Building a 3D printer is easy. Getting the details right to build a great 3D printer is hard, as this is where most companies fail. Why?
For example, on this printer, the bed is a three-point mount (two wheels for adjustment at the front of the printbed) and the printer's bed levelling dialogue doesn't show the height difference that needs to be adjusted (which most 3D printers do). It does show how much it needs to be turned, and the bed levelling wheels have 1/8th turn indicators, making it easy to get it perfect.
In short, instead of an arbitrary number like 0.3mm that has no meaning to the user, they tell the user to turn this knob 1/4 of a turn. An instruction the user can follow.
** Why is this so outstanding? It doesn't cost much, but it improves the user experience. Are companies blind to these improvements because the engineers are experienced, or is there a lack of testing during development?**
By the way, years ago I did such a fix/modification myself on a Tronxy XY2 pro by adding indicators on the wheel for 0.2mm height difference so I could convert the number to rotation: https://www.printables.com/model/301670-replacement-bed-leveling-wheel
It’s extremely expensive to produce things with tight tolerances. Cheap 3D printers have gotten away with it by making things “good enough”. Which why you got this the other way around;
0.3mm is easy to measure with the right tool like digital indicator. On the other hand, quarter turn on a knob might adjust 0.3mm on one bolt, but 0.5mm on another.
Also as mentioned, ABL, cheap and can be DIYed. Cheap / printed parts can warped over time, bolts can shaken loose, etc. ABL just put these out of the equation.
I largely agree with what you're saying, but was surprised to see that you called out that much variation in thread pitch. I would absolutely expect a lot of variation in the ability to measure z - especially since most printers rely on microstepping here. Thread pitch on the other hand is generally way more consistent. I am not a machinist, but it would be interesting if one chimes in. I don't know what to ask Google to get some data here, but I strongly suspect there's a term to use.
Tight tolerances will exponentially skyrocket production cost, period.
While the coffee grinder example is true, this is not at all due to tolerances in the thread pitch used to adjust the distance.
This is because of the tolerances on the burrs themselves and the sideways alignment varies too much, and why it's better on more expensive models.
Threads are controlled to a fairly tight ISO standard, which can be done very cheaply because of the simple helix shape and ridiculously large quantities.
Burrs on the other hand are much more complex shapes, in much lower quantities, which is why tight tolerances are expensive on coffee grinder burrs and not on simple thread pitches on standard bolts.