Originally Posted by
mklotz
The intended application suggests you don't need tenths (or even thousandths) accuracy in locating the center so perhaps a device based on my math post above might work.
On a metal bar, mount two identical pins about six inches apart on the centerline of the bar. Draw a line on the bar midway between the two pin locations. This is your "chord generator".
Push it against the circumference of the disk until the pins touch. Lay a machinist square on the top of the bar, align with the center mark and draw the bisector on the disk. Repeat. The center is the point where the two lines cross.
It's just a gedanken-design but it might inspire you in your build.