Finding the Center of a given Circle or an Arc:
During making a drawing of a mechanical part, we often deal along with circles and arcs that constitute its outlines, and contain to determine their radii and centers. In diagram (a) displays a bracket whose rib has the shape of a concave arc. To find out the center and the radius of this arc, mark three arbitrary points A, B, and C on the arc and join them with two chords AB and BC as shown in diagram (b). Bisect AB and BC and draw perpendiculars by the middle of each chord AB and BC. The center of the arc will be at the point O that is intersection of these two perpendiculars. OA is the radius of the arc ABC.