I have a pre-Prusa Mendel and my crappy threaded rod leadscrews seized up, so I decided to move to ACME threaded rods. After procuring some 5/16ths rod and hex nuts, I found that even though the rod was the same diameter as my previous triangualar threaded rod, the nuts had a larger outer diameter and wouldn't fit in the existing Mendel nut traps. So, here is a redesigned nut trap with the right dimensions for a 5/16 ACME thread heavy duty nut.The original was done in Art of Illusion, which I've always found very unwieldy, and its completely non-parametric. So I did a redesign. I've been curious to try out QTmium ( https://github.com/hmeyer/qtmium ) so that's what I used to design it. Its fully parametric if you want to modify it to fit other nuts.Only for an original Mendel, not Prusa Mendel. Print 4 of these and replace the x-vert-drive-4off.par pieces. Designed to hold a 5/16 heavy duty ACME thread hex nut, for example: http://www1.mscdirect.com/cgi/NNSRIT2?PMAKA=03124013 Then replace your Z leadscrews with ACME threaded rod and enjoy smooth z movement.