The effect of shrink fitting ratios on tool life in bolt forming processes

Increasing the service life of cold forging dies with respect to fatigue is a key issue in the highly competitive tool manufacturing market. In industry, achieving a lower level of stress in a die insert is known to be preferable when deciding the amount of shrink fit for a stress ring. However, it was found that stress amplitude is a more significant factor in increasing the cold forging tool life with respect to high cycle fatigue. With higher shrink fitting ratios, undesirable pre-stresses were induced in the tool insert, resulting in an increased stress amplitude between the forging/extrusion and unloading/ejection stages in a bolt forming process that lowered the high cycle fatigue life of the die insert. In the present investigation, the effect of the shrink fitting ratio on the level of die stresses, including the pre-stressing due to shrink fitting, was numerically investigated for a two-stage hexagonal bolt forming and gear extrusion operation. The study shows that optimum values of the shrink fitting ratios tended to increase the high cycle fatigue life based on the level of the stress amplitude in the die insert.