A review of ballising: Cold working and its effect on fatigue life

Ballising is a relatively unexploited method for the sizing and finishing of bores by cold working and consists of forcing an over-sized ball through a bore manufactured previously by a conventional method such as drilling, etc. Work carried out at the National University of Singapore has shown that the roundness and surface finish are superior to that of conventional methods and the method has the advantage also of superior diametrical precision. Sufficient research work has been done to enable ballising to be used as a reliable production tool, employing aids such as sizing nomograms, developed in NUS, which predict the final hole-diameter to high degree of accuracy. In this paper, practical aspects of ballising, such as its automation into single stroke, multiple stroke, and in combination with other cutting tools into one single-stroke operation, are considered and examples of ballising employed in the expansion of thin-walled tubes and in the forming of interference-fit joints are discussed. The stress analysis of bores that have been cold worked by ballising is studied. The effect of interference on the elastic-plastic zone and the induction of beneficial residual stress is treated theoretically and finally, the effect of ballising on the fatigue life is discussed.