It's difficult to find the center of cylindrical turning stock and other circles with only a straightedge and pencil.

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An inexpensive and simple jig quickly and accurately pinpoints the center of a circle. Cut a 90° notch from 34 " stock. Screw a straightedge over the notched base, as shown, to create a 45° angle. Place the circle against the guide and scribe one line. Rotate the circle about 60° and mark another line. To check the accuracy, rotate the circle once more. If the lines form a small triangle, the center is inside the triangle.
—George Harenberg Chincoteague, Va.