Tube Eccentricity Allowance
Seamless tubing is made by rotary piercing a solid round billet. In this rugged operation, it is rarely possible to center the hole exactly along the centerline of the former billet outer diameter. This characteristic is known as "eccentricity." It is defined as the distance between the centers of the circles prescribed by the outside and inside diameters. This eccentricity spirals through the tube and naturally occurs in all seamless tubing manufactured by rotary piercing.

Only the wall thickness is affected by tube eccentricity. The theoretical or average wall thickness is reduced at any one point and the wall is increased by approximately an equal amount at the point directly opposite. This continuing effect is controlled by the manufacturing process, and is measured as a variation of the wall dimension with a plus or minus tolerance. This tolerance is customarily expressed as a percentage of the theoretical average wall.

This wall variation is one of several seamless tubing characteristics that influence the size of tube required to satisfactorily machine a part. Details as to how to design a tube size follow in the next section.