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I'm using the new Elegoo Neptune 4 Pro printer. I have the issue with both Cura and PrusaSlicer alike (I'm a Cura user).

The issue is simple, instead of printing a perfect circle, the final print will have a diameter range of around 0.3 mm. For example, if I print a 15 mm diameter cylinder, the longest diameter will be 15 mm but the shortest will be 14.7 mm.

I measured these with my caliper and I simply turned the cylinder around in the calipers; the cylinder will push up against the calipers as it reaches the thickest part of the cylinder and then come down as it goes to the thinner part.

It's not visible to the eye; it looks like a very nice and perfectly round cylinder. But I can feel it with the calipers. The height is dead on 25 mm as designed.

However, I don't get this result from the holes inside that same print. The holes are perfect circles and measure the same diameter roughly no matter what angle I measure them (at least I believe so, measuring the holes can be a bit tougher).

Calipers are accurate and go to two decimal places for mm.

I've tried:

  • Tightening up the belts (a lot - caused the deviation to go from 14.7-15 mm to 14.53-14.8 mm, still roughly a 0.2-0.3 mm difference)
  • Bed leveling
  • Vase Mode
  • Arc Welder (even though it technically shouldn't do anything)
  • Axes calibration

I remeasured and found that the thickest part of every cylinder is not directly on the Y-axis, instead, it's diagonal. The bottom right and top left of the cylinder is the thickest portion while every other angle is roughly the same.

I found the same was true for the cube I made. Measuring from the bottom right to the top left resulted in 26 mm, but the bottom left to top right resulted in 25.67 mm.

Calibrated the the Y-axis (the only one that was off), issue persists. Again, this 0.3 mm deviation exists in the cubes printed as well, and always diagonally. Bottom Right to Top Left is always about 0.3 mm larger than Bottom Left to Top Right. Looked at an old calibration cube I made with my Ender 3V2 previously and the same measurements were only off by about 0.03 mm.

This is very strange, I think I may simply return this printer for a replacement.. I have no idea where to begin investigating this issue.


I took some measurements. I found that the right and left sides of the printer (which are separate pieces of aluminum) were roughly 0.2-0.3mm in difference from their distance to the aluminum center piece.

Now, without taking the entire printer apart, I can't say for certain that this measurement even matters, its possible that things aren't attached the way they seem from the outside.

However, the measurement perfectly aligns with the error margin on the prints. It also correlates to the X/Y axis misalignment, as the measurement indicates the bed X-Axis is slightly tiled up on the right side by 0.2-0.3 mm. Had it indicated it was tilted up on the left side, the print errors wouldn't have made sense as the diameter would have increased/decreased on the wrong diagonals.


They agreed to a replacement after I sent pictures of the inconsistent diameter and measurements on the printer itself. I will update on if the issue is gone on the new printer once I have it.

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  • $\begingroup$ Have you calibrated both X and Y axes? $\endgroup$
    – agarza
    Sep 17, 2023 at 13:42
  • $\begingroup$ @agarza I heard the axes did not need to be calibrated. But after a few more tests, I believe I should. I'll try it and get back. $\endgroup$
    – zapshe
    Sep 17, 2023 at 21:00
  • $\begingroup$ If you are just looking to print things that don't require precision alignment then no, calibrating the axes is not required. But you would be better off calibrating for better-quality prints. $\endgroup$
    – agarza
    Sep 17, 2023 at 22:20
  • $\begingroup$ @agarza Well, I finished calibrating and the issue remains. There's about a 0.2-0.3mm variance in the smallest and largest diameter measured on a single cylinder. $\endgroup$
    – zapshe
    Sep 18, 2023 at 1:07
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    $\begingroup$ If you print a larger circle - as large as your bed -, does the error stay the same (i.e. 0.3mm) or does it scale alongside the print? The only other thing causing symptoms like that should be a slightly misaligned X vs. Y axis (them not being at exactly 90° to each other). You can try printing a "+" or "□" shape and measuring the corners to check that. $\endgroup$
    – towe
    Sep 18, 2023 at 5:53

2 Answers 2

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Take a builder/carpenter/machinist square to your machine frame and double check the bed rail (y axis) is perpendicular to the gantry (x axis). The only way I can think of for this sort of distortion is if they aren't actually perpendicular and your prints are skewed diagonally.

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  • $\begingroup$ Thank you. I believe that the x and y axes are indeed not perfectly perpendicular. I've been speaking with support for over a week now.. But once I have a new machine, I will update on if I have the same issues with the new one. $\endgroup$
    – zapshe
    Sep 26, 2023 at 19:18
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It's all but confirmed that the printer was defective. I got a new printer and loaded up the sample G-code it comes with, but the bed's belt snapped 10 minutes into the print (it was only finger-tight). This makes two Neptune 4 Pro printers in a row I've received with defects.

I can only imagine that their quality control is terrible or I have terrible luck. They're also not replying to my emails for the next week due to a week-long holiday. This is an entire month wasted on defective printers.

After some shenanigans, I got a new belt on it. The belt didn't snap as I thought, instead, the metal that was crimped on it failed to hold the belt and it slipped out. This was when I noticed that the Z-axis movement was very noisy. To my surprise, I find that the lead screws were entirely unlubricated.

I printed new cylinders/cubes. The measurements are inconclusive, which probably means the machine is fine. On a 15 mm cylinder I found nearly a 0.2 mm variance, diagonally. This time, the bottom left to the top right (opposite of the last printer). A 30 mm cylinder showed a definite 0.4 mm variance on the same diagonal.

I tried readjusting the X-axis, but nothing changed. I pulled out a cylinder that my old Ender 3v2 printed (25 mm diameter), the variance was <0.1 mm, a perfect circle (ignoring the Z-seam).

This printer has not been worth the time spent on it, let alone the hassle. What a shame. The printer is fast and has beautiful quality prints otherwise. But it's worthless if they can't properly align the axes at the factory.

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