This is a thrust bearing holder I made to improve the quality of my MendelMax 2 parts. I think the lack of a bearing to hold the Z screws in place at the bottom. It seemed like hanging the Z screws by spring-like helical couplers wasn't helping, so I put them in a very slight compression with the help of this part.
This part requires two 688 bearings. I am misusing the term "thrust bearing", but given the such light forces involved, a regular ball bearing is more than adequate.
An isolator is required for this part to work well. I recommend this one: http://www.thingiverse.com/thing:750610
This seems to improve part quality a lot.