Simple Circle Jig - For Perfect Circles
Cut or sand perfect
circles with this simple jig you can make in an hour!
you are, building your latest flying model masterpiece, when a perfectly round
shape is needed for a cowl ring, nose ring, or a firewall for an engine nacelle.
If you're like me, you whip out the old circle template or drafting compass and
lay out a nice circle on the wood. No problem, fire up the scroll saw or bandsaw
and there it is... right?
times, the layout stage is the last time you'll see a perfect circle. Beyond that,
committing wood to machinery usually results in circles with slight flat spots
or some egg-centric shapes :-). Unless you have the hands and coordination of
the best surgeon, a round shape done freehand will not be very round.
your buddies will point out that flaw right away.
solution is fairly simple. Build a circle shaping jig.
So what is a "Jig" exactly?
jig or fixture can loosely be defined as: A home-made clamping, holding, alignment,
or material handling device that may manupulate, align, or hold raw materials
for processing by shop machinery, in a way that is not native to the machine's
normal mode of operation, to achive a unique result or margin of safety.
nifty little jig is simply a method of fastening an adjustable pivot point to
a sander or bandsaw for cutting and sanding circle shapes in wood, soft metal,
basic shop machinery has slots milled somewhere in the work table for a miter
gauge. If you're lucky and have a bandsaw, scroll saw, and sander - you might
just be able to use this jig on all of these machines. If you're even luckier,
all of these machines will have the same size miter slots. If you are truly blessed,
the slots will all be the same distance from the blade or sanding disk. If you
are this lucky, I suggest you go buy a ticket for the state lottery.
jig simply fits into the miter slot of my bandsaw and sander.
of the circle jig starts with making the miter slot blocks. These blocks should
be a snug fit in the miter slot so the vibrations from the machine don't allow
the jig to slide away. I made my blocks out of a scrap of pine. They are a snug
fit, and rise slightly proud of the surface of the table by around a 64th of an
reccommend that you make yours out of pine or poplar. If you use an extremely
hard wood like oak or maple, you may lose some of the repeatable friction fitting
properties that the slightly soft pine provides.
DO NOT, UNDER ANY CIRCUMSTANCES USE BALSA TO BUILD THIS JIG!
you'll need a good flat board to use as a table. I used 3/8" aircraft grade
birch plywood for my table.
decided to make the table 7" wide x 12" Long to accommodate most of
the circles I would have to cut for all but the most monstrous giant scale airplane
project. The ultimate useful circle limit on my sander is nearly 20 Inches in
the distance from the cutting surface to the edge of the miter slot. Subtract
1/8" for table edge clearance from the cutter. You want the table edge to
be set back from the cutter, blade, or sander about 1/8 of an inch. This is for
safety and to allow sanding dust and chips to escape.
a line that distance from one short edge of the table. Scribe another line the
width of the miter slot farther away from the cutting surface. These are your
guide lines for gluing your blocks. Glue the blocks 1/2" away from the sides
of the table to allow for finger pull areas when you want to pull the table free
from the machine.
the blocks to the table with slow setting epoxy, thick CA, or CA gel. Make sure
no excess glue forms fillets around the block. It will interfere with proper seating
of the table in the slot.
you should have a piece of scrap .047" wire, or something similar, laying
around from a pushrod or control linkage. Cut a piece 1 1/2" long and bevel
the ends round on the sander.
a straight center line down length of the board. A hard lead mechanical pencil
was used on mine. I made a sharp, dark line so that it would not rub off.
a .054" hole in the board. This hole should be located on the centerline
of the table at the point where you the center radius would be for your circle.
the switchplate project, I marked mine at the 2" mark and the 1 7/8"
mark, centerpunched, and drilled them out on the drill press.
I might scribe another center line 1/8" inch to either side of the existing
centerline. This would be used for my 1/16" offset points since it's impractical
to drill all the holes along one line.
table is now done and ready to be tested.
Here's the first
piece I intend to cut. It is a piece of maple that will become a switch mounting
plate for my garage door opener button.
this small project, I'll free-hand cut the circle close to the line on the scroll
saw, then I'll sand it perfectly round on the circle jig. I could have cut it,
using the jig, on the band saw, but I didn't feel like swicthing to a smaller
blade for one little project.
scribed a 3" circle on a blank of 1/4" maple then I roughly cut the
blank to shape. I left less than 1/16" inch of extra room to be sanded off.
drilled a .054" hole where the point of the compass went.
you don't want a hole through your workpiece, drill only halfway through from
the back and use a shorter pivot pin, or use a short, pointed pin as a pivot.
it slow and easy sanding the circle.
would suggest that you sand one side of the circle almost up to the line without
any pivot pin, then turn off the sander and seat the pin in the table. Hold onto
the workpiece while turning on the sander.
the piece slowly through a whole revolution, then make a quick pass again to chase
over the whole thing again.
Here are a few short clips showing the jig in operation.
are 1.4Mb - 15 Sec.)
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there it is, a perfect circle.
you look at the larger images, you might also notice the nice clean, sharp, edge.
If you look even closer, you'll notice something else interesing. I discovered
my bandsaw is 1" further away from the front edge of the jig. I marked one
side of the scale for the bandsaw, and the other side was marked for the sander.
I'm just glad they were an even inch in difference.
is the finished product. It's the new wallplate for my garage door opener button.
a bit spiffy for the garage, but it matches the trim moldings and solves the problem
of the button falling off the wall from worn out drywall holes.
edges were routed and the piece was stained. A hand rubbed wax finish was applied
to seal the wood.
- Update: Tanzer Cabin Wing - Firewall Project|
in the process of building a large model airplane that needed two identical 1/4
by 8 3/8 inch in diameter firewall pieces that would have to be
laminated together for a
1/2 inch firewall. Here are a few photos and a movie
of my band saw cutting operation
using the circle jig.