When rounding metal sheet sections for forming can bodies by means of a rounding machine, a pre-rounding element is used, whose lower part is deflectable with respect to the upper part by means of an actuating element. Thereby, the joint between the upper and the lower part is executed as a solid body joint whose section modulus in bending during the deflection is different along the joint. Thereby, a uniform impact on the metal sheet may be reached. Furthermore, the separating gap between the upper and the lower part is arranged obliquely, thus reducing the bending of the lower part.