Summary

This is a universal joint which uses four 1/8 x 3/8-inch bearings mounted in the central ball. This turns more smoothly than a simple plastic pivot would. I printed this with PLA on the Ultimaker 2. You can see it in motion at https://www.youtube.com/watch?v=rqXTtEzYYBk

Instructions

Print one ball and two clevis parts. The ball takes 4x mini-bearings (1/8 x 3/8 x 5/32 inch) pressed into four faces, and 4x M3 screws + 4x M3 (8mm long) hex nuts which fit in the two arm pieces and go into the ID of the bearings. Not an exact fit with metric screws, but fairly close. I think the easiest bearings to find are the "R2ZZ" type, about 10 for $10 on ebay. I had a bag of "SSR-2ZZ" from a surplus place, those are apparently used in aircraft.

Each clevis has a hole for 2x M3 setscrews for the input and output shaft, holes slightly undersized so the screw will bite into the plastic. The shaft hole is sized for a 6 mm rod, but you can adjust that in the SCAD file of course. If you edit the SCAD, don't forget to download the M3_screw file also or you'll miss the pockets for the M3 nuts in the clevis faces.

Universal Joint with minibearings