Zero-Clearance Inserts

Zero-Clearance Inserts

Make ‘em by the bunch and you’ll
save a bundle.

By Tim Johnson

Zero-clearance throat inserts are the best simple upgrade you can make to your tablesaw. They’ll make your saw safer and give you better cutting results with less hassle. A zero-clearance insert minimizes the size of the blade opening. Raising the blade through the specially made insert blank reduces the opening to the exact size of the blade’s kerf.

When you see the difference that zero clearance makes, you’ll want to have a dedicated insert for each sawing operation you do—regular cuts, thin kerf cuts, dado cuts (one for each width!) and even angled cuts. Keep extra blanks on hand, and you’re always ready to replace a worn-out insert. (With repeated use, the zero-clearance slots get wider and less effective).

Don’t let the low cost fool you. These $5 inserts are stable and flat, with dense cores and durable surfaces. Like the $20 commercial versions, they can be leveled and adjusted for a perfect fit. They include retaining pins to keep them in place and finger holes so they’re easy to remove.

Our inserts for ripping and crosscutting are designed to be used with your saw’s splitter and blade guard. Store-bought blanks lack this important safety feature. A $50 investment buys enough material to make nine blanks—enough to last you a long time.

 

Zero clearance all the time

We’ve tailored these inserts for three different sawing operations. Inserts for ripping and crosscutting have dadoes underneath to accommodate 10″ diameter blades.They also have slots for the splitter and blade guard assembly. Inserts for bevel cuts have even wider dadoes underneath. Inserts for dadoes have no  alterations and are the easiest to make.

These 3-3/4″ wide inserts fit the throat openings in most contractor’s and cabinet saws, but are too thick to use with benchtop saws. Measure your saw before you begin, to make sure the width is correct.
Besides your tablesaw, the only other tools you’ll need are a drill press, an edge-guide-equipped router and a hand-held drill.

 

Better than store-bought

Aftermarket insert blanks are available for about $20 apiece. If you make them yourself, they’ll cost less than $5 each. The trick is to make a bunch of them at once.

At a full-service lumberyard, buy one 25″ x 48″ sheet of plastic laminate and one 4×8 sheet of tempered hardboard. You’ll only need a 2×4 section of tempered hardboard, but most lumberyards only sell it as full sheets. Tempered hardboard is more dense and much stiffer than regular hardboard, so it makes better core material for the inserts. You’ll also need carpenter’s wood glue, a box of 1/2″ 6-32 flat-head (Phillips) machine screws and a couple of 8″ x 16″ concrete blocks.

This four-layer laminated sandwich
makes cost-effective, first-rate
blanks.

Four-ply construction

Cut both the hardboard and plastic laminate into nine pieces, all
15-7/8″ long. Keep the hardboard pieces as wide as possible at 7-7/8″
Cut the plastic laminate pieces 8-1/4″ wide (Fig. A).

Stack the pieces in four four-layer sandwiches. You’ll have single
leftover pieces of hardboard and plastic laminate. Cut these two pieces
in half, lengthwise, and stack the four pieces to make a single
“half-sandwich.”
Before gluing, scuff-sand both faces of the
hardboard pieces. This quick sanding flattens any ridges and dulls the
shiny surfaces so the glue will adhere.

While gluing each four-layer sandwiched blank, keep the hardboard
cores centered inside the edges of the plastic laminate. Use cauls and
concrete blocks as clamps (Photo 1).

After gluing, cut each blank in half to yield two blanks that are the
same width as your saw’s stock metal throat plate (Photo 2). The
plastic-laminate faces are wide enough to give you a little cushion, but
you won’t have much left over, because most throat plates are 3-3/4-in.
wide. Trim the single half-sandwich blank to the same width. Square the
ends of all nine blanks, leaving them slightly long.

A “master” insert blank serves as a
setup tool and routing template for
making a pile of inserts all at once.

Always keep a master insert

Dedicate one insert as a template for routing your blanks to size
(right). Use the same blank as a setup tool, by recording layout marks
on it (Photos 3 and 4 and Fig. B). Every insert has four holes for the
leveling screws and a finger hole. Inserts for use with a 10-in. blade
require clearance dadoes and splitter slots.

Once you’ve routed both ends of the master insert (Photo 5 and inset),
test-fit it in your saw. It should drop in easily, with just a bit of
front-to-back and side-to-side play.

Anatomy of a general-use insert. This is a view of the
bottom of a basic insert, showing the blade clearance dado,
which allows the insert to clear the blade when it is
fully lowered, two sets of screws for making the
insert flush with the table and snugly fitting in
the saw, a slot for the splitter, and a
retaining pin, which anchors the insert
in the throat.

Inserts for everyday use

The vast majority of tablesaw cuts employ a standard 10-in. blade set at
90 degrees. Making zero-clearance inserts for this setup is relatively
simple (Photos 6 through 8).

All marks are made on the bottom side
of the blank. Mark all your blanks, so you don’t get confused and drill
or rout on the wrong side. Drilling a bunch of blanks is easier if you
equip your drill press with a fence and stop blocks. If you have a
plunge router, you don’t have to drill start and stop holes for the
clearance dado (Photo 6). Countersink the 3/32-in.-diameter holes for
the leveling screws a bit at the bottom and halfway through from the
top. Drill the 3/4-in.-diameter finger hole from both sides, too, to
keep it from blowing out.

Rout the clearance dado (Photo 7). The start hole is oversized, so
you can turn the router on after it’s in place.

To saw the splitter slot, set the blade height at the center of the
stop hole and use guidelines drawn from the hole’s edges to locate the
stop blocks (Photo 8).

Most stock inserts have a retaining pin in the
back. Installing wooden pins is simple (Photo 9).

Adjustment screws added to the side and front of each insert
allow you to remove virtually all of the side-to-side and front-to-back
play (Photos 10 and 11).

After removing the play and leveling the
insert, you’re ready to cut the zero-clearance kerf (Photo 12).

Fig. A: Cutting Diagram for Laminated Blank Stock
(makes 9 inserts)

Fig. B: Measure the Blade Location

Measure from the
edge of the throat
opening to the center
of the blade (X).
Then measure from
the center of the saw
arbor to the front of
the opening (Y).

Fig. C: Inserts Require a Clearance Dado

Inserts require a clearance dado
underneath because a 10-in. blade only barely
retracts below the table. Measure the distance “Z,”
and use it when cutting the clearance dado in the
bottom of the insert.

Fig. D: Determine the Throat’s Recess By Measuring from the Tabs

This story originally appeared in American Woodworker September 2002, issue #95.

Purchase this back issue.

Click any image to view a larger version.

1. Laminate the blanks on the flattest surface in your shop.
After scuff-sanding the hardboard, carefully stack the four
pieces, spreading glue on all the interior surfaces as you go.
Press the stack under cement blocks, making sure the pieces
stay aligned.

2. Saw the blanks in half. Then trim each half-sized blank
to the same width as your saw’s stock throat plate.

3. Trace the stock throat plate’s profile, including the
blade and splitter slots, while holding it centered on a blank and
flush at the front.Then mark the locations of the leveling
screws.

4. Locate the blade-clearance dado, splitter
slot and finger hole on the master blank, using your
measurements from Fig. B. Measurement “X”
indicates the centerline of the blade-clearance dado
and splitter slot.“Y” allows you to mark the dado’s
start and stop locations, which extend 2-in. fore and
3-in. aft.“Y” also locates the finger hole.

5. Rout the master blank, using the
stock metal throat plate as a pattern.
Hold them together with double-faced
tape and clamp them securely to your
bench before routing.

Avoid the retaining pin and slot.
After routing the front, pull the stock
insert off, flip it end-for-end and
remount it. Use the smooth front end
of the stock insert to rout both ends
of your master blank.

6. Drill start and stop holes for routing the clearance
dado. Drill the smaller stop hole for the splitter slot from the
same fence setting, and go all the way through.

7. Rout clearance dadoes using an edge guide. Center the
bit on the guideline.

8. Saw the splitter slots, using stop blocks pinned to an
auxiliary fence.

CAUTION: The blade guard must be removed to make this cut.

Modifications for a Shallow Throat

Some tablesaws
have thin throat
plates, which require
simple modifications to the
inserts we show.To tell if your
saw requires shallow inserts,
measure its throat recess (Fig. D). If
the depth is less than 1/2 in., you’ll have
to make your inserts thinner.

Rabbeting the edges is the quickest way (photo
below left), although you could also drill recesses
for the tabs with a Forstner bit. Make the rabbets
slightly wider than the length of the tabs.You may have
to use shorter adjustment screws, depending on the
depth of the throat recess.

Flanges all around the throat mean
you’ll have to rabbet all four sides of your insert.

Rabbet the edges to make the inserts thinner.

For Bevels

Zero-clearance inserts for angled cuts have to
accommodate a tilted blade.They require a much wider and
longer clearance dado on the bottom side and an equally wide
slot for the splitter.

For Dadoes

Zero-clearance inserts for dado cuts have no
splitter slot because you can’t cut dadoes with the splitter
installed.There’s no clearance dado on the bottom either,
because dado sets are never more than 8-in. in diameter.

Final Touches

9. Install retaining pins to keep the inserts from lifting out
of the throat. Glue a piece of maple into a saw kerf, using the
pin on your stock insert for location. Plane or sand it flush.

10. Clamp the edge while drilling holes for the adjustment
screws, or you may split the hardboard core.

11. Get a perfect fit from side-to-side and front-to-back with
edge-mounted adjustment screws, two on the side and one at
the front. Countersunk leveling screws keep the insert flush to
the table surface.

12. Hold the insert in place when you raise the blade.
Clamp a board to the fence, flush to the insert and positioned
so only the blade’s outer face is exposed. Lock the fence, start
the saw and raise the blade to the desired height.

Throat-Plate Woes

The throat plate, or insert,
that came with your saw is
fine for general work, but it
does create some problems.

 

This offcuts get stuck, forcing you to stop everything
to remove them.The stock
throat plate has a wide opening
because it’s designed for both
angled and vertical cuts.

The bottom edge tears out, especially when you
crosscut plywood, because the
stock throat plate’s wide slot
leaves it unsupported.

Kickback potential. Stock
one-size-fits-all throat plates for
cutting dadoes have enormous
openings that make it
impossible to process short or
narrow pieces.

Zero-Clearance Solutions

Zero-clearance inserts improve
cut quality and make your saw
safer to use.

 

Zero-clearance slots are
no wider than the blade, so offcuts
can’t get stuck and tear-out
is dramatically reduced.
Another benefit:You can use the
saw kerf to line up your cuts.

Zero-clearance is safer. A perfectly sized opening won’t
allow the workpiece to sag,
catch and kick back.