My goodness! What's wrong with using a machinist center square. This thing should be in everyone's tool box or chuck it up in a 4
jaw, eliminate runout and mark with your tailstock. I can't believe you are asking the question.
Steve

"Spehro Pefhany" wrote in message news

Sorry for the metal content, but there might be someone here who knows
the answer!

I would like to find the center of a 6" metal disk (center the mill
spindle precisely over it).

Here is my plan...

Use an edge finder and find 3 points at roughly 45 degrees to the
center by moving only the X axis, then only the Y axis (forming
a right triangle) with the hypoteneuse over the disk. Go halfway on
both X and Y between those pairs of X and Y numbers.

Does this sound like a reasonable plan?