The paper mentions that the rough surface of sintered powdered metal 3D printing won’t work (presumably way to much sliding friction). It also suggests the possibility of using that printing technique and surface machining it after printing.

there are some emerging metal 3d printing techniques that might overcome that, like rapidia

By far the easiest way to deal with this would be to just print them slightly oversized, add some abrasive paste, and then 'run in' a set using some balanced pattern of movement, tightening the clearance as the parts wear in until they're sufficiently smooth.