They make fancy and expensive gizmos for finding the center of circles, but chances are you already have all you need for that task, sitting on your bench. Grab your dividers and place one point on the circle. Then adjust the opening to what you guess is about the distance to the center. Now scribe a very light arc onto the metal, with one point fixed on the circle. Move the fixed point about 1/3 around the circle and repeat. Then swing the last arc from a point about 2/3 around the circle from the first point, so that you now have 3 arcs coming close to the center. They either overlap closely, miss each other closely or meet in the middle. No matter what the out come it is now very easy to find the center of those arcs, which is the center of the circle.