A method for manufacturing a lamination for electro motor rotors that have skew or helical slots by forming at least one boss on a face of the lamination and reducing a thickness of a first portion of the boss. The reduction in thickness may be by carrying out a localized narrowing of the thickness of the first portion and the boss may be manufactured by pressing the lamination. The localized narrowing may be carried out by misalignment of a die relative to a punch within a mould.