Abstract: A method for preparing a cylindrical separator from sheet material and for inserting the separator in the cylindrical anode space of a semi-finished cell, in which the separator is wound from a sheet material around a mandrel to have a diameter smaller than the diameter of the anode space so that the sheet is temporarily affixed to the outer surface of the mandrel by providing vacuum through the mandrel wall, and the so obtained winding is inserted axially in the cathode cylinder. When the insertion is ready, the vacuum is disrupted, and the wound structure expands and presses against the cathode wall.