How to use a centre head
Step 1 - Place combination square set on object
Place the combination square set on the circular object with the centre head attached.
Step 2 - Mark diameter line
Mark the diameter of the object along the rule.
Step 3 - Mark second diameter line
Move the combination square set and mark a second diameter line (you may choose to do this at roughly 90 degrees to the first line). Where the lines cross each other should mark the centre of the object.
Step 4 - Identify centre of circle (if necessary)
Sometimes the object may not be an exact circle. When this happens, marking more than two diameter lines may show that they do not all cross at the same point. You can then make a judgment on where the centre actually is.