If you're talking about the front rotors, they should never be un-riveted from the hubs, imho. If they need servicing/turning, it should be done with the hub attached.
The rear rotors is an entirely different deal....not much you can do to turn them except to drill the rivets, and separate from the hub, unless you have an on-car lathe.
Re-riveting the rotors back to the hubs is not a necessity. It's an adventure in graphic language, and an opportunity to expand your level of patience. It also requires some serious tooling to do it properly(ie: safely).
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Joel Adams
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