withattribution

BLTOUCH BED LEVELING 

G28 instructs the printer to home itself to the X an Y endstops and the Z sensor determines the homing of the Z axis; i.e. when the sensor triggers, this is not necessarily (and most commonly) not the position where the nozzle is at Z=0.

G29 determines the shape of the bed by probing the bed. This will set the shape of the bed with respect to the sensor trigger point as described earlier. The Z-offset (set by M851 Z-x.xx is needed to set the offset between the nozzle and the sensor trigger point (to the bed).

The sequence to determine the offset is:

M851 Z0; // Set the Z offset to zero height
G28;     // Home Z in the middle of the bed