(by Ethan Anderson)
Let’s start with terminology. This is super important.
When we level the bed, we are adjusting the angle of the bed along the X and Y axes, such that as the print head traverses the bed in these axes, it remains at a constant height above the bed. On the Monoprice Select Mini, this is accomplished by moving the print head around to the four corners of the bed and adjusting the screws for each corner. Loosening each screw raises its corner, while tightening lowers it. Leveling the bed is a separate task from adjusting the overall height of the bed, setting the first layer height, etc. Do not mix these up.
Bed leveling does not address the overall height of the bed, even though these tasks are accomplished using the same four screws on the Monoprice Select Mini. This is, in my opinion, the worst part of the printer. Let’s take a look at all the major components that contribute to what we call “bed height” and see why.
All of these are relative, and dependent upon manufacturing tolerances and assembly tolerances. That’s why it all has to be adjustable - because no two 3D printers can be manufactured and assembled exactly the same. On most 3D printers, there is a screw that adjusts #2 from the list above. This means that you can level your bed without worrying about its height, then fine-tune the height of your print head relative to the bed when it homes independently.
On the Monoprice Select Mini, this is not possible. That means that after leveling the bed, you have to go back and either loosen or tighten all the screws collectively to adjust the overall height of the bed relative to the print head in its Z-homed position.
This process can be painstaking, and may be exacerbated if the Z-axis limit switch in your printer is not secured tightly enough (see Inconsistent height of first layer from print to print under Issues List - Hardware).
This is a setting you’ll see in your slicer (Cura, Simplify3D, slic3r, etc.). It refers to the height at which your printer will print the first layer, assuming your bed is leveled and at the right height to do so. This is entirely relative. The difference between a 0.2mm first layer height setting and a 0.3mm first layer height setting will be 0.1mm, but your actual first layer heights could be 0.27mm and 0.37mm in reality, if your bed is not set at the right height. It is incredibly useful to have a set of digital calipers in order to check the height of your first layer. You can eyeball it, too, but unless you know exactly what you’re looking for, it’ll be tough. A number is a number.
It should be pretty clear at this point what you need to do in order to achieve a correct first layer, but let’s look at what’s actually going on and causing the issues associated with a bad first layer. It’s important to understand what your slicer is doing when it’s taking your settings into account. Let’s say your first layer height is set to 0.3mm in your slicer. With that information, it knows how much plastic (in volume) it should be extruding per length traveled in order to fill the space exactly right.
But what if your bed is too high (too close to the print head on the first layer)? Your slicer and your printer don’t know that - they assume everything’s set up correctly. So the printer extrudes too much plastic for the available volume, since the volume of a shorter layer is less than that for a taller layer. That extra plastic has to go somewhere, so it goes out to the sides, and sometimes forms mountains between the paths traveled by the extruder. The effect is that holes are too small and outlines are too big, and you lose dimensional accuracy.
What about when your bed is too low (too far from the print head on the first layer)? Again, your software and hardware are oblivious, so not enough plastic is extruded to fill the available space. This is where you get adhesion issues. The surface area of plastic in contact with the bed is considerably less than the total surface area of the model in contact with the bed (the extrusion is round, so we’re relying on a layer height less than our nozzle diameter in order to squash it flat against the bed). If the bed is way too low, such that the distance between the nozzle and bed is greater than the nozzle diameter, it’s unlikely that you’ll get any real adhesion at all.