I needed to cut some small, thin pieces of wood on my tablesaw and so decided to build a zero clearance insert.

Your tablesaw came with an insert from the factory that would accommodate the blade at full height and at any angle. This insert will accommodate only a straight vertical blade, but the opening is only as wide as the blade itself, so small stock is much less likely to be pulled down into the saw.

Because there is a smaller opening, a zero clearance insert can increase the amount of dust expelled on top of of the table, however that's an easy compromise to make: without it, the workpiece can drop into the saw as it's being cut.

TOOLS:

Tablesaw... obviously!
Jigsaw, scrollsaw or bandsaw.
I used my scrollsaw and I think this gives the best results, however any of these 3 will work.
Measuring Caliper
I have a $6 one and should upgrade, but this is a great tool to have and I've used it very frequently since I got it.
Drill
5/8" or 3/4" spade bit. (This is for the finger hole)
Hammer
Misc. Clamps & scrap wood for clamping

MATERIALS:

Wood for the insert
I used 5/8" Melamine faced MDF. It has to be made out of reliably straight, smooth material, such as MDF or baltic birch plywood with a veneer of plastic laminate over top; or a straight hardwood such as maple, which could be sealed with a floor finishing product for maximum durability and smoothness.
Paper, masking tape & spray adhesive
This is if you want to use the same measure & template method I am.
2 small nails; ie finishing nails, 14 - 16 gauge.

If you want to use the screw-height adjustment method: (See the first step)

4 small / short machine screws
1/2" hardwood dowel
1/2" or 33/64" drill bit
A drill bit only slightly smaller than the small machine screw shaft diameter.

Step 1: Remove the Existing Insert & Measure

Most people remove the insert and trace around it. Because I have AutoCad, I'm going to measure it and print a template instead. This will help make the widths more exact, and should minimize the amount of sanding required to get it to fit.

I measured across (3.75") and along the existing insert at the centre (13.5"). The ends look like half circles (not ovals, etc) and so are easy to reproduce in AutoCad.

I used 5/8" melamine faced MDF for the insert, but the support arms on the table saw top are only 0.5" down from the top; so I'll need to cut away 1/4" of material away from the base in order to get 1/2" thickness.

(My original plan was to drill out 1/2" diameter circles, glue a piece of hardwood dowel into the MDF, and recess in four small machine screws to control the height / depth. However, the first time I tested it in the saw it was nearly perfect, so I just shaved a bit more off and left it alone.)

After measuring, I produced a paper template in AutoCAD. Because I have an 11x17 printer, I could fit it on one page. I haven't included it here as I'm not sure if 3.75" x 13.5" is a standard size, or if other tablesaws are all different sizes. Let me know if you'd like it (PDF).

Step 2: Mark Support Arms

So on the underside of the existing insert, there are 4 protruding screws. These machine screw ends sit on support legs on the table. To mark their locations; I took a pencil and scraped it onto the screws; then pressed the existing insert down onto my paper template; which marked all four locations.

Step 3: Glue Template to Wood... With Tape

So I wanted to glue the template onto the MDF; but I didn't want to mess up the top surface of the MDF with glue. So I put down 4 strips of masking tape, then put a quick blast of spray adhesive down onto the tape, which allowed me to glue the paper to the MDF.

Double sided tape may work too, but I'm pretty sure masking tape is easier to remove.

Step 4: Cut Out the Perimeter

I cut both long edges off the insert wood with the tablesaw, then cut the circle with my scrollsaw. I don't have any pictures of this step, so you'll just have to imagine it.

Test fit it, which will likely result in some sanding. I used a hand sanding block as it was nearly perfect; which is probably due to me using the measure & Template method instead of the trace method.

Step 5: Cut the Blade Slot & Recess the Base

Making sure the blade is straight vertical & fully recessed; remove the factory insert and set the new one in top. It may go in a tiny bit before it hits the top of the blade; check if it's relatively level.

I clamped the insert in place with the rest of the MDF board and also one of the half round cutout scraps; then turned the tablesaw on and raised it until it was just poking through the top. (While doing this, I held down on the insert with a block of scrap wood).

After cutting, I turned the saw off, removed the clamps, and dropped the insert into the slot. It was sticking up 0.125" because the support arms are 0.5" down from the top.

I removed the insert and set the saw blade 1/8" up from the top of the table; then cut out portions of the bottom of the insert that had supports or parts of the blade mechanism close beneath. This saw has a metal dust collection enclosure around the saw, so I had to remove quite a bit.

Step 6: Test Fit Again

I test fit it again and it sat nearly perfectly flush. This is when I decided not to use the screw depth-adjustment method the original insert has, but just dado out a bit more depth to get it flush.

You want to make sure that a workpiece doesn't get caught on the edge of the insert as you feed it in, and then also that it doesn't get caught on the back of the saw table as it comes off the insert. Because of this; I raised the saw blade 1/4 of a turn and took a bit more material off the front of the insert.

To make the insert easier to remove from the table, I stuck a piece of masking tape onto one of the edges so it stuck up above the table. I could then pull on that to remove the insert.

Step 7: Safety Pins & Finger Hole

On my factory insert there are 2 little pins that press up against the underside of the table on the outfeed side of the table saw. These pins hold the insert in place if for any reason it catches on the back of the blade - which is going upwards when it exits the insert - and prevent the insert from being ripped out of the saw and thrown at the operator.

I measured the location of these pins and drew a guideline on the insert, then hammered 2 small finishing nails in just below the guideline.

I took the insert over to my drill press and put a 3/4" diameter hole into it so that it would be easier (well: easier than the 'almost impossible' it was getting to be when I checked the depth the last time) to remove the insert. I should have placed the hole closer to the infeed side, because the two pins hold the outfeed side in place a bit too well when you're trying to lift this out of the table.

Thanks for reading!