In the cross section of the circular steel pipe there is a phenomenon in which the outer diameter is not equal, that is, there is not necessarily perpendicular to each other, the maximum outer diameter and the minimum outer diameter, the difference between the maximum outer diameter and the minimum outer diameter is ellipticity Roundness). In order to control the ellipticity, some steel pipe standard provides the allowable index of ellipticity, the general provisions of not more than 80% of the outer diameter tolerance (after consultation between the supply and demand sides).