The mechnical expander is designed for the inner & outer diameter calibration and accomplishment of a circular circumference for the big welded pipes used for mains for energy agents. so as to correspond to the international norms & specifications. The mechanical expander consists of a plastic, step by step, deformation of the pipe achieved by pushing radially jaws to the outside by means of cones. The cone travel is driven hydraulically. The expansion leads to a pipe diameter increase of up to 1.5% in comparison to its initial value.