Rotating Rings with a Heart of Steel

I was impressed by the original idea of RarelyEvil Rotating Rings. But I think it must have a solid center ball. I provide the 30mm ring with a cap, so it is easy to put a 20mm steel ball to the center.

I have made the first and last layer with a bigger gap to prevent melting the rings together. Designed for layer thickness of 0.2mm!