Simple cylinder with end flange, to increase diameter of 6mm threaded rod to fit in 8mm hole (for example, common 608 skate bearing). 6mm threaded rod can be screwed into it securely (on my print anyway) and the adapter is a firm press-fit into the 8mm ID of a bearing. I have two versions: one with a hex-head flange at the bottom, one round. Printing on Ultimaker 2 in PLA with 0.1mm layer height the 8mm OD was good, the 6mm ID was tight and the threaded rod bites into the plastic. Now Better: the ID was opened up about 0.2 mm, so the adapter relies on external nuts to hold it in place, instead of forcing the threaded rod's threads into the plastic like a tap. That allows the rod to be more accurately positioned on-axis, instead of the threads wanting to tilt it as it is threaded in. That is the "6mm_Better" file.