Notes on Laying Out an Arc Without a Compass

This is especially useful when you need to draw a smooth arc around a point, but you don't have access to that point; for example, when there is an object in the way, or the center is off the material you're marking.

You need to know three points on the arc. You must have two small nails, and two straight-edges long enough to reach between the two most distant points on the arc.

Drive tiny nails into the two ends of the arc.

Arrange the two straight-edges so each touches two points, and they overlap. Connect them firmly together at this angle, and don't let them move - use glue, tiny nails, whatever. (C-clamps don't work because they prevent you from laying the straight-edges flat on the material.)

Sliding the two straight-edges along these two nails, hold a pencil-point at the angle where the two straight-edges meet. The pencil will draw an arc between the three points.

References

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© 2003 W. E. Johns