Zero-Clearance Insert

After completing the riving knife modification, I needed to replace the blade cover insert to accommodate the knife’s position behind the saw blade.

I had been using the stock insert, which has a full opening to accommodate a dado stack. This setup does not support the wood fibers along the edges of the cut, and will allow tearout along the ends of the cut pieces, resulting in a frayed edge on the final product, particularly when cutting across the grain.

A zero-clearance insert has a slot that is only as wide as the blade itself, and thus the wood fibers are fully supported all the way up the the blade itself (almost as if with a pair of scissors), making for the cleanest possible cut.

Not only are zero-clearance inserts preferable to factory inserts, they are the easiest type of custom insert to make since the slot for the blade is created by simply cutting through the blank insert with the blade installed in the table saw.

I started with a set of phenolic zero clearance kit from Rockler. This kit is about $30 and contains two rectangular blanks from which you can cut inserts to fit your saw. I could have purchased a pre-shaped insert but they are more expensive ($30 for one insert) and this seemed like a fun project.

Per the instructions that came with the phenolic blanks, I started by tracing the stock insert and cutting the the blank close to the final size. The instructions recommend using a band saw for this cut, but I don’t have one yet (regularly refreshing my search on Craigslist) so I used my scroll saw. This was marginally successful, but I burned up a good blade and the blank was jumping all over the place. Honestly I think this step would be better handled with a hacksaw if a band saw is not available, or at least by switching the blade on the scroll saw to one without any reverse teeth.

The idea of the first step is to leave minimal material for the the router bit to remove in the next step, which is to use a copy-bit and the stock insert to finalize the shape.

I used turner’s tape (fancy double-sided tape) to attach the stock insert as a template to the rough-cut blank and used a 1/4″ copy-bit on my router table to remove the extra material and bring the outer diameter of the phenolic blank to its final shape.

I almost had a minor disaster while doing this. I noticed a slight change in the sound coming from the router bit, so I shut off the router and realized that I the bit was slowly lifting out of the collet and was begining to cut into the metal of the stock insert. There was no visible damage to the router bit (Carbide is tough!), so I re-seated the bit and really tightened it down. I was able to finish the routing without any other issues. On a side note, I’m now convinced that I need to set up some dust collection on my router table…

The phenolic blank is thicker than necessary for my saw, so I set up my drill press with a large forstner bit to remove enough material for the insert to sit flush.

Once I had achieved the correct thickness, I drilled out and countersunk the holes for mounting the insert.

I stripped out one of the screws while testing the fit, which gave me a chance to use a screw extractor bit that I had never tried before.

The extractor worked well enough, bit it really guts the screw so I cut a slot in the screw to re-use it as a flat head until I find a replacement.

After this, I re-installed the blade on the table saw and cut the slot in the new insert. This had to be done in 3 stages:

      • I very cautiously lowered the insert on to spinning blade to remove enough material be able to set the insert fully flush with the table top with the blade installed. Since the insert is unsecured, it is very important to do this slowly from BEHIND the saw. This way if the insert is ejected it flies away from you. since, at this point in the process, the top of the insert is completely symmetrical, I actually started cutting with the insert backwards and had to flip it around and start the cut over:
      • I secured the insert and raised the blade with the table saw running until the blade reach its full depth:
      • I extended the cut to make room for the riving knife. I actually did this with a hand saw, Stanley FatMax:

With everything finally installed, I quickly cross-cut a piece of rift-sawn white oak to see what sort of cut quality I would get. I made the cut freehand, with the blade set too deep,, but still got a decent quality cut with minimal tearout.

I’m happy with the results, and I am done with this project.


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