The Spring Back of Metals

Abstract

Abstract A generalized and simplified mathematical analysis and derivation for a spring-back correction curve for pure bending is given. Extensive test data, especially for “springy” materials are shown. Qualitative reasons are given for their departure from the theoretical curve. An optimum empirical curve is plotted, showing a band of most probable values. The utilization of this general spring-back curve is described and a worked example is given. Whereas no data have been accumulated on brass and bronze materials, it is believed that these will fall into the bands set by data on aluminum, nickel-base, titanium, and ferrous alloys. It is recommended that all tooling for metals whose ratio of yield stress to elastic modulus S/E lies between 1.1 and 5.0 × 10−3 be treated in accordance with the spring-back correction factors found in Fig. 15. As soon as this ratio S/E and the material thickness have been determined a chart of tool radius R versus resultant part radius r may be prepared, e.g., Fig. 16. This chart is then used by the tool designer to specify the tool shape. Conservation of thickness and length are a necessary assumption. Whereas many sheet-metal-forming operations do not require spring-back correction, there is a large savings in “tool-development” time in those cases where the spring back is large and cumulative.