Neat: (from Make: Online)
If you’ve ever struggled to locate the center of a circular plate or a piece of round stock using a ruler or a square, you know it can be a tricky proposition.
But this cleverly-designed gauge from Robert Larson Woodworking Tools makes short work of the job, and does it more accurately, to boot. Basically, it’s just a small framing square with a built-in 45-degree rule: Fit the workpiece against the square, mark the bisector, rotate it about 90 degrees, and do it again. The lines will intersect right in the center, no matter the exact angle between them, whether you draw two or twenty.
The gauge actually has two sides—one with 90-degree jaws for use on round, square, or octagonal stock, and one with 60-degree jaws for use on hexagons.
Good old angle bisectors. “When are we gonna use this?”