Abstract: Manufacturing method, welding arrangement and door frame structure for a door frame (21) of a windmill tower (10), wherein a hole (15) is made in the tower shell (10) for the door and a door frame is placed in the hole and welded on the shell. In the method, the shape of the door frame for the shell and the shape of the surface of the shell at the opening (15) to be made are measured with a measuring sensor (40). Based on the measurements, a cutting torch (41) is controlled so that an opening corresponding to the shape of the door frame is cut in the shell and bevels (16, 17) are cut in the edge of the opening on both sides. The door frame is welded on the shell with a weld (24), such as a double bevel, double J or square butt weld, from the inside and the outside of the shell.