Circle and arc layout
Two shop tricks for drawing curves without a compass big enough to hold them: a trammel arm for a full circle, or the chord-and-rise sagitta method for a shallow arch.
Layout
in
Trammel arm length
12"
Diameter
24"
Circumference
75 3/8"
About laying out circles and arcs
- How do I draw a circle bigger than my compass?
- A trammel: a strip of wood with a pivot point at one end and a pencil at the radius. This gives you that radius from either a diameter or a radius you already know, so you can mark the strip once and swing the whole circle.
- What is the sagitta method for drawing an arc?
- It finds the radius of a shallow arch — a valance, a corbel, an arched rail — from two numbers you can measure directly: the chord (the straight width at the base) and the rise (how far the arc bows up at the middle). A 24" chord with a 4" rise is a 20" radius circle, which you can then strike with a trammel or a thin strip pinned at both ends and pushed to the rise.
- Why is the radius always bigger than the rise?
- The rise is only the sliver of the circle closest to the chord; the centre of that circle sits further back, behind the arc. A shallow bow — a small rise over a wide chord — comes from a very large, gentle circle, which is why architectural arches are so often drawn from a radius far bigger than the arch itself looks.