There are a couple of possible approaches:
- use a circle finder to mark the center and then use a pointy tool to manually set zero at that point — while not done for a circular piece of stock, this is shown in:
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get the positions of the edge along each axis and do the math to get the center — this is fraught with the possibility of error, and I wouldn’t recommend it unless you then verify the math carefully
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secure a sacrificial blank and machine a pocket just a bit larger than the circle which has the origin set at the center, then secure the circular stock in the center of the machined feature — this is probably the best option