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This is an old thread, but it's linked to the STICKY thread, so I'll add this as a useful relevant reference:

"Leveling" your printer

A more detailed but similar process:

  • level the X carriage bars (This section may not be required)
    Consider leveling your bars as 'step 0', and worth at least eyeballing to check for level prior to adjusting the bed.

    • Home the printer using the touchscreen or issue a G28 from a controller host.
    • Either wait a minute for the motors to power down, or issue an M18.
    • Make the X carriage bars parallel to the case of the printer.
      Bars should be within <1mm of the same height.
      Turn the Z motor couplers to adjust the bars at both sides (avoid touching the leadscrews with your salty human fingers)
      Either use a measuring tool like calipers to ensure both sides are equidistant from the case (not the buildplate) , Or a convenient object, or one of the multitude of tools made for this purpose, like https://www.thingiverse.com/thing:2768165 The pressure of the bar resting on the objects is relevant, but only accounts for a fractional millimeter of inaccuracy even if it's imbalanced on either side, so not incredibly important. Then remove the spacers.
    • Note: You may NOT need to do the above steps every time, but because z-slouch can happen after even the slightest vibration or touch... it's worth checking each time, even if all you do is eyeball that the lower bar is parallel to the case. I home the printer, pull the hotbed all the way forward, and with my eye up 30ish cm over the front of the hotbed and in the centre of the bar, compare the gap on either side between the bar and the rear edge of the case. If the gap looks the same it's probably still fine and you can omit the previous steps.

    • To completely avoid ever needing to do this again: install a second Z axis endstop, this way the firmware automatically levels both sides of trhe X carriage every time you do a G28. Trust me, if your mega doesn't already have one, you really want to install one and flash the appropriate firmware. It is a massive improvement. Frankly I think anyone making an i3 without dual Z endstops needs to be punished.
  • Tighten the hotbed thumbscrews as far as they will go without crushing the springs excessively.
  • G28/Home the printer.
  • Consider adjusting the Z endstop screw inside the right side of the gantry. (This section may not be required)
    If homing the printer drives the nozzle into the bed, even with the bed at it's lowest position, then you need to adjust and inspect the Z endstop for damage. It rarely needs to be adjusted unless you have changed the length of your j-head or altered the geometry of your hotend somehow, so this step is more about inspecting the z endstop for damage, and making sure the adjustment screw is centred on and contacting the switch cleanly. Sometimes this endstop can get bent, and this is a common cause of driving nozzzle into bed, despite frequent re-leveling.
    The screw is located above the extruder motor inside the rear right side of the gantry, and protrudes down past the extruder onto the case, where the endstop is located.
    Adjusting the z endstop adjustment screw s an iterative process, where you may need to repeat the following steps a couple of times:
    • Raise the Z a bit using the menu/octoprint/repatierhost/whatever. You want the endstop switch to disengage. Like maybe see it's indicator LED go off/on (depending on what kind of switch it is), or hear the microswitch un-click. SO you know the adjustment screw is not activating the switch.
    • Adjust the Z endstop screw in the desired direction.
    • G28 Z or otherwise home Z on the printer......
    • Until your nozzle is about 1mm above the fully tightened hot bed.
  • Heat the hotbed
    • If you need to clean any goop off the nozzle then heat the nozzle too, but turn it off before you do the actual leveling so the filament doesn't dribble and you aren't burning holes in your bed surface.
  • Move the hotend to one corner of the bed and adjust the thumbscrew, repeat the following for each corner.
    • My preference is to use my eyes to set zero gap. I do not use paper/feeler gauges (see attached image/rant), but these methods are also useful if you aren't comfortable with "eyeballing". Use whatever method you like, but just remember that the printer is expecting ZERO distance between the nozzle and the bed, not a gap.
    • With a light behind the nozzle, wearing your closeup/magnifying glasses, or just using your youthful 20/20 eyesight; put your eye down near parallel with the bed surface, adjust the thumbscrew till you see the gap between the nozzle tip and the bed diminish. Ideally you want there to be no gap whatsoever, however you do not want to the nozzle to be pressed into the bed at all. So back it off and re-lower it till you have almost no gap, and cannot adjust it closer without the gap vanishing.
      refer to: attached: 2018-03-28--10-47-43.jpg
    • Repeat this process several times for each corner in succession, or on diagonal opposite corners until you no longer need to adjust the thumbscrews at all.
  • Sometimes I will print a 4mm wide stripe around the outside of my bed, or a single perimeter stripe, and after removing the test print then adjust the thumbscrews according to my calliper assessment of the printed layer. Usually I do not. If your bed is bulged in the middle you may like to print a different test, with elements in the middle etc.
  • Clean the bed surface. Use isopropyl alcohol (rubbing alcohol), or similar. If there is any trace of grease on it from your greasy mammalian hands then you have not cleaned it properly and prints may fail.
    • DO NOT use acetone (nailpolish remover) on the old style buildtak surface, and especially not on the ultrabase, or you will destroy it.

Each time you do this you will get better at it, eventually it's easy but always a little annoying.

Anycubic i3 Mega - z sync blocks