Straight Talk About Circles
Finding the Center
Easy-to-use center-finding tools, such as the one shown in the opening photo or a centering head for a combination square, are readily available from woodworking-supply dealers. The devices work well on rounds up to 7" or 8" in diameter.
But you'll need to rely on layout methods to find the center on larger discs. The easiest way to do it (though a method prone to some error) is to stretch your tape measure across the diameter of the circle, and make a mark at the middle, as shown above. Then, move 90° around the edge, and repeat. Extend the marks until they cross, pinpointing the center.
If you prefer greater accuracy, try this method. First, draw a chord on the circle, shown as line AB in the illustration below. (A chord is a straight line that extends from one point on a circle to another, but doesn't pass through the center.) Then, draw a perpendicular chord at each end of the first one, shown as lines AC and BD in the drawing below.
Next, draw diagonal lines between the two perpendiculars, shown by the broken lines. The point where the diagonals cross marks the center of the circle.
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