I work with mixed media and sometimes I need to find the center of a disc. If I made the disc myself, it was laid out from the center, but if it's say a round baking pan or a lid, I need to find it.
Step 1: Methods and Parts
One method of finding the center is with two squares, but that takes alignment, coordination, and the aforementioned two squares. Another method involves a compass to make several smaller circles and draw lines through their intersections. Yet another method involes drawing tangents, which are square to each other and then connecting to corners diagonally. Worst of all ... equations ... yuck.
Anyway, my solution uses plywood and hardboard (of course).
Step 2: Cutting the Parts
I made mine from 3/4" plywood, ripped to 1 1/2" widths. Two true miters were made with a miter sled on a table saw. I then cut these to length (7 3/4") at the same time using a miter saw.
These two pieces got glued to the hardboard as seen above. I squared to one side and then trimmed off the excess on the table saw after the glue dried. I cut the hardboard out of the V notch using a band saw. I also nipped off the points a little (you'll see) so I wouldn't stab myself in the future.
Lastly, attach another 1 1/2" strip to one side. I used screws and no glue just in case I needed to replace it after time, because I could drop it or something. For me, this piece was 18" long, but can be whatever you want.
Step 5: Mark the Center
Just slide the disc into the V notch, mark a line, rotate the disc, mark a second line.
It can also quickly find the center of squares, as long as the extension can reach the middle of the stock, which is much quicker than using a ruler to connect both corners and making more sanding work for yourself.