(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).
How To: Bed Leveling by Chris Brown
Not an issue regarding bed leveling. After carefully adjusting the bed level and height for a correct first layer height and getting a successful print, bed or print head seems to change in height, causing subsequent prints to have a first layer that is either too high or too low.
Solved by removing the front panel of the upper chassis and tightening the loose screws holding the Z-axis limit switch to the side panel of the upper chassis.