Dealing with the hole in Seamless Tubing

An important question, but not an insurmountable problem

Many good machining applications for seamless tubing have been defeated by the problem of eccentricity or more accurately stated, the lack of understanding and how to allow for this condition in ordering machining stock.

Because seamless tubing manufactured by an extremely rugged piercing or extruding operation, it is rarely possible to get the hole in the exact center of the tube. Consequently, the inside diameter of the tube may not be exactly concentric with the outside diameter of the tube, as illustrated below:

Eccentricity does not affect the outside or inside diameter, sectional area, weight per foot, or the mechanical properties of the tubing, and it has very litter effect on resistance to bending and torsional stresses. However for machining applications, eccentricity is an important consideration.

To the machinist at the lathe, a discussion of eccentricity mat seem superfluous because he has always used a drill smaller than final hole size to allow for drill run out and finish bore; and he added a little stock to the outside diameter to take care of the run out due to inaccurate centering and lack of straightness, However, for machining seamless tubing, allowances have to be made for eccentricity in calculating the size of tubing stock to be ordered.

In additional to allowance for eccentricity, there is an allowance for "clean-up". The reason for this is that tube surfaces may contain minimal amounts of undesirable imperfections, decarburizations, or camber, or the part may simply require a surface better quality than ordered from the mill

Tubing specialists, either at the mill or steel service center, can be most helpful in calculating starting sized for tubing that is to be machined, and it is recommended that buyers or specifies consult with these specialists before specifying their tubing requirements. However, there is no mystery to calculating starting size, as demonstrated by the following exmaples, and buyers may wish to do their own calculations.