I was starting a project this past weekend (Bluestem Hand Drum Banjo) and the first step -- following cleaning the garage to find a horizontal surface to layout the parts -- was to "Cut 6-inch radius at end of top board BEFORE gluing the sections together", and the only compass I had would expand to just under 5-inches, so my project was stuck before I'd even started. I needed a drafting compass that was at least 6-inches, possibly more for any future projects.
Inspiration at Work
The following week at work I was thinking about the problem and came up with this simple and rather accurate tool from office supplies I had on hand:
- Ruler (I had clear plastic, flat is best)
- Two Small Binder Clips
- Clear Tape
- A Thumbtack
- A Pencil
Step 1: Assembly: a Bit of Tape
1) Wrap the ends of one arm from each Binder Clip with about 2-inches of tape, it should wrap around 3 or 4 times.
2) Align the two tape-wrapped arms and poke a hole through both with the Thumbtack; the more accuracy you have at this step, the more accurate the compass will be.
(sorry for the poor focus on the close-up)
Step 2: Assembly: Ruler Alignment
3) Place one Binder Clip on the ruler so the edge is just on the zero mark, and the other with the same side edge on the desired radius. Both Binder Clips should be flat against the ruler.
Step 3: Draw Your Arc
4) Use the Thumbtack on the zero mark Binder Clip hole and a pencil in the other Binder Clip hole to draw your circle or arc.
Step 4: Example & Thoughts
Here's a quick example of a few arcs created with the compass. It's easy to choose the radius length, because you have a working ruler built in, and the whole thing works because the Binder Clip arms and holes in the tape are the same distance from the Binder Clip edge. On reflection, this should also work with a yard-stick should you need an even larger radius.