Recently egbertfitzwilly posted an Instructable on Finding the Center of a Circle. WirelessMonk posted a link to a machined steel center finder. Rimar mentioned making one. I decided to try making one that would be very precise.
Step 1: Materials
Step 2: Cut the wood at an angle
Step 3: The second wood cut
Step 4: Flatten the steel
Step 5: Pound it flat
Step 7: Clamp while drying
Step 8: Attach the steel
Step 9: Use
No allowance has been made for the thickness of a pencil line. I like to use a knife to score a very fine line. Make three or more score lines from different positions around the work piece to see if your center finder is truly accurate. If it is not completely accurate, you will get a small triangle, which also marks the center rather well. I chose steel so it would stand up to the wear imposed on it from the knife I use for marking.
If you want to try another version of a center finder made without laser cutters, etc.; try this link.