A zero clearance insert for your table saw, so called because the slot is cut by the blade itself, and not machined to some arbitrary width, can be very useful in your work. For one thing, it can help prevent wooden shrapnel or other anomalies by keeping small cutoffs from wandering into the throat of the saw between the blade and the insert. For another thing, zero clearance plates provide more support to the workpiece; useful when cutting small pieces. Once you learn this technique, you can make inserts for your dado head cutter and your molding head cutter that will enhance the safety of their use as well.
I used ½" Baltic birch plywood (which is actually more like 15⁄32"). Originally I cut one out of 1⁄8" BB, and it fit fairly well, but
First I ripped a piece of BB to the width of the insert and then used the original metal insert to trace the shape of the insert onto the BB. I then rough cut it with a band saw, to within 1⁄8" of the lines. Making several trips between the sanding drum and the table saw, I was able to get a really nice fit on the insert.
You need to stop here make a decision. If, despite my admonition, you used 1⁄8", you can put it right to work. If you made the wise choice and did it with ½" you can start the process of adapting it for use. However, if you went to all that effort why not exercise Choice 3 and mark the piece you just made with big black letters: Pattern, and set it aside.
Assuming you chose the wise course, rough cut another piece of BB as before and double-stick tape it to your newly labeled Pattern. Then using a flush trim bit in your router, rout the rough blank to the shape and size of the pattern keeping the pilot bearing of the bit against the pattern. Split out the pattern from the new piece, put the pattern away and proceed to the next step (you may have to do some light sanding to get the blank to fit perfectly).
Adapting the insert to use requires some inspection of the saw and a choice of methodology for the next step.
Yours may be different, but my saw has a ledge a little more than 1⁄8" below the surface of the table. The ledge varies in width from about 3⁄16" along the sides, to about 5⁄8" at the ends. Also, it is somewhat irregularly shaped at the ends. After unplugging the saw! set the insert in place and reaching underneath trace the shape of the ledge onto the underside of the insert. This requires a little bit of gymnastics, and you won't be able to get a really accurate line on the side toward the arbor flange.
In short, there are two ways to accommodate for the ledge: the first is to simply make several passes on the router table against a fence cutting out more and more material that interferes with a flush fit. I made my first ½" insert this way. The second is to make another pattern the size of the hole inside the ledges, register it to the main pattern and blanks and use it with a pattern bit to rout the ledges.
Method 3A: if you have a router table, chuck a mortising bit or straight bit into the router and set up a fence that leaves about ¼" exposed. This dimension should be a little more than the width of the shelf in the table saw throat along the sides. Set the bit height equal to or a little less than the depth of the shelf and run the insert along the two sides. Now that I know how to do it, I'd set the depth for 1⁄32" less and then once the insert will fit the outline of the saw opening, from executing the following steps, go back and try deeper depths until it is perfect.
There is no point in trying to fit the ends exactly, so just move the fence to expose more of the bit, and nibble away at the ends until the insert fits in the throat. This will take several trips between the router and the saw.
If you don't have a router table, you can accomplish the same thing by hand, although with not quite the consistent edges as on the table. It really doesn't matter, as this cutting doesn't relate to the fit in the outline of the throat, anyway.
A couple of points;
The first thing to do is take a piece of baltic birch routed for use as an insert. That means you made it from the Pattern. Before you split it off from the Pattern go to the drill press and drill a couple of 1/8" holes. The location isn't real critical, although you'll certainly want to be at least ½" in from any of the edges. Probably right down the center and a couple of inches from either end is best. Once you've drilled the holes you can split the pieces apart.
Now, mark both the Pattern (lets call it Pattern, main from now on) and the other piece (we'll start calling it Pattern, inset) so that you'll always be able to orient them properly with regard to the holes you drilled. If you manage to flip one of them over in subsequent steps, you'll mess up the necessary alignment.
Take Pattern, inset over to the saw and set it in place (in the proper orientation. Reach underneath the saw (Unplugged!—did I need to say that?) and mark the edge of the ledges in the blade opening on the bottom of the Pattern, inset. Then take it to the bandsaw and cut it out, along the lines. Refine the fit with the sanding drum—it doesn't have to be as precise as the Pattern, main does—in fact, somewhat loose is good. The function of Pattern, inset is to define clearance rather than fit.
What you've now done is made a Pattern for the insert itself, and a Pattern for the ledge it rests on that are automatically aligned by the two holes. Did I mention the roll pins you need to buy? 1⁄8"x≈¾ to 1" in length. You will use the pins to align not only Pattern, main and Pattern, inset, but also the blanks you'll be making and turning into inserts.
To use, place Pattern, main over the top of a rough cut blank of ½" baltic birch and smack the roll pins (left in place) lightly with a hammer or mallet. Mark the blank with the proper orientation. Take the blank to the drill press and drill out two 1⁄8" holes where the roll pins left their mark. Come back and attach the Pattern, main to the blank by driving the roll pins into the freshly drilled holes, and rout to fit with the pattern bit. Remove the blank from Pattern, main (leaving the roll pins in Pattern, main) and refine the edges of the blank at the drum sander for a snug fit.
Now attach Pattern, inset (to which you have previously attached two roll pins of its own) to the bottom of the blank (again making sure of proper orientation), and with a top bearing pattern bit, rout the edges of the blank to the proper depth. You'll have to fuss with depth of cut until you get the insert to fit with a lip ≈1⁄8" + when you are done, adjusting the cut for a dead flush fit (you won't even need to remove Pattern, inset as you test the fit), but when you're done, voila, you have a new insert.
Every time you make a new insert (whether you wear them out or you need them for dado sets or TK blades), you will have to test fit for depth of cut unless you make more than one at the same time. I suppose you could make a gauge for the depth of cut, but I never got the process refined that far. You will be able to easily turn out custom, well fit inserts for so long as you own your saw.
When getting ready to make the slot for the saw, I found that while the blade retracts low enough for a 1⁄8" insert, it doesn't clear a ½" insert. There are two ways to solve this problem. Probably the best solution is to put a smaller blade, such as from your dado set, or your circular saw on temporarily and use that to cut the slot. Put the insert in place, slide the fence over part of the insert, making sure you don't put it over the blade, fire up the saw, and raise the running blade through the insert.
If you use a thin kerf blade, however, the dado blade will probably be thicker, defeating the concept of zero clearance. So, what I did was flip the insert over (end for end) so that the top surface is down in the throat. This makes it the same depth as the insert you are replacing. Then follow the procedure described above, except the fence won't sit nicely over it. It'll do well enough to keep the plate from rising, however. Be sure not to flip it side for side, as you will wind up with your blade slot on the wrong side of the insert when you put it back.
The final touch is to drill a small hole (5⁄64") in the end toward the motor that angles up from underneath into the insert. Drive a 4d finishing nail into the hole and clip the head off. This emulates the pin on the real saw inserts that keeps the end of the insert from rising up due to the motion of the blade. In real life, I never had a problem with that when I had the 1⁄8" insert and no nail in use.
An hour's work and you feel like you have a new saw.
Last updated: 22 August 2010