I had the opportunity yesterday to find the center of a circle on my CNC mill. I needed to center it without the aid of an indicator mostly because I don't have a way to mount the indicator into my milling head. I don't have a probe to touch off yet either, so I did it by hand. When I actually did the job, I drew the coordinates out in CAD, it was quick at the time. I figured there must be a way to calculate, and that would be much faster next time. Here is a link to an Excel file that does just this. The only catch is the none of the points can be on horizontal or vertical lines. So when you move your Y and Y axes to go to the next point, you must move both axes. Moving X or Y until you touch off the other side of the circle won't calculate. This is useful for CNC or simple DROs. Keep in mind that the Radius and Diameter are related to the probe center, not the diameter you are touching off.
Enjoy.
www.dieselrc.com/download/circlecenter.xls
Enjoy.
www.dieselrc.com/download/circlecenter.xls