How to Rout a Large Arc

How to Rout a
Large Arc

Need a big, smooth curve?
This method will do it!

By Jim Baumann

Curved lines in any project are both a blessing and a
curse. They’re a blessing for the eye of the beholder, but a
curse for the poor guy who has to make them.

Executing a perfect, fair curve isn’t easy—particularly
a large one. If you cut one freehand, you’re often
faced with hours of sanding to true it up. I build custom
fireplace mantels, which often
require a horizontal front board
with an arc that follows the insert
below. I can’t afford to use such
a slow, inexact method, so I’ve
adopted a much more efficient and
precise approach using a router.
The results are so good that I hardly
have to sand at all.

 

Calculate the radius

An arc can be elliptical, oval or circular,
or some combination of these three shapes. I’ll only
deal with the simplest one—an arc that’s a segment of a
circle. Using the equivalent of a giant trammel (a compass
that uses a beam instead of two legs), this type of arc is
fairly easy to make with a router.

Let’s say you have a pretty good idea of how large the
arc should be. You know how tall it will be (its rise) and
how long it will be (its run). In order to calculate the
trammel’s length, you’ll need to figure out the radius of
the circle that defines the arc. And in order to pivot the
trammel, you’ll need a method to pinpoint the circle’s center.
No problem.

Using the measurements of
your arc’s rise and run, calculating
the circle’s radius is a simple
math problem:

 

 

Once you know the radius, it’s
pretty easy to find the circle’s center—
but we’ll deal with that later.

Although your arc may not be
very long, the circle that it’s based
on might be huge. In this article,
I’ll be routing an arc with a rise of
only 4″. From endpoint to endpoint, its run is 42″. Doing
the math, I’ll need to rout an arc with a radius of 57-1/8″—
almost five feet! My method will work for making an arc
of virtually any size, but it’s particularly well-suited for big
ones like this.

 

Make the arm

To rout the arc, you’ll be mounting your router on the end
of a long arm. Making the arm is very simple—it’s just
a straight piece of wood. Once you’re done with the job,
there’s no need to hang on to the piece; you can just put it
back in your woodpile.

So how do you connect the arm to the router? I use
a very simple system that should work with any router
that accepts a fence. You don’t need the fence itself; you
just need the rods or bars that the fence slides on—the
parts that slip through the router’s base. (If you don’t have
these parts, you’ll probably be able to find or make substitutes.)
To connect the rods to the arm, you’ll be making a
wooden device that clamps around the rods (Fig. A).

You’ll have to adjust the dimensions of this clamping
device to suit your router, of course. Both parts of the
clamp are the same thickness, and that thickness is a critical
measurement. Why? Th e bottom of the clamp must be
even with the bottom of the router, or slightly above it, so
the clamp doesn’t bump into your workpiece when you’re
routing the wide part of the arc (Fig. B, below).

To determine the thickness of the clamp’s parts, measure
the distance from the center of the rods to the base
of the router. Subtract 1/32″ or so from this measurement,
then mill the clamp’s parts to this dimension.

The two halves of the clamp must firmly grip the rods
when you tighten the clamp’s screws. Th e best way to
ensure this is to insert spacers between the halves when
you clamp them together and drill the holes for the rods
(Photo 1). (I made my spacers from a craft stick, similar
to a tongue depressor, but any piece of wood will do.)
Th ese holes should be the same diameter as the rods—
not smaller or larger. When you’re done, test the holding
power of the clamp by fastening it to the rods (Photo 2).

Next, fasten a T-plate to the top of the clamp. Th e exact
position of the T-plate doesn’t matter—I off set it so that its screws didn’t run into the rods or clamping screws.
Make the arm from a stout piece of wood. (You don’t want
this part to fl ex when you’re routing.) Mine is 7/8″ thick
by 2-1/2″ wide. Cut the arm the same length as the arc’s
radius, more or less, then drill a hole for the pin that the
arm will pivot on (Fig. B). Th is hole should be perpendicular,
so use a drill press. Position the hole about 4″
from one end of the arm. I used a 6d nail for the pivot pin,
which required a hole 3/32″ dia. Fasten the T-plate to the
arm (Photo 3).

 

Adjust the arm

Before you set up the router, you’ll need to come up with
some method of anchoring your workpiece so it can’t
move. You’ll also have to lock down a second board, which
will act as a pivot point. Let’s call that the “pivot block.”
Depending on the radius of your arc, the workpiece and
the pivot block might be quite far from each other—my
arc requires them to be about fi ve feet apart.

My workbench isn’t large enough for this arrangement,
so I set up a platform on sawhorses, using a ratty sheet
of plywood (Photo 4). Screwing the workpiece and pivot
block to the plywood guarantees that they won’t move relative
to one another. It might be possible for you to fasten
these pieces to your fl oor, but the platform method has
an additional benefi t: It puts everything at a comfortable
working height.

Cut your workpiece about 4″ extra long, then mark
the endpoints of the arc, leaving extra space at both ends
of the board (Photo 5). Fasten the board to the plywood,
countersinking the screws (Photo 6).

Next, cut two sticks the exact length of the arc’s radius
(Photo 7). Miter both ends of each stick. Cut a piece of
scrap for the pivot block—it should be the same thickness
as your workpiece and about 8″ square. Use one of the
sticks to position the pivot block (close is good enough),
then fasten the block to the plywood (Photo 8).

Use the two sticks to locate the center of the arc
(Photo 9). Trace around the pivot-block ends of both sticks
(Photo 10). Drill a hole in the block for the 6d nail
(Photo 11). Insert the nail through the arm and two
washers, then into the pivot block (Photo 12). Next,
make a support stick that’s about the same length as your
workpiece. Its role is to keep the router level, so its thickness is important: It should be the same as the pivot
block plus the two washers. See Fig. B for a cross section
of the whole setup.

Install a straight bit in your router (I use a 3/8″). Adjust
the bit’s height so that the end of the bit is fl ush with the
router’s base. Slide the router onto the rods on the other
end of the arm, then position the router so the edge of the
bit touches the arc’s endpoint (Photo 13). Swing the router
over to the other endpoint—the bit should touch that, too.
If it doesn’t, you’ll have to go back a few steps and reposition
the pivot block in order to drill a new nail hole.

Lower the bit to cut about 1/4″ deep, then turn on the
router and swing it through the entire arc (Photo 14).
Repeat this process a few times—the last pass should cut
1/16″ or so deep into the plywood so that you can cleanly
separate the waste from the arc (Photo 15).

Fig. A: Exploded View

Fig. B: Arm Setup

Click any image to view a larger version.

1. Start by making a pair of blocks that will clamp onto the rods for
your router’s fence. Place two 1/16″ spacers between the blocks,
then drill holes the same size as the rods.

2. Fasten the clamping blocks onto the rods. You won’t need the
fence—you can set it aside.

3. Make a stout arm that’s about the same length as the radius of
the arc you’ll rout. Fasten the arm to the clamping blocks with a
T-plate.



4. Assemble a plywood platform for routing the arc. The plywood
shouldn’t droop—you may need to support it with 2x4s or these
torsion beams.

5. Mark the endpoints of the arc on your workpiece.

6. Fasten the workpiece to the plywood. Allow about 2″ of waste
on both ends of the board to provide room for the screws.

7. Cut two sticks that are the exact length of your arc’s radius. For
precise measurements later on, miter their ends.

8. Fasten a block to the plywood to serve as a pivot point for
routing the arc. Use one of the radius sticks to position the
block—close is good enough.

9. Locate the center of the arc. First, place the radius sticks on the
endpoints of the arc (see inset). Support each stick with a block
that’s the same thickness as the workpiece.

10. Position the other ends of the sticks so
that their points meet. Trace around the
miters.

11. Drill a small hole at the intersection of the
lines. This is the exact center of the arc.

12. Insert a nail through the arm that’s
attached to the clamping blocks. Place
some washers on top of the pivot block,
then insert the nail into the block’s hole.

13. Adjust the position of the router by sliding
its base along the rods. The outside edge
of the bit should touch one of the arc’s
endpoints.

14. Swing the arm and begin routing the arc. The first pass should
only be 1/4″ deep or so. To keep the router level at both ends of
the arc, support the arm with a long stick.

15. Reset the depth of cut a few more times until you cut all the way
through your workpiece. The result is a perfectly smooth, true arc.

This story originally appeared in American Woodworker December/January 2014, issue #169.