A manufacturing assembly for manufacturing a metal part comprises a flattening module having a trolley and at least one pressing roller. The flattening module is movable along a hemming of the metal part. The flattening module comprises at least one pressing actuator that is arranged to move the at least one pressing roller in a determined pressing direction relative to the trolley and to place the pressure roller bearing against the free surface such that the at least one pressing roller exerts pressure on the free surface in said determined pressing direction. The pressure increases auto-adaptively with an altitude of the free surface in a direction opposite the determined pressing direction.