Cut precise curves with a router trammel
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Photo 1: Lay out the trammel
Draw the sides and ends of the trammel. Trace around the base
plate and drill holes for the mounting screws.
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Photo 2: Rout a perfect circle
Screw the trammel to the center of your workpiece and cut out
the circle with your router. Make two or three passes in a counterclockwise
direction.
Whether you're building a frame for an
arched opening, making curved brackets
or fashioning arch-top casing, marking
and cutting curves is part of the
process. In this article, we'll show you
several techniques and tips for marking,
cutting and fine-tuning curves.
Some methods are best suited for rough
curves. Others are refined enough for
furniture making. Choose the technique
that works best for the project at
hand.
This simple router trammel is easy to
build and allows you to cut a perfect
circle. For circles up to 6 ft. across, use
a piece of 1/4-in. plywood, MDF or
hardboard that's about 4 ft. long and at
least as wide as your router base. Start
by removing the base plate from your
router and clamping it to one end of the
trammel material. If you want your
trammel to be stylish, trace around a
coffee cup to make a nice-looking
rounded end. Then draw tangent lines
connecting the circles and cut the sides.
If you don't care about looks, simply
make a long rectangular trammel.
Trace around the base plate and use the mounting holes as
a guide for drilling holes in the trammel (Photo 1). Cut out the
trammel and drill a 1-1/2-in. hole in the center of the router
end to clear the router bit. Countersink the mounting screw
holes so the screw heads won't tear up your workpiece.
Attach the router to the trammel with the base plate screws.
Screw the trammel to the workpiece, centering it on the
circle you want to cut out. Mount a straight plunge-cutting
bit in your router and set the router bit to cut about 3/8 in.
deep for the first pass. A plunge router works best, but if
you don't have one, hold the router above the wood and
start it. Carefully plunge it into the wood and begin moving
it counterclockwise around the circle (Photo 2). Complete the circle,
then readjust the depth and make another pass until you
cut all the way through.
Plastic wood template
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Bend plastic wood
Bent plastic wood makes a perfect curve. Clamp blocks to a 2x2 and spring-fit a length of 1x2 plastic wood
between them. Adjust the position of the blocks to change the
curve.
Often you can simply “eyeball” the best curve for the job by
bending a piece of wood and using it as a template. But
variations in wood grain can result in inconsistent curves.
Here's a tip to make this technique even better. Use plastic
wood or a plastic molding instead. It bends very uniformly
and yields near-perfect symmetrical curves. Azek, Fypon,
Kleer and Versatex are several brands available at home
centers. Choose a thickness that'll bend to the curve you
need. For gradual bends or wide curves, use 3/4-in.-thick
material. For tighter bends (those with a smaller radius),
use a 1/2-in. x 1-1/4-in. plastic stop molding or something
similar. Support the ends of the plastic wood with blocks
attached to a strip of wood. Adjust the position of the
blocks to change the curve.
Draw large curves with a giant compass
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Drill two pencil holes for parallel curves
Clamp the board and screw the compass to the workbench. Use a scrap to elevate the
compass to the same height as the board you're marking. Make sure the pivot point of the
compass is at 90 degrees to the center of the board.
Grab any narrow board or strip of plywood
and drill a few holes—voilà,
instant compass. Drill a pencil-size
hole a few inches from the end of the
board. Then drill a screw-size hole at
the pivot point. The distance between
them should be the radius of the curve,
if you know what that measurement is.
Otherwise, just use the trial-and-error
method, drilling a series of pivot holes
until you can swing the trammel and
draw the right-size arch. It's easy to
draw parallel curves too. Just drill two
pencil holes spaced the desired distance
apart.
There's no limit to the size of the
arch you can draw. If your plan calls for
a 10-ft. radius, find a long stick and use
the floor as your workbench.
Cut gradual curves with a circular saw
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Circular saw technique
You don't need a jigsaw to cut corves. When you're forming gradual curves, you can speed things up by
using a circular saw instead.
The first tool that comes to mind for cutting curves is a jigsaw,
but if the curve is gradual, try a circular saw instead.
It's surprisingly quick and easy to cut a smooth curve with
a circular saw. This method is for cutting rough curves.
Don't try to make furniture with this technique. The trick is
to make sure the curve is gradual enough that the blade
doesn’t bind. If you try this method and the blade binds or
starts to heat up and smoke, switch to the jigsaw. The thinner
the material you're cutting, the sharper the curve can
be. Set the blade depth so it barely projects through the bottom
of the wood.
Stack and sand for matching parts
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Sand tight curves
Use a random orbital sander for tight curves. Line up the parts and clamp them together. If they're small parts
like these, clamp them to the workbench to hold them in place.
Sand the curves smooth with a belt sander or random orbital
sander.
When you have several identical curved parts, the best way
to sand them is to stack them and sand them all at once.
You'll save time and the parts will all match perfectly. The
wider surface keeps you from rounding off edges. If the
parts require a lot of sanding, a belt sander is a good choice.
If you don't have to remove much wood, try a random
orbital sander. The key to success is to keep the sander
moving at all times to avoid creating any flat spots. Check
your progress by running your hand over the parts. Mark
high spots with a pencil so you’ll know where more sanding
is needed.
Use a pattern and a router for irregular curves
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Photo 1: Make matching parts with a pattern
Trace out and rough-cut your part. Attach the pattern with hot-melt glue. Use a top-bearing
pattern bit to follow the pattern and shape the part. Move the router counterclockwise
around the pattern.
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Photo 1A: Close-up of pattern bit
A pattern bit has the bearing mounted on top to follow a pre-cut pattern.
When your plan calls for cutting
curved parts and you need to make two
or more, first shape and sand a perfect
full-size pattern from a
piece of 1/2-in. medium-density
fiberboard.
Then use a router with
a top-bearing pattern
bit to cut out the parts.
Here are a few tips
for routing with a pattern bit.
First, use the pattern to mark the shape.
Then remove excess material by cutting
about 1/4 to 1/8 in. outside the lines
with a jigsaw or a band saw. Elevate the
workpiece to avoid cutting into your
workbench. We used Bench Cookies
(available at rockler.com). But hot-melt glue and scraps of
wood are another option. If you're cutting
material that's thicker than the pattern
bit is deep, cut as deep as you can.
Then remove the pattern and use the
part as the pattern to complete the cut.
Mark an arch with two sticks
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Photo 1: Arch drawing set-up
Set up for arch drawing with two sticks. Drive nails at the ends of the baseline and at the height of the
arch. Lay one stick across two height nails and lay the other from
the center height to the end of the baseline. Connect the sticks
with short pins or hot-melt glue.
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Photo 2: Slide the sticks over the nails
Nestle a pencil into the crook of the sticks. With one end of the
sticks resting on the baseline end nail and the other on the center
height nail, slide the sticks along the nails to draw the arch. Repeat
on the opposite side to complete the arch.
Here's a quick way to draw an accurate
curve if you know how wide and tall
you want the arch to be. Let's say you
want to draw an arch that's 3 ft. wide
and 9 in. high. Drive two nails at the
ends of the 3-ft. baseline. At the center
of the baseline, draw a perpendicular
line and make a mark 9 in. above the
baseline. Drive a nail at the mark. At
one end of the baseline, draw another
perpendicular line and make another
mark 9 in. above the baseline. Drive
another nail at this mark. Photo 1 shows
how to arrange and connect two sticks
that you will use to draw the arch
(Photo 2).
