Low cost technique to cut large circles/arcs

[ElectriFan]

I needed to cut some sheet foam whose diameter was larger than anything I had available at home to attach my hobby knife to.

So here is a technique I came up with last night to cut a circular piece of foam with no special equipment (cost = ZERO).

1. mark the center point of the circle on the foam.
2. cut a strip of foam about 1/2" wide and about 2" longer than the radius of the circle I needed.
3. take a push pin with it's steel shaft about 3/8" long (average office push pin) and pierce a hole in the strip about 1" from one end.
4. make another hole on the strip at the other end: distance between them should be the radius.
5. stick the pin thru the first hole of the strip and into the center mark of the circle and hold it down lightly.
6. stick a pen point thru the other hole till the tip just pokes out the other end and draw the circle for reference.
7. then stick a new hobby knife tip (blade surface to be perpendicular to the radius line on strip) thru the same hole as the pen till the tip pokes out about 1/4".
8. start cutting the circle with light pressure on the knife - 3-4 passes should cut a nice clean edge.
9. save the strip for future use :)

I needed a donut shape, so I just repeated the procedure with a smaller radius on the same strip. I expect this technique can be used on balsa, ply, paper, etc too. One can also improvise and cut a beveled edge by sticking the hobby knife in at an angle.

[submanmike]

Your technique is very good for zero cost. For more accuracy, I got a device from JoAnn's Fabrics for about $4. It consists of two aluminum cilinders with rectangular holes cut through them. They slide over a standard wooden yardstick. One piece holds a shaft with a point, the other holds a pencil or a hobby knife. Cuts circles very accurately up to about 78" diameter (using a metre stick). For small circles, use a shorter stick, or for about $8, get a fabric circle cutter that is in 1mm steps from about 1/2 inch to 12 inches. It is concentric plastic circles that allow the inner one to spin. A standard swivel tip knife fits in the holes in the inner circle, and cuts beautiful circles. I have done 1/32 inch ply this way. I also borrow them from my wife's sewing supplies. But that's ok, she uses many of my tools to help her sew.
Good luck,
Mike