Abstract: A plastic frame and a frame are provided in a screw-on structure. A through hole is formed in the plastic frame and a screw hole is provided in the frame at a position corresponding to the through hole. Furthermore, a screw is provided, the screw is inserted in the through hole to be screwed down the screw hole. The maximum inner diameter of the through hole is made larger than the outer diameter of a shaft section of the screw. Three ribs, which protrude toward the center of the through hole to contact the shaft section of the screw, are integrally formed with the plastic frame on an inner surface of the through hole. A far end shape of the ribs is formed in a shape along an outer surface of a virtual cylinder whose diameter equals the outer diameter of the shaft section of the screw and whose center axis matches the center axis of the screw.

Abstract: A plastic frame and a frame are provided in a screw-on structure. A through hole is formed in the plastic frame and a screw hole is provided in the frame at a position corresponding to the through hole. Furthermore, a screw is provided, the screw is inserted in the through hole to be screwed down the screw hole. The maximum inner diameter of the through hole is made larger than the outer diameter of a shaft section of the screw. Three ribs, which protrude toward the center of the through hole to contact the shaft section of the screw, are integrally formed with the plastic frame on an inner surface of the through hole. A far end shape of the ribs is formed in a shape along an outer surface of a virtual cylinder whose diameter equals the outer diameter of the shaft section of the screw and whose center axis matches the center axis of the screw.