerging from the top of the board on the far side of the blade. So again, since the teeth that go down, almost never cut through uncut wood fibers, how exactly does a zero-clearance insert on a miter saw prevent tearout?
Thanks for the detailed explanation. I'm just speaking through my own experience, but since I started making zero clearance inserts, the bottom and edges furthest away from me have been coming out much cleaner. That's because the fibers are being supported on the bottom of the material where the blade passes through. When I added a zero clearance fence, I haven't had any tear-out where I usually would before. Once again, that's just my own experience.
