These are the tools I mention in my video “Saturn IB Part 4 – Centering Rings”
Router circle cutter: https://www.amazon.com/gp/product/B00009K77A/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
I am not an affiliate for any of these vendors. Feel free to shop around. There are also many tools by other manufacturers that will achieve the same result, but these are what I use.