Zero (chip-out) tolerance
Anyone who's ever crosscut oak plywood knows how face-grain chip-out can ruin an edge. Once the damage is done, you're forced to either fill those voids or accept the flaws on your project.
But you don't have to live with chip-out. A shop-made zero-clearance insert replaces your tablesaw's factory-supplied throat plate--and its wide gap that allows unsupported wood fibers to tear away during a cut. Because you cut the blade slot with the blade you're using, the zero-clearance insert fully supports the fibers.
It's a good idea to use an insert for every blade and every cut you make. Plowing a 3/4"-wide dado? Use a custom-fitting insert to stop chip-out. How about a 1/2" dado? Make another insert. Cutting a 30° bevel? Get an insert just for that. You can easily make insert plates, so cut out a dozen blanks and keep them handy for every time you change blades or bevel angles. After using an insert for a specific setup, mark it with that setting (and blade) and save it so you'll have it for the next time you make the same cut.
How to make inserts fast
You can buy pricey, premade phenolic inserts, but we like to make our own zero-clearance inserts from 3/8- or 1/2"-thick Baltic birch plywood. This stable material proves strong, and doesn't have voids between plies. Medium-density fiberboard (MDF) also makes a good insert, but lacks the strength of plywood. Hardwoods, although strong, can shrink or swell with seasonal changes in humidity, and don't work as well as plywood.
With a pattern bit installed in your router table, use your saw's original insert to make duplicates. Some insert plates have antilift tongues [Photo A] right, or lateral-adjustment screws--with these you need to create a pattern to use for making copies. To do this, trace your insert plate onto a blank of plywood or MDF, smoothing the tongue or screw areas.
Cut close to the line at your bandsaw, and then sand the pattern until it fits snugly into your saw's throat. If you want the antilift benefit of the tongue, you can add this to your inserts by cutting a groove on the bottom side and then gluing in a thin strip of hardwood that protrudes under the tabletop.
Cut out rectangular plywood blanks slightly larger than the pattern. Using cloth-backed, double-faced tape, secure the blank to the pattern, and then bandsaw to within 1/8" of the pattern. Next, rout the blanks to shape using a pattern bit or flush-trim bit in your router table [Photo B] below right.
Customize the inserts to fit your tablesaw
Now that you have the blanks cut to shape, make a finger hole (for removing the insert) by drilling a 3/4" hole through each insert. Keep it at least 1" to the side of where the blade will project through the insert.
On many saws with 10" blades, the blade retracts only 1/4" or so below the table surface, photo right, meaning your unkerfed insert blank won't sit flush with the tabletop. You've got three options here: First, use a smaller diameter blade--like one of the outer blades from your stacked dado set, or a blade from your portable circular saw--to cut a relief slot that your 10" blade will fit into. (Unless the blade you use has the same kerf width as your 10" blade, don't raise it high enough to break through the surface.)
Precut the blade kerf
Your second option is to adhere the blank onto the metal insert plate, clamp it in place, and then slowly raise your spinning 10" blade until it just pokes through the plywood blank, photo right. Now separate the two plates and install the zero-clearance insert over the blade.
Routing a relief channel
With option #3, rout a 1/4" channel along the bottom of the insert deep enough to give the blade initial clearance, photo right, when you place it in the saw. However, do not rout deeper than half the thickness of the insert. More than half would weaken it and create a potential safety hazard.
Jigsaw a riving-knife slot
You'll also have to cut relief holes or slots for blade guards, splitters, or riving knives. You can do this with your tablesaw and 10" blade--with the original insert installed--if the relief exits the back of the insert. If it does not, use a jigsaw, photo right. You might also have to relieve the bottom face for such things as the arbor assembly and flange, or for parts specific to the original insert, photo below right. Machine these shallow relief areas at the drill press with a Forstner bit or on a router table.
Now level the insert with the tablesaw top
If your insert sits too high, drill or rout relief areas on the bottom where it sits on the throat tabs or rabbet. If your insert sits too low, add leveling screws so you can adjust the fit perfectly from the top. (Use either setscrews or machine screws.) To do this, either transfer the screw locations from the original insert, or use thumbtacks to mark them, photo right. Now drill shank holes, and countersink or counterbore on the top face. Add the screws, center photo right, and then raise or lower them to make the insert flush, photo below right.
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