The Simple Circle Jig - For Perfect Circles
Cut or sand perfect circles with this simple jig you can make in an hour!


So there 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?

Most 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.

Yeah, your buddies will point out that flaw right away.

The solution is fairly simple. Build a circle shaping jig.

Ok, So what is a "Jig" exactly?

A 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.

Our 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, or plastic.

Most 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.

My jig simply fits into the miter slot of my bandsaw and sander.


Construction 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 inch.

I'd 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!


Next, you'll need a good flat board to use as a table. I used 3/8" aircraft grade birch plywood for my table.

I 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 diameter!

Measure 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.

Scribe 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.

Glue 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.



Hopefully, 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.

Scribe 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.

Drill 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.

For 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.

Later, 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.

The table is now done and ready to be tested.



The Switchplate Project
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.

For 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.

I 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.

I drilled a .054" hole where the point of the compass went.

If 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.


Take it slow and easy sanding the circle.

I 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.

Rotate the piece slowly through a whole revolution, then make a quick pass again to chase over the whole thing again.


Ok, movie time!
Here are a few short clips showing the jig in operation.

(Both are 1.4Mb - 15 Sec.)






And there it is, a perfect circle.

If 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.


Here is the finished product. It's the new wallplate for my garage door opener button.

It's 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.

The edges were routed and the piece was stained. A hand rubbed wax finish was applied to seal the wood.


July, 2004 - Update: Tanzer Cabin Wing - Firewall Project
I'm in the process of building a large model airplane that needed two identical 1/4 inch thick
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.