Adjustable Circle-Cutting Jig

Adjustable Circle-Cutting Jig

Cut perfect circles
up to 4' across!

By Richard Tendick

Have you ever tried to cut a circle on a
bandsaw, freehand? It’s quick, but it sure
isn’t exact. Dollars to doughnuts you’ll
get fl at spots or bumps that are a pain to
sand out. Big circles are really tough to
get right.

Fortunately, there’s a clever solution.
If you rotate the wood on a fixed
pivot point, like a record on a turntable,
you’ll be able to make a circle that’s just
about perfectly round, needing very little
cleanup work.

Jigs that can provide this pivot
point have been around for a long time,
but I’ve never been satisfied with their
design. They’re hard to mount, hard
to adjust and can’t handle large pieces.
This jig solves all three problems. No
bolts or clamps are necessary to mount
it on your saw. Using just a screwdriver,
the pivot point can be adjusted to any
radius you desire. And it makes circles
as small as 1-1/2" across all the way up
to 48" in diameter.

 

Make the sliding block

As with all jigs and projects, purchase
the hardware first (see Hardware
Shopping List, below). Begin building
the jig by starting with the block
that holds the pivot pin. The block is
composed of two pieces (A and B) that
are clamped together with a machine
screw. Although these pieces are quite
short, it’s best to make them from
blanks that are about 12" long, for
safety when machining them. First,
mill both blanks to final thickness and
rip them to final width.

Using a router table or a dado set,
cut rabbets on both sides of the bottom
piece (B, Fig. B, below). Next, drill
a shallow counterbore for the head
of a T-nut in the center of this piece,
opposite the rabbets. (It doesn’t matter
where you drill the hole along the
blank’s length; you’ll trim off the ends
later.) Drill a 1/4" dia. hole through the
center of the counterbore. Place the
bottom piece above the top piece (A)
and continue the hole. Countersink the
top piece to receive a 10-24 machine
screw. Install the T-nut and clamp the
two pieces together with the screw.

Make the pivot pin. You can use
1/8" dia. welding rod or cut off the end
of an old 1/8" drill bit. Test the fi t of
your pin in a 1/8" dia. hole—it should
be tight. Drill the hole for the pivot pin all the way through the top piece and
partially into the bottom piece. Cut
both pieces to final length.

 

Make the top

Cut the top pieces (C) to fi nal size.
Mill the rabbets that will receive the
pivot-pin block (Fig. C). (Note that
the top and bottom rabbets are cut at
different depths.) On the right piece,
cut a notch for receiving the bandsaw’s
blade (Fig. A). On both pieces,
drill holes for the rare-earth magnet
cups. The depth of this hole is critical,
so drill some test holes in a scrap
piece that’s exactly the same thickness
as the plywood you used for the top
pieces (see “Drill Test Holes for the
Magnets,” right). For maximum holding
power, it’s important that the magnet
be flush with bottom of the jig.
Drill the holes for the machine screws
that will hold the cups, then fl ip the
pieces over and counterbore holes for
the T-nuts using a Forstner bit. Install
the magnet assemblies.

Make the cleats (D) and miterslot
bar (E). Cut a 1/16" deep dado in
the top center of each cleat to provide
clearance for the pivot-pin block (Fig.
A). Make the bar so it fi ts snug in your
saw’s miter slot. Chamfer the bottom
edges of the bar.

Fasten the cleats and bar to the
tops. To space the top pieces, install the
pivot-pin block at one end of the rabbets;
place a cutoff from the block at the
other end. Clamp across the assembly,
then install the cleats. Install the bar
last—it must be perfectly square to the
jig. Finally, install the pivot pin.

When cutting a large circle on
your bandsaw, you may fi nd that the
cast-iron table will tip, no matter how
hard you’ve tightened the trunnion
knobs. Th e fi x is simple: just install
a shop-made device that clamps the
table at 90°. I’ll show you how to do
that at AmericanWoodworker.com/
WebExtras
.

Source

Note: Product availability and prices are subject to change.

Lee Valley, leevalley.com, 800-871-8158,
Rare-earth magnets, 3/4" x 1/8", #99K32.11; 3/4" mounting cups,
#99K32.54.

Cutting List

Hardware Shopping List

Fig. A: Exploded View

Fig. B: Sliding Pivot Bar

Fig. C: Cross Section of Top

This story originally appeared in American Woodworker February/March 2012, issue #158.

Click any image to view a larger version.

Easy to mount.
Just slide the jig across the bandsaw’s
table until a bar drops into the saw’s
miter slot. Done!

Stays put.
Two large rare-earth magnets hold the
jig—enough force to support three full
paint cans. No legs are necessary.

Easy to adjust.
To change the size of the circle you want to
cut, just move this sliding block. The wood
rotates on a pin sticking up from the block.

How It Works

1. Align the pivot
pin
with the
front of the blade
when you place
the jig on the
saw. Next, move
the pivot-pin
block the correct
distance from the
blade (the radius
of the circle you’ll
be cutting). Lock
the block in place
by tightening its
clamping screw.

2. Make a square
blank
the same
size as the circle
you’re going to
cut. Drill a shallow
hole in the center
of the blank, on
the bottom side,
that’s the same
diameter as the
pivot pin. Place
the blank on the
pivot pin, flush
with the blade.
Turn on the saw
and rotate the
blank to cut the
circle.

Drill Holes for the Magnets

Drill the holes
for the magnet
cups very
carefully. The
depth has to be
just right, so the
magnets will be
perfectly flush
with the surface.
Make some test
holes in a piece
of scrap before
you drill into
your jig.