Mendel90 is a great design, but the 8mm threaded rod for Z drive has tendency to wobble. In addition X axis is directly supported on motors only, which leads to little changes in Z position.
Set below lets to hang 8mm rod on ball bearings and prevent its wobble. X axis is more stable in Z direction.
More info: printerzero.blogspot.com/
Remark: Designed for 10mm rod