I've noticed that there are a variety of preferences for holding a rim blank to the lathe for turning. They seem to include making a chuck with a large grip range by using bowl turning accessories like Oneway's "Jumbo Jaws". Others make a jam chuck in which the workpiece makes a jam fit into the compression chuck and still others simply glue the rim blank to a sacrificial plate.
In all cases, the goal is two fold;
1) turn the rim to a consistent and accurate diameter
2) don't end up with the rim flying off an lodging in your chest or head.
I was thinking about stirring the pot by asking which of these methods was best, but instead, I'll ask why each prefers the method they use. It seems like they all work, though gluing the blank to a piece of ply seems diabolically simple, but if it's glued in a way that makes it easily seperated, what's to stop it from seperating while it's being turned. I know on some of the things I've turned, the initial truing process and be rather jarring. As you can tell, I'm very new to both turning and rim turning, so eager to soak up the expertise here.