Prints Small, Circles Oblong

celadon_one · January 19, 2021

On the surface, this looks straight forward, but allow me to give some history.

User Story
I want to be able to print objects I design, with a +/- 0.5mm dimensional tolerance, and have circles and regularly polygons print with a consistent radius and all lines and angles within the polygon are congruent.

Symptoms
See the photo attached to this post. About 2 weeks ago circles began printing ever-so-slight out-of-round.

Evidence

This is a standard S5. It's about 2 years old, and has around 250hrs of print time on it. It's always printed small objects smaller than they should be (some times as much as a full mm) but for the most part that's not a problem as long as holes are actually circles or regular polygons since any scale deviance in size can be resolved in the model or with some simple hand tools.

I asked around and did some reading and most suggested tightening belts or re-calibrating. I opted for the latter--I'm a bit remiss to do any disassembly on this machine. I calibrated the Z-axis and got stuck. The printer would fail to start the XY calibration, complaining about a height difference. While searching for an answer, I noticed there was a firmware update from 5.8.1 to 5.8.2. Did that.

A post on this forum lead me to realize it might be a problem with the lift switch and so I watched that carefully and found it was missing the bracket to flip the switch. I recalibrated the lift switch and could now print the XY calibration pattern.

The thing I noticed at this time was that the Y portion is exactly the same size as on the paper calibration print out (in the Y direction), and all the tick marks match up with the numbers on the paper. The X axis, however, is smaller that the calibration rectangle (in the X direction) and starting at the left-most position (-), gets further away from the numbers as the you look to the right (+). Still, using this guide it looks like +1 is the way to go, but that doesn't help.

If X and Y adjustments are the same [(0.5, 0.5),(1.0,1.0), or (1.5,1.5)] circles are ovals, and polygons are skewed, but right angles with sides perfectly on the X and Y axis are straight. If the X adjustment is larger than Y, I get a right angle trapezoid when printing the skirt, but printing a rectangular object prints a perfectly right-angled parallelogram as one would expect.

I've attached a photo of the before changes (before Z calibration) and after changes (current).
Before is Ultimaker White Tough PLA, from extruder 1(AA 0.4); the same setup I've used for a couple years.
Current is Polymaker Teal Polylite PETG (CPE) from extruder 2 (AA 0.4);
Changes:

Z-axis alignment
Firmware update
Calibrate lift switch
Calibrate XY
New Filament (first time using non-ultimaker filaments) using settings as recommended by SandervG in this post.

Conclusion

Something I have done has taken me from bad to worse and I'm outside my realm of expertise to resolve it.
I am not concerned about problems with infill or material adhesion at this point.

celadon_one · January 21, 2021

This 20x20x20 cm cube comes out pretty well. X and Z are spot on. I really couldn't ask for any better precision. Given the accuracy of those, the measurement for Y surprised me. If X and Z were off by half of that deviance, I'd call it close enough for FDM. But this is repeatable. PLA, Tough PLA, ASA, and this CPE.
Smaller than 10 cm, and X is smaller than the dimensions in the model. Larger than 10 cm and X about right, but Y is larger than the model by about 10% of the model's dimension on the Y axis. Print speed (30mm/s to 60mm/s) does not affect the tolerance by anything my tools can detect.

Larger circles, hexagons, etc have less noticeable skew, but I attribute that to the woefully inaccurate nature of sight and our brains tendency to normalize what we see.

Any help with diagnosing this is much appreciated.
xyzCalibration_cube.stl