The ABC’s of ZCI’s

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This article is from Issue 53 of Woodcraft Magazine.

A zero-clearance insert puts your stock throat plate to shame.

By Paul Anthony

Your tablesaw has a big mouth, and it may be getting you into trouble. Here’s the problem: The stock throat plate that came with your saw has a large opening to allow the blade to tilt. That’s good for making beveled cuts, but the wide opening means that there’s no underlying support for the wood fibers. So, the wood tends to tear away at the sides of the kerf as the blade teeth come crashing through the underside of the stock, leaving a nasty looking cut that comments rather profanely on your workmanship.

Fortunately, there’s an easy, inexpensive fix–switching out your stock throat plate for a zero-clearance insert (ZCI). These replacement throat plates are available commercially in a variety of materials (phenolic laminate is among the best), or you can make your own. Regardless of the type of ZCI you use, the basic installation and working principle is the same: After placing the unslotted ZCI in its opening, the saw blade is raised up through it, creating a slot that perfectly hugs the sides of the blade, offering workpiece support to prevent exit tear-out.

In addition to improving cut quality, the tight slot in a ZCI serves as a convenient cutline reference and prevents narrow offcuts from jamming in the blade opening. Perhaps more importantly, a ZCI affords a platform for mounting a shop-made or aftermarket splitter, in case you “lost” or dislike the stock splitter that came with your saw.

You’ll want a dedicated ZCI for every cutter of a different width that you use regularly. That may include one for regular-kerf blades, one for thin-kerf blades, and perhaps several for common dado head configurations. I have one ZCI that I use for 45° miter cuts, but I bring my stock throat plate back into play for any intermediate angles.

Whether  you make a dead-simple solid-wood ZCI, or a batch of perfectly flat, height-adjustable, splitter-accommodating versions, you’ll be glad for your saw’s new tight bite. Expect kind words about your work.

Making a ZCI

The simplest approach to making a ZCI is to thickness-plane a piece of straight-grained hardwood to match the depth of your throat plate opening, and then shape the wood to fit. Unfortunately, a wooden ZCI can warp, compromising accuracy. It can also break if dropped, and it tends to wear over time at the intersection of the blade teeth and the front end of the slot. Plywood is more stable, but not necessarily dead flat. For the flattest, most stable shop-made ZCI’s, I use 1⁄2"-thick MDF (medium-density fiberboard) covered on both sides with plastic laminate, which also provides a low-friction surface.

Cut an MDF blank about 1⁄2" wider and longer than your saw’s throat plate recess. Also cut two pieces of plastic laminate the same size. Apply the plastic laminate to both sides with contact cement, pressing it firmly in place with a roller or bullnose board (Photo A).

Rip just enough from one edge of the blank to flush up the laminate and the MDF. Then mark the blank for width. A snug side-to-side fit in the recess is critical, so mark directly from the insert opening, then make a nibble cut and test the fit (Photo B).

After ripping the blank to width, use the stock throat plate to mark the rounded ends (Photo C). Also drill a finger access hole. Use a bandsaw or jigsaw to cut a bit proud of the lines, and then finish up with a disc or belt sander, testing the fit with the saw blade removed.

You may need to rout away the areas on the underside that contact the metal tabs or lip at the bottom of the recess (Photo D). If you overcut, either shim as necessary with masking tape or install leveling screws.

Mastering The Shape

Rather than sanding the curved ends of each plate, you can finesse one perfectly fitting “master” blank and dedicate it as a template-routing pattern. Attach it to another rough-bandsawn blank with double-sided tape or hot-melt glue, and finish up the curves using a flush-trimming bit in your router table.

Slotting a ZCI

Sawing the slot on a ZCI involves raising the spinning blade up through the insert. However, on many saws, the teeth of an installed full-size blade reach nearly to the saw table surface, which may prevent a ZCI from fully seating in the recess. Most commercial versions include a starter slot to accommodate the top end of the blade, but shop-made ZCIs require a different approach.

To cut the slot, first back off the height-adjustment screws on your stock throat plate. This will create a shallow recess into which you can place your shop-made ZCI (Photo A). Clamp a strong, straight beam across the blank to one side of the blade location, and then slowly raise the spinning blade until it just peeks through the surface (Photo B). Lower the blade, remove the stock plate, and install the zero-clearance blank. Again, clamp it down with the beam, and slowly raise the blade to complete the slot (Photo C). Finish up by cutting the splitter slot with a jigsaw (Photo D), or as shown in Photo C on page 33.

Note: If fitting a shop-made ZCI to your particular saw requires routing the full length of the edges on the underside, do that after cutting the slot as described above.

A Clamping Bridge

Clamping a beam across your saw top to hold down a ZCI for slotting can be kind of a hassle, especially if an outfeed table abuts your saw. Instead, you can make quick and easy work of securing the ZCI by using a simple hold-down. Just grind opposing flats on a 3⁄8"-dia. barb-less T-nut so it slides in a miter gauge slot, and then make a simple wooden bridge with a center hole to accommodate a short section of 3⁄8"-diameter threaded rod with a wing nut.

Installing leveling screws

For accuracy, a ZCI (or even a standard throat plate) must sit perfectly level with the saw table top. Otherwise it can compromise the intended depth of cut for joints or impede workpiece travel. Although you can shim a ZCI with tape or other materials to level it, a better way is with screws, and it’s best if they’re accessible from the top side of the plate so you don’t have to remove it to make adjustments.

For my shop-made ZCIs, I prefer Allen-head set screws. They’re easy to install, allow for fine height adjustment, and don’t create large recesses in the surface to trap sawdust and other detritus. For the typical throat plate, 1⁄4-20 by 1⁄2"-long set screws work nicely. For thinner plates, use shorter screws.

To install them, you’ll need a 1⁄4-20 tap and a handle for it, both commonly available at hardware stores for a few bucks (Photo A). Using a 3⁄16" bit in the drill press, bore through holes at the appropriate locations on your throat plate (Photo B). Use the tap to cut the threads for the set screws (Photo C). If you haven’t used a tap before, don’t worry. It’s as easy as keeping the tap relatively plumb while applying good downward twisting force for the first 1⁄4" or so. After that, you just turn the handle to finish cutting the threads.

There is no need to use thread-cutting oil as you would when tapping a hole in a piece of metal. Just be sure to thread completely past the taper at the end of the tap (Photo D). Install the set screws using an Allen wrench, and you’re done (Photo E).

ZCI-mounted splitters

Mounting a splitter

A splitter is crucial to tablesaw safety. Its primary job is to keep the workpiece against the rip fence and away from the rising rear teeth of the blade, which can otherwise catch the piece and violently kick it backward toward the operator. Unfortunately, the stock splitters on many saws–particularly older models–were removed and often misplaced because they were so troublesome to attach and detach for making non-through cuts like dadoes.

A ZCI presents the perfect opportunity to install a splitter on a saw that’s missing one. Micro Jig’s MJ splitter (available from Woodcraft) is a good commercial option for a ZCI-mounted splitter, or you can make your own from wood or aluminum. Whatever type of splitter you use, keep in mind that its effectiveness depends on proper alignment. Always mount it so that it aligns with the sides of the teeth facing the rip fence.

A wooden splitter

It’s easy to install a wooden splitter in an MDF, wood, or plywood ZCI because the splitter is simply glued into the blade slot. When doing this, use a fresh ZCI, as the slot on a used one may have widened a bit over time.

Slot the ZCI, and then saw a splitter from the end of a piece of hardwood scrap to fit the width of the slot exactly. You can make the splitter whatever height you like. A projection of 3⁄4" or so is fine for general work. Cut the piece to about 11⁄4" wide. Chamfer or round over the leading edge of the splitter, and then glue it in place (Photo A). Make a test rip to ensure that the workpiece doesn’t bind between the splitter and fence. If it’s a tight squeeze, sand the side of the splitter with fine sandpaper wrapped around a truly square wooden block (Photo B).

A Wooden Splitter For Thick Stock

The shortcoming of a splitter that’s installed in the blade slot is that it’s only good for ripping moderately thick stock. A blade raised past a certain point will start to cut away the splitter. For ripping thick stock, install the splitter in an aft slot cut, as shown in Photo C (opposite page). Trimming the bottom corner of the splitter to match the slope at the end of the aft slot will locate the splitter as close as possible to the blade while maintaining maximum strength of the ZCI.

An aluminum splitter

A splitter can easily be made from 1⁄16"-thick aluminum angle (available at hardware stores and home centers.) Install it in a separate slot aft of the blade slot. To cut the aft slot, flip the ZCI upside down and sideways, and slip it over the blade. Carefully slide the rip fence against the edge of the ZCI just enough to make contact without applying pressure against the blade (Photo A). With the ZCI upside down, mark a line 1⁄2" from the end of the blade slot (Photo B). WARNING: Unplug the saw when setting up to cut the aft slot.

Raise the blade fully in order to cut the end of the slot as perpendicular as possible. Use a blade guard, and clamp a featherboard to the fence. Saw to your line, keeping the ZCI pressed firmly against the fence (Photo C). Don’t back away from the spinning blade; just hold the ZCI in place at the end of the cut, shut off the saw, and wait for the blade to stop before retracting the piece. Then, with the ZCI right side up, use a bandsaw to cut away the sloped section of the slot, which will allow the splitter to sit a bit closer to the blade (Photo D).

Cut a 11⁄4" length of aluminum angle. (A carbide tablesaw blade will do the job nicely.) Drill clearance holes in one leg for mounting screws (Photo E). Press the splitter against the rip fence side of the slot and mark for the screw holes, offsetting them slightly so they’ll pull the splitter toward that side (Photo F). Drill pilot holes and install the screws. Flip the ZCI over, and make sure that the splitter is pressed entirely against the edge of the slot. If necessary, enlarge one or both holes to allow minor adjustment. Finish up by filing opposing chamfers on the leading edge of the splitter. Use thick paper to protect the ZCI (Photo G).  

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