This fascinating tool, made by Detroit-based precision tool manufacturer Cogsdill, is called a DBFM (Diamond Burnish Face Mill tool).
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It mounts into a standard milling arbor, and each of the posts have a spring-loaded diamond insert in the tip. When the tool comes into contact with the metal workpiece, the springs force the diamonds against it, and it then produces a mirror-like finish.
As an example, the top half of the workpiece below shows a merely milled surface, whereas the bottom half has been finished with the DBFM.
Titans of CNC Machining got their hands on a DBFM, and used it to finish a disco ball that they milled out of a chunk of aluminum:
Source: core77