Delrin nut on perfectly straight ACME rod alternately tightens and then loosens

What would cause the delrin nut to alternately tighten and then loosen as it travels along the ACME rod (turned by hand)? I’m at the point during assembly where I’m supposed to install the ACME rod and, prior to installing it, in order to match what was shown in the image, I attempted to turn the rod inside of the delrin nut by hand to re-position the nut on the rod. What I found was that, although the ACME rod isn’t bent the least little bit, something is causing the nut to tighten or bind at regular intervals along the length of the rod. For the first 8 turns needed to fully insert the rod into the nut, there’s no resistance – it turns nice and smooth. By 12 more turns though, I can barely turn the rod at all. By turn 31, it returns back to the initial state of no resistance. Then, at turn 43 it starts tightening again…

Clearly, I don’t want my Z-axis stepper to be overburdened from simply re-positioning the cutter along that axis. Any idea what’s going on here? I’ve never heard of this happening before. I can understand that a nut might be too tight along the entire length of an ACME rod, but alternating every 20 or so turns? Am I in the Twilight Zone? And yes, I’ve already tried running the nut over the rod using a drill but doing so hasn’t helped at all. I’m completely stumped.

It does it for some reason. Maybe tread count is not perfectly matching with acme tread. Best suggestion is, you better use your hand drill, put a little silicon lube, hold delrin nut with your hand and go up and down until you feel free turn. That’s the way we all did. Like breaking the engine.

Thanks Alan. Did yours alternately tighten and loosen or was it just too tight for the entire length of the ACME rod?

It was getting tighter on close to both edges. Middle of Acme was looser. I ordered and received replacement, replacement was the same plus it was bent on bearing end.

Same issue 3 in a row. Whoever is Inventables’ supplier is not up to spec! @Zach_Kaplan O’Merciful One please fix. Frustrating issue.