About this 3D model

I don't know about other Ultimakers, but that one has a long Z screw which is on rather springy coupling. Because of that I got sometimes layer "missing" (second picture, before - after), so I design a cap for that screw. You put in that cap 8 mm ball and a M3 screw on top. Then you just tighten the ball against the screw to reduce flexibility of that coupling. I also included old version of that cap, which is bulkier.