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A zero clearance insert is a replacement for the insert on your table saw. Zero clearance means that the insert is as close as it can be (the blade kerf) to the blade. It does a couple of things. It prevents small pieces from lodging between the saw blade and the insert. It provides an exact measurement from the saw blade to the your fence or wherever your measuring from the cut of the blade. And, it helps stablize your blade and prevent tear out. For a dado, the key benefits of a zero clearance insert are that it helps prevent tearout because there is wood backing your piece as the blades past through the piece and it helps stabilize the blades. You can either buy or make your own insert for your saw blades, but you will have to make your own zero clearance insert for your dado blades (and you will have to have different inserts for different widths of dadoes). If you use one on your saw blade, make sure you remove it before you tilt your blade and replace it with the original so there will be enough clearance for the blade to tilt.
To make one, trace the profile of your original insert on a piece of stable hardwood or plywood that is the same thickness as your original insert. Bandsaw the profile and insert it in your saw. (you'll probably need to do a little sanding your a snudge fit--be careful, not too tight, if using hardwood because it will expand with humidity) Drill a finger hole somewhere in the insert so that you can remove it easily. Clamp it firmly in place with some 2x4 or something similar but don't cover the area where the blade will come through. Slowly raise your blades through the insert to the depth you want your dado to be. There may be an easier method but this is the way I do it. I think I've seen strings on this site that offer suggestions.
I am speaking second-hand but I think you may have to start with a smaller blade because you may no be able to get it in position with a 10 inch blade on. You can switch to the 10 inch after you have a slot started with the smaller blade.
Originally posted by messmaker: ........... but I think you may have to start with a smaller blade because you may no be able to get it in position with a 10 inch blade on.
Messmaker raises a good point if you're cutting a conventional ZCI slot. A circular saw blades works great for this purpose. You don't need to worry about this when cutting a dado ZCI slot as most dado blades are either 8" or 6" anyway.
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