Today I only have a small update regarding my most probably final Z-probing setup, while tomorrow I’ll release my review of the “Masterspool”!
As explained 2 posts ago, I wasn’t sure yet what to do. Just to recap, the issue was that in total I’ve already bought two inductive sensors and both don’t really work out for me:

  1. LJ12A3-4-Z/BY PNP DC6-36V I bought this item after watching a video that explained this sensor would work fine at 5V and also have a decent detection distance. You can watch it here, it’s very informative: Auto Bed Level Inductive Sensor for 3D Printers. The reason why I don’t want to use it is simply its size: When you read ‘LJ12XX’ you know the physical size of the sensor is 12mm in diameter. This doesn’t fit a regular Prusa hotend and for one, I didn’t want to modify the original CAD files, re-print and reassemble the whole X-carriage and on the other hand I’m not sure that the 12mm sensor will pick up the ~9mm~ leveling points of my MK42 heatbed correctly.

  2. LJ8A3-2-Z/AX-5V NPN This sensor does have the right footprint, but because of the smaller size it’s rated at 2mm whereas the aforementioned one detects up to 4mm. The bigger issue is that 2mm is not even in the reach of what I get. I even tried increasing the voltage incrementally and measuring the sensor’s output with a multimeter, but the sensor is really made for 5V - not only advertised like that - and the most I could get out of it was around 0.4mm (measured with sheets of paper).

Since the last post, I found out that the original Prusa “PINDA” sensor is also rated at only 2mm! I thought my issue with the LJ8A3 was just because 2mm is the maximum value and that it is to be expected not to reach it, so you would need like a 4mm sensor to reach 2mm of clearance.
I’m now pretty sure that my item is either simply bad quality or broken as everybody I know with a Prusa printer told me his probe is actually almost reaching the rated distance so I decided to invest 13€ plus shipping to get a genuine “PINDA” probe from Zaribo: PINDA2 Sensor - Zaribo Research & Development.

In case you’re wondering why you would want as large a detection distance as possible: I usually print everything at 0.2mm layer height with a first layer height of 0.3mm to help adhesion. The absolute max I could reach with sensor nr. 2 was 0.4mm, but for the readings to be reliable and for the nozzle not to smash into the heatbed when leveling, I couldn’t go higher than 0.3mm.
One one hand, it’s extremely difficult to make adjustments by hand on such a scale. On the other hand, 0.3mm of clearance is exactly the height of my first layer, so even when putting it down to 0.2 via slicer settings, the probe would almost always eventually catch onto the printed model, which in turn caused layer shifts when the printer had to ‘pull’ the probe over the edge of the model. Also, the usual procedure of setting up mesh / auto bed leveling is to have the probe a bit lower than needed and then adjust the nozzle into negative Z values at the beginning of a print with live Z offset. Of course, I didn’t have nearly enough clearance available to play with for this to work.

As part of my tests with the 8mm sensor, I watched and followed along the video that I mentioned last time: MK42 Heated Bed Upgrade - Log - Stock Marlin - Chris’s Basement and it was really pretty simple to get the printer to level at the exactly right points. It took me around an hour and worked reliably ever since.
I’d also recommend to get the Octoprint plugin Bed Level Visualizer which displays the surface of your heatbed in a 3D interactive view. This also helps to identify if one of your Z motors has to be adjusted so that the X axis is level with the heatbed. Just a little general side note: you don’t want a non-level X-axis to have to be counteracted via auto-bed leveling or your heatbed leveling screws, if that’s what you use.

So now I’m waiting for my new inductive probe and hoping that this will do the trick and that it’s not maybe my heatbed’s leveling points that are not conductive enough. We’ll see and I’ll report back asap!
Probably, that will also be the final post about my MK3 including stuff like the bill of materials (BOM) etc.