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I've actually solved this, but I think its still a useful question which I don't think is easy to answer with existing questions.

As soon as I'd built my ANET-A8 (Prusa i3 DIY kit), I found I was having problems with the extruder crashing into the bed. Although I thought I'd adjusted the bed leveling OK, the calibration seemed to keep getting messed up.

I tracked this down to two factors. First, I was winding the extruder head up some distance before loading the filament and starting a print. Second, at roughly half-way up the axis, the right-hand thread seemed to be getting stuck (more often when moving up than down).

What wasn't clear (and not mentioned in the building instructions) was what might cause this problem.

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  • $\begingroup$ Is it correct to use the prusa tag for clones? $\endgroup$ – Sean Houlihane Nov 21 '16 at 10:47
  • $\begingroup$ (thanx for creating it) $\endgroup$ – darth pixel Nov 21 '16 at 11:00
  • $\begingroup$ @SeanHoulihane There's no such thing as a "clone". The Prusa design is open-source and in my opinion, "Prusa" refers to any number of designs. $\endgroup$ – Tom van der Zanden Nov 21 '16 at 12:54
  • $\begingroup$ @TomvanderZanden Thanks for clarifying - I edited the tag :) $\endgroup$ – Sean Houlihane Nov 21 '16 at 13:10
  • $\begingroup$ In many open source projects the name is trademark, while the designs are copyleft, the company still wants to maintain their brand (and reputation). However, I don't know Prusa's feelings on it. $\endgroup$ – HaMMeReD May 11 '17 at 19:16
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I assume you did everything according to the instructions but here is a checklist of what could be possibly wrong:

  1. Friction - check if you can rotate/move parts without lot of resistance
  2. Screws - check if screws on couplings are tight and they don't slide over a shaft or thread
  3. Stepsticks - check if they are cooled properly and similar (as there are two of them)
  4. Carriage nuts on threads - check if they do not slide out of their nests while [the x-axis] carriage goes up
  5. Filament - check if filament unrolls without resistance which can eventually cause [the x-axis] carriage to hang.

IMO #2 and #3 are the most possible cause

Diagram of issues 1, 2 and 5

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  • $\begingroup$ caret nut is a new term for me. $\endgroup$ – Sean Houlihane Nov 21 '16 at 11:01
  • $\begingroup$ well - here is top left elem (prusa i2) where such nut sits 3dprinter.my/cart/images/… more or less the same is in prusa i3 $\endgroup$ – darth pixel Nov 21 '16 at 11:11
  • $\begingroup$ I don't think caret nut is a real word. It is probably a mistranslation. $\endgroup$ – Tom van der Zanden Nov 21 '16 at 12:53
  • $\begingroup$ With (2), I had the spindle and the thread pressed tight together, the coupling was unable to bend... $\endgroup$ – Sean Houlihane Nov 21 '16 at 15:46
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    $\begingroup$ @TomvanderZanden - I think caret should actually be carriage $\endgroup$ – Greenonline Nov 21 '16 at 16:58
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So after reading the instructions more carefully, ANET do cover this in their operating instructions under troubleshooting. On closer inspection, I realised that the brass nut following the right hand thread seemed to be out of alignment with the stepper at the bottom. Fiddling with the flexible coupling helped a bit, but what I needed to do was place the assembly at the tight point, slacken the 3 bolts marked SHCS in the diagram, then tighten them again. I was expecting I might need to open out the holes, or re-make the white (factory printed) part, but this wasn't necessary.

From ANET manual

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    $\begingroup$ ha! so there is caret nut :) it's good to hear you've solved it (btw: my suspicion about 1. assembling according to instruction was priviledged) ;) and 2. that there is a problem was priviledged too) $\endgroup$ – darth pixel Nov 21 '16 at 11:42
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    $\begingroup$ I don't think caret nut is a real word. It is probably a mistranslation. $\endgroup$ – Tom van der Zanden Nov 21 '16 at 12:53
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    $\begingroup$ @darthpixel - I would wager that caret actually means carriage, as in the x-axis carriage. Mostly because of the phrase in Tom's answer, which can eventually hang caret, which probably should be which can eventually hang [the x-axis] carriage $\endgroup$ – Greenonline Nov 21 '16 at 16:57
  • $\begingroup$ where did you find this under troubleshooting, I think Im having the same issue. Can't find what you are refereing to. $\endgroup$ – user6668 Apr 4 '17 at 21:38
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    $\begingroup$ Found it in the A8 Operation instruction-1.1.pdf file provided on the SD card, page 66 $\endgroup$ – Charly Koza Jan 20 '19 at 12:50
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From http://3daddict.com/anet-3d-printer-common-mistakes-fix/

The motor shaft and the threading rod must have space between them in the flexible coupler.

That means unlocking the coupler from the motor shaft and moving it up a bit, in the end the threaded rod nearly touches the printer top hole instead of having like a 1cm gap. This will allow the coupler to flex, and thus should handle small misalignments of the brass nut.

That's about point 2 of @darth-pixel answer, but before locking the screws, make sure to have empty space between rod and motor shaft

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