Described herein is a new method for operating a hemming head of the type comprising a hemming roller (101) and a second, opposed, roller (102), which are rotatable about respective axes contained inone and the same plane. The method envisages variation of the position of the second roller that is opposed to the hemming roller as a function of a signal indicating the force exerted by said secondroller on the metal sheet and/or as a function of a signal indicating the position of the head along the machining path.