Here is what I did to make table saw zero clearance inserts for my Porter Cable PCB270TS
First I cut some scrap ½ inch plywood to the same basic dimensions as the factory insert. Using small screws attached the insert to the wood:
I then trimmed the corners and using my router table and the factory insert as a guide, cleaned up the ½ inch plywood to have the same outside dimensions as the factory insert.
I then made a template to mark the leveling screw holes. I drilled the holes from the top, if there is any tear out it will be on the bottom.
I then routed out about 3/8 inch material from the bottom leaving about 1/8 for the top. I used the factory insert as a template, so It is not exact. I went about 3/8 inch in around the ends and ¾ inch in from the sides.
This is not necessary, but I put on a coat of schlack and sand down.
I lowered my blade all the way down, inserted the insert, placed the fence over it and slowly raised the blade slowly until it was completely raised and through the insert. I had to use a 7 ¼ inch blade and then a 10 inch. The 10 inch would not lower enough to allow the insert to seat. Make sure your blade will not hit the fence when it comes up.
I like the knife behind the blade. So using a ¼ inch bit, the smallest I have. I drilled out an area long enough for the knife.
And there you have it:
