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Draw an Arc Without Access to its Center

Saw this in a print copy of “Fine Woodworking On Proven Shop Tips” (in the previous millennium); but have not been able to find it on their website. It was sent in by Thomas Baird of Woodland, CA; attributed only to a Danish carpenter of his acquaintance. I've snootied the language up a bit.

Method to draw a circular arc of known displacement and chord length; where there is no access to the center.

Mobirise

Place a nail at the beginning, the end, and the point of maximum displacement of the curve (defining the chord of the arc.)

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Erect a parallel to the chord at the point of maximum displacement. Place a fourth nail on that line.

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Fix together two straight pieces of rigid scrap so that one piece lies against the point of max. displacement (at one end) and on the end of the curve at the other. While the other lies on the parallel line; at the point of max. displ. 

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Remove the fourth nail.
A pencil/scribe held at the crook between the two pieces of scrap. . . 

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. . .will describe a uniform curve (circular arc segment) when the edges are guided against the nails.

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Flip the sticks over to mark the other half of the arc.

If the displacement is greater than the radius of the arc; the technique draws a bubble (and may require the use of automatic self-lengthening sticks.) 

Mobirise
USPS

PO Box 14
Monticello, NY 12701-0014