A calibration system for automatically resetting the effective vertical Z coordinate corresponding to the tip of an extruder of a printing head with filler material filament for 3D printers provided with a printing plane movable along the vertical Z axis, wherein the printing head is installed on a carriage for moving the extruder on the horizontal plane XY, includes at least one touch probe unit associated with and operatively connected to the extruder, specifically arranged between the carriage and the extruder itself, including: at least one movable support, at least one touch probe, at least one extruder support, at least one coupling system, at least one carriage, at least one extruder. The movable support is substantially a mechanical spring-operated sliding system, configured to allow the extruder to travel the path needed to activate the touch probe during the touching operation and to return to the printing position.