This is to help new users understand the Wanhao Duplicator 6 leveling script and ensure top prevent a nozzle crash.
The official PDF instructions are located here as of today http://www.wanhao3dprinter.
So the fundamental problem is really 2 things, I don’t really like the process, but it was how Ultimaker did it in the source firmware, but the second part is the instructions do not detail a key step but there are notes about the function.
Steps / Summary
The most important part is understanding the control knob can raise and lower the build plate at KEY steps in the sequence and that on the critical second pass leveling, there really should be clear instruction for the user to raise the build plate back to the limit switch, or the script should be edited in the firmware to rehome Z back to the limit switch.
The first pass leveling instructions say to use a 1mm gap. This is mostly a non-critical step and it’s all about ensuring the bed is not so far out of level as to cause a nozzle strike. Basically it’s an eyeball visual step to ensure that nothing horrible happens on the fine tuning leveling step.
So here lies the problem step. After you run the first pass past the 3 leveling points, because the LCD can only display so much text, the instructions are not clear on a required user action. The user must turn the OLED control knob at the base of the printer to raise the build plate exactly back until the limit switch just clicks BEFORE adjusting the wing nuts to change the bed leveling height. Again, this is the mistake and the instructions could be clearer and/or, really the script needs changed to take the user action out of it.
If the user does not turn the knob and raise the build plate leaving the Z axis some unknown distance away from the Z homing switch, then when the user adjusts the wing nuts and leveling gap, you are adjusting to the incorrect and much too high of a physical position. This then guarantees the bed will slam into the nozzle because the bed is now higher than the physical homing limit relationship.