Check out this amazing video of a Gear Ring 3D printed in Sterling Silver by Shapeways. The design was 3D modeled in Autodesk 3D Studio Max uploaded to Shapeways to be 3D printed in Sterling Silver in multiple parts then blackened with ‘liquid smoke’ and assembled in place to make the mechanism work.
You cannot currently 3D print moving parts in metals such as Stainless Steel and Sterling Silver but you can make articulated mechanisms in both Acrylic and Nylon. Take a look at each of the material pages for specifications but you can usually heave moving parts in Acrylic (depending on the geometry) with a 0.4mm gap between parts and in Nylon (depending on the geometry) you can have moving parts with a 0.6mm gap. Any parts that are closer or touching will be fused together into a solid form.