What it is
Bed leveling is the most basic calibration you can and should perform on your 3d printer.
When you level your print bed the goal is that the nozzle of the printer (the hot part that the filament comes out of when you're printing) has the same distance to the build plate on all 4 corners and that this distance is rather small (about 0,2 mm or one layer height).
If your bed is not level you will either "print in the air" and your filament will not stick to the build plate or you will scratch on the bed with your nozzle and no filament will come out. Or maybe even both in different corners of the bed.
Bed leveling is a task that must be performed regularly, because the level may change slightly when you're removing prints or the build plate from your bed.
What I did
For my first attempts in leveling the printer bed I followed this video guide:
In this video the paper method is demonstrated, that means you slide a sheet of paper between build plate and nozzle and from the feeling you get you can judge if the distance is right.
How I did it
I use a mix of the paper method and test prints. My biggest problem with the paper method is, it has to be just the right amount of "nozzle scratching on paper", which of course in the beginning you don't know. I changed to 2 sheets of paper for this method. That way I can get more "feedback" in the form of the nozzle scratching on the paper while still having the right distance.
I also do regular bed level test prints. There is one for the ender 3 linked in the video description on youtube (link to ender 3 bed level test).
This test prints 5 single layer squares in all four corners of your bed and one in the middle. You can then judge if the square has gaps between the lines (nozzle too high) or if the lines are squished together or not printed at all (nozzle too close to the bed). When the square is just a smooth surface the nozzle distance in that position is just right. But keep in mind that when you change the other corners that might influence this corner again.
What I learned
- Leveling your bed and keeping it level is really important - nothing is more frustrating than a print coming loose in the middle of printing because of a badly leveled bed.
- At first it was really hard to level the bed, because it depends so much on "feeling" and I had no reference. But after I had done it a few times it became much easier - so don't despair!
- Using 2 sheets of paper instead just one for the paper method helped me to get a better feeling of the nozzle distance.
- Do the paper method first, then do a bed level test print. It's much easier to fine tune your level with a test print.
- There is also a sensor that you can buy and attach to your printer that will automate the leveling (at least the finer part): ANTCLABS BLTouch But I haven't tried that yet.