This is a handy trick that I learned for finding the center of a circle.
I made my girlfriend and I phone holders with steel discs for their bases. I had to drill and thread a center hole in each base for the articulating arms.
To figure out where I needed to the drill the center of the hole at, I used the method detailed below.
- Sharpie marker
Make two equal length lines parallel to each other on the circle. In my case, I chose to use 3" long lines. So on the top of the circle, find where its width is 3" and make a straight line across. Then on the bottom of the circle, find where its width is 3" and make a straight line across there, too. You can use any width you'd like, as long as they're the same length.
Once you have two equal length lines parallel to one another, you can then connect them as pictured. You're making an X between the two lines.
You're all done. Where the two lines of the X meets will be the center of the circle.