Bed levelling is critical to getting a good print. If the first layer doesn't print right the rest won't either. Ideally you want an accuracy better than .1mm which is about the width of typical paper. Default cura settings print the first layer .3mm thick to compensate if your bed is off by .1 it should still work. But if you are off by .2mm it might not.
How to level:
Levelling here is defined as setting the Z height and also levelling. It's one procedure that does both at once.
Optionally heat up the nozzle to 180C if first layer is .1mm or less because a cold nozzle shrinks and you will be setting the bed to the wrong height. Make sure tip of nozzle doesn't have any plastic on it or you may level to the wrong height.
Home the z axis only. If you must home all 3 then you need to disable the steppers once it's done so you can move the print head by hand. Move the head as close as possible to each of the 4 screws in turn. Once at a screw tighten the screw and then slip a piece of paper between the nozzle and the print bed. Make sure the paper slides very freeley. Then loosen the screw until the paper gets slightly stuck. You want the paper to easily be able to slide in and out under the nozzle with one hand pushing the paper. If the paper gets stuck it's probably too tight under there.
Repeat this procedure for the other 3 screws. Then go back to the first screw and repeat on all 4 screws again. Then repeat on all 4 screws again. Then again. It may take you 20 minutes to do this the first time but the second time you do this it should take much less time because you are both better at it, faster at it, and because there isn't much to adjust the second time.
If you levelled with a cold nozzle you are done. If you levelled with a hot nozzle you should then loosen the 4 screws 1/8 of a turn to compensate for the thickness of the paper.
Once done levelling rotate the z screw by hand to keep the nozzle off your bed. This makes it less likely to damage your bed surface and gives the nozzle room to leak.
On a new ultimaker repeat this procedure before every print (at least every hour) because the print bed can move/droop like a new guitar string. After many months the droop slows down.
the print bed is too high. You can fix that two ways: either adjust the Z end switch, or you can adjust the bed level with the 4 screws holding it. But the second way is more for fine tuning, to make sure it's leveled. yours seems way off so I'd re command adjusting the end switch first: http://wiki.ultimaker.com/Ultimaker_rev.4_assembly:_Mounting_the_electronics#Step_10:_Checking_and_aligning_the_Z-homing_switch
the Z switch is the little black thing located inside the box, on the back panel, at the top, on the left side of the lift. it can be adjusted from outside, but unscrewing the two bolts
Once this is done, you should be good for a while. You should also try running Cura's bed leveling wizard after that, it will teach you how to do it.
I set Z switch once and never had to mess with it again, I only adjust the bed level with the screw every now and then, when I have messed with the bed or moved the printer around
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