Manually level CR-10 V2 with BLTouch

Question

I am using a Creality CR-10 V2 with BLTouch installed. I referenced many articles but seems like I am constantly getting inconsistent information regarding to bed leveling. If I am not wrong, I need to manually level it using the bed screws and a piece of paper before deploying BLTouch. But I have so many questions regarding to manual leveling.

My attempt is like: (doing everything at 210 ℃ nozzle and 60 ℃ bed)

• Firstly, the auto home function of stock firmware with BLTouch support and without BLTouch support behaves very differently:

  • BLTouch support: stops at X=103 Y=150 Z=10 (center of the bed, nozzle very high from the bed)
  • Without BLTouch support: stops at X=0 Y=0 Z=0 (front left of the bed, nozzle almost touching the bed if manual leveling is done before)
  • I have BLTouch installed, so for this I simply invoke a Z-=10 command after auto home.

• After Z-=10 command, the Z is showing 0, but the nozzle is so high from the bed. If what I have learned is not wrong, I need to set a Z offset.

• So I manually move Z by -0.1, while dragging paper under nozzle until I can feel the drag. (Question: do I stop when slight drag is occuring, or I need to make the nozzle scratch the paper really hard? For action below I assume the former.) Take note of the current Z value, which is Z = -2.4. Z = -2.5 will completely chew the paper, and Z = -2.3 does not have drag at all.

• I set the Z Offset option to -2.40. (Question: Does the second decimal digit really does anything on Marlin firmware? I tried -2.45 and -2.47 but does nothing. Until I set to -2.5 it begins to chew paper like when I manually set Z = -2.5. ) I save the setting to EEPROM, and do auto home again.

• This time auto home stops on Z = 12.41. (Question: why additional 0.01? Shouldn't it be homed at Z = 12.4?) Because the additional 0.01, I am not able to adjust Z to 0 now (motion only provided accuracy down to 0.1). If I do everything I can do for now to lower Z, Z will eventually be 0.01, and the nozzle still have a small gap (around ~1 mm measured by eyes) to the bed. Why?

• If I continue to lower Z to negative value, Z = -0.1 would still does not have any drag, and Z = -0.2 will drag quite hard. After quitting from Z adjust screen to information screen with a Z = -0.2 setting, it shows Z is now -0.28 rather than -0.21. Why 0.08 now?

• So what should I do now if I want to manually level it before auto mesh leveling? Everything seems so random to me and I am not able to observe a pattern.

• Also, if doing manual levelling (ignore the current BLTouch situation), should I do the paper drag on Z = 0 or Z = 0.1? I am getting mixed information on this.

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I don't know what I did, but now auto home behaves weirder. After auto home, the Z displays 12.4 for a sec, but then immediately jumps to 0 while the nozzle is high in the air. Until I manually adjust the Z axis the nozzle starts to move to the "absolute" position. (i.e. If I adjust to Z = 1, the nozzle moves down to Z = 1.) I wonder whether this is the normal operation or stops at Z = 12.4 after auto home is the normal operation. I am so confused now.