Tests of magic numbers, part 1

 There's some talk about "magic numbers" in 3D printing, the idea being that the Z stepper motors are more precise when aligned with full steps (or somewhat with half steps). Others say that it doesn't matter with modern precise printers[1], and that since the stepper motor starts at some random state, using magic numbers won't help. With my Prusa i3 mk3s, I certainly have a modern and precise printer, so let's find out!

First, the magic number. Prusa i3 mk3s steps/revolution at a step angle of 1.8 is 200. By marking the Z threaded rod and moving the Z axis, I found that one revolution is 8mm. 8mm/200 means the magic number is 0.04mm - confirmed by this thread. One should thus always use layer heights that are a multiple of 0.02mm (because a half magic number is still OK, with the motor evenly balanced between two magnets).

Now to deal with the random initial state. The initial state could be reset by briefly disabling the stepper motors. That would offset the Z level by at most 0.01mm, less than I adjust the Z level by, and thus not a concern for bed levelling. So if I try with and without magic number layer height and with and without aligning, I should be able to determine if it really makes a difference.

This is the required GCode in the 'Before layer change G-code' section:

; Briefly disable Z motors before start to align magnets. 

{if layer_num == 0}

M18 Z

G4 P10

M17 Z

{endif}

First I tried with just plain DasFilament PLA and my current nozzle, because that's what was in there.

Here's a shaman with 0.2mm layers and alignment, two detailed areas zoomed in:

Details in larger:

The same shaman with 0.19mm layers and no alignment:

Details in larger:

There are some differences to be seen, but you have to look carefully. My current nozzle has been used for a while, including with UniCoFil filament and sparkly filament. Maybe it's a bit worn. and maybe the DasFilament filament doesn't match the settings as perfectly as Prusament does. 

So next I'll switch to a fresh 0.4mm nozzle and freshly unpacked Prusament and do a test piece with 0.2mm and 0.1975mm - one microstep away from a full step, presumably the worst case. Then I'll try with 0.1175mm unaligned and 0.12 aligned, probably the finest a 0.4mm nozzle can realistically do. Then I'll switch to my slightly used 0.25mm nozzle and go extreme: 0.04mm (yes, that's 40 microns) and 0.0425mm layers.

[1] Some of the posts in that thread get the numbers quite wrong, which is why I went to manually confirm the mm/revolution.

Last miniature test, I promise

Having had some luck with the 0.04mm layer (yes, that's 40 microns, less than what many resin printers advertise), but also having read that thinner layers may need higher temperatures, I tried to vary that. Also, due to stringing, I tried to vary retraction. Results were mixed, shown  belowwithout any cleanup.

This one (front and back) is (accidentally) at 230°, with 1.2mm retraction.

This is also at 230°, with 2mm retraction:

And finally, at 235° and 3mm retraction, an accidental skeleton king:

Clearly, 235° is too much, though it should also be mentioned that these were done after a failed attempt at a temperature tower that broke loose from the print bed. 230° is also a bit high, I liked the 225° better, the print was smoother. And 3mm retraction is quite obviously a bad idea here. 

This temperature tower was done at 0.16mm layer height, still 0.25mm nozzle, and it's really hard to tell any difference between the temperatures. Actual temperatures are, from the bottom, 225, 215, 205, and 195. There is a bit more bridging droop at 225, and the overhang seems a bit less regular at 205 and 195. The most interesting difference is that at 195, the surface is matte rather than shiny. 

Full-size front shot, layers clearly visible:

These shots were taken in my new mini-softbox, a cut-open plastic container. It can bounce the light enough to give reasonable shadow,s while the highlights of irregularities are easy to see in the sun. Lens is a 60mm macro at 1:1, f/16 on APS-C.

Now back to a regular 0.4mm nozzle and some normal prints.

A good, long service

One of the first complicated pieces I made, way back in early 2016, was a replacement foot pedal for one of our trash cans. Finally yesterday it met the end of its life, breaking off partway through:

Three and a half year of being stepped on multiple times a day is not bad, though. I shall give it a second chance using either epoxy or contact cement, but I'm proud of how well it held up.

In other news, the printer has had software issues after getting updated. The Display ETA plugin for OctoPrint was adding malformed text to the M117 commands, which made the printer reject it. Apparently plugins are not entirely harmless.

I also had a nasty crack in the extruder block:

This was causing the gears to move apart a bit and start skipping, the noise from which is how I eventually noticed. Notice how the crack follows the print, then follows the edge of the circle down to the nut trap (there's another nut trap on the left of the picture that the crack also went through). If I had printed for much longer, it would probably have fallen apart entirely. As it was, I was able to use good old-fashioned contact cement to extend its life:

I suppose one could actually pre-treat it with contact cement, to fuse the layers a bit better.

But it's not all doom and gloom. I haven't been keeping up with the posts, but that doesn't mean I haven't printed things. Here are a few of my successful outcomes:

I added some feet to a container in the bathroom, to avoid water collecting underneath (and to prevent sagging). I sprayed them with PlastiDip for extra stability and glued them on with contact cement. Disappointingly, when  I took this shot, I notic ed one of them was loose.

I printed a bunch of minifigs for our DSA campaign, several of which I painted. They must have been blessed, for the ambush we first employed them in went above all expectations - three shots took down three guards, the rest surrendered.

Team Green vs. The Red Rangers

Truth be told, my printer isn't nearly good enough for proper figurine printing. The heads are basically blobs with faint hints of facial features. I'm curious to try HeroForge, though it's expensive enough that I'll only do it for my own character, not a whole army.

I printed some new hangers for my plant pots, this time in white PETG, which should be more resistant to heat. Unfortunately, I forgot to mirror one, so now they hang kinda funny. And I only had a small sample of PETG, so I couldn't just reprint.

I'd previously made a base for our salt- and pepper-grinders, so they wouldn't dirty the table. But when moved, they had a tendency to fall off, so for safe-guarding during transport, I added the obvious thing - tentacles:

They were done by defining just one third-degree polynomium, which I then copied and rotated. It took some fairly arbitrary tweaking, but with only a single test print I got a usable holder. Does your kitchen have tentacles? If not, maybe it's time you ask yourself why not?

We have an extendable table from Ikea with a simple plastic locking system. Unfortunately, one of the lock parts fell out and disappeared, but printing a new one turned out to be quite easy:

And even earlier in the year, I made a sturdier version of the drilling guide for the magnetic poster sticks I had been wanting to make for years. This one worked. I'm still amazed that my printer can print a horizontal beam into thin air - the fan duct certainly helps make that possible, but still.

I'm slowly starting to consider replacing this old workhorse with a more modern machine that doesn't need as much care. Levelling is still a sore point - I have a probe mounted (as seen below) but not connected, I have nothing telling me when I run out of filament, nor if the motors actually move the amount I tell them to.

Finally, here's a small video of my machine at work, printing a test piece for a little holder in the kitchen. I like the sound of the movements, it's very futuristic. As is the ability to design and print useful pieces in your own home. We're truly living in the future.