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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.

zapshe
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  • Have you calibrated both X and Y axes? – agarza Sep 17 '23 at 13:42
  • @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. – zapshe Sep 17 '23 at 21:00
  • 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. – agarza Sep 17 '23 at 22:20
  • @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. – zapshe Sep 18 '23 at 01:07
  • Is your hotend properly tight on its carrier plate and the plate on its axis? I.e., if you try jiggling the nozzle, does it move at all? – towe Sep 18 '23 at 05:30
  • @towe Everything is nice and tight from the factory. Bed doesn't wiggle, hot end is firm. Also made sure the motors didn't have wiggle to them, everything was nice and solid. – zapshe Sep 18 '23 at 05:41
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    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. – towe Sep 18 '23 at 05:53
  • @towe Good idea, I'll try that and also check that the stand is properly aligned to the printer. – zapshe Sep 18 '23 at 05:58
  • @towe I removed then reinstalled the top portion (holding the X-Axis gantry) while making sure to keep them level and the variance decreased to 0.21mm from the previous 0.3mm, showing that may very well be the problem. I also looked over the facts, and a slightly tilted X-Axis is the best explanation for these diagonal issues. I'll reinstall again later, but it's possible the holes were drilled just every so slightly wrong and is a factory defect, as it shouldnt be possible to misalign it during assembly. – zapshe Sep 18 '23 at 06:59
  • Remember this isn't accurate CNC! Small variations in the 0.2/0.3 mm range are very common, there are many uncertain parameters in the positioning and extrusion (even change in filament diameter may be of influence). As already stated, whether this error is consitently scaling with the diameter of the print (hinting to changing the steps per mm) or a printing variance should be easily found by printing a larger object. you can look into skew compensation which is a default feature in Marlin firmware. Print https://www.thingiverse.com/thing:2563185 and measure diagonals. – 0scar Sep 18 '23 at 08:28
  • @0scar I don't expect CNC accuracy, but this thing can't print a circle. My Ender 3V2 never had this problem. 0.2-0.3mm variations are due to plastic expansion/contraction, which is common in printing holes. But this is very different, my last Ender 3V2 printed circles and squares without randomly oscillating diameters. – zapshe Sep 18 '23 at 10:06
  • 3D print circles from STL data are not actual circles, they are n-gons, and as such have, with low resolution, at times *massive* differences between the flat-flat and peak-peak measurements. – Trish Sep 19 '23 at 10:27
  • @Trish The issue appears in all prints. I also make sure to have lots of vertices to get the smoothest cylinder possible from my print. – zapshe Sep 19 '23 at 21:28

2 Answers2

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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.

SF.
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  • 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. – zapshe Sep 26 '23 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.

agarza
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zapshe
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