Quote Originally Posted by mklotz View Post
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.
I like that "chord generator". Akin what tool gunsmiths use to mark centerline of shotgun ribs for beads, woodworkers use a larger version to bisect material. Both are identical to your example; with a hole mid-point twixt the dowels, sized for transfer punch of choice.

Math and geometry are fine, and I use them regularly. Plates too rusty to scribe with nice old M&W dividers.
But constructing a center finder, I can do wearing gloves and cold weather gear.
It's 21F degrees outside and a couple colder in the building; Wednesday 17F. In the spring I anticipate 375K BTU boiler to arrive and plumb iron radiators I've collected. Several tons of machinery make excellent heat batteries. Next year I'll be working down there any time I dang well please!