A combination square is a layout tool that wise construction professionals keep handy in their tool box. The versatility of a combination square comes from the interchangeable heads you attach to the combination square blade. An important square head, the centering head, allows you to find the center of circles and squares. A centering head resembles a "V," with a locking thumb nut located near the point of the "V." The square blade rests in the center head, with one side of the square blade sitting on the center of the head.
Slide the combination square blade into the center head. If the blade does not slide into the center head, loosen the lock nut to open the center head.
Inspect the edges of the metal circle. Remove any burs or irregularities you find on the edges of the circle with the 80-grit sandpaper.
Place the edges of the center head against the edge of the metal circle and loosen the lock nut to extend the square blade across the surface of the metal circle. Tighten the lock nut to secure the square blade to the center head.
Scribe a line across the face of the metal circle with the awl, slide the center head and scribe a second line with the awl. The intersection of the two lines is the center of the circle.
Perform Step 2 on the edges of the metal square.
Place the inside corner of the center head on one outside corner of the metal square and scribe a line across the surface of the square. Move the square to the nearest corner and repeat the procedure. The intersecting lines designate the center of the square.
Remove the center head and store it for finding the center of a circle or square later.