Update: Slightly resized the tension bearing holder and added a part to hold the bearing in. As well as a part to hold the pivot bearing in. (I had a problem with them staying in when testing)
Update: 02-11-18 : Added the option for a 6001ZZ bearing for hobbed bolts that are 12mm.
4 608zz Skate bearings
6 M3x20mm bolts
2 M8 or 5/16th nuts
1 M8 or 5/16th bolt at least 25mm long
Mount bolt to be hobbed in extruder and mark position with permanent marker to get exact position. Mount bolt in device and line up with tap temporarily in position. Use washers to align. Tighten hobbed bolt in place tight, but not so much that it no longer turns. Place bearing on tap, place tap in drill. Place tap in pivot mount, and into back tension bearing. Tension as needed to get better results. Make sure you use lubrication. I've found 3 in 1 oil works best for me.
Make sure to put nut in place for tension to work properly.
Designed with Blender