Closed-die coining – an upper bound analysis

Abstract

The final stages of many forging processes involve the flow of material into corner cavities or other surface details within a die. The process at this stage is essentially a ‘coining’ operation, and is usually characterised by high forming forces and relatively little material flow. These forces can be particularly severe under closed-die cold forging conditions, as in, for example, the manufacture of specialised headed fasteners, and can lead to problems of premature tool failure or the production of faulty products. This paper presents an upper bound analysis of such a ‘coining’ stage during a cold forging process. The theoretical analysis is based on an upper-bound formulation derived previously by the author, but, as will be shown, can be extended with ease to investigate the deformation of a family of similar workpiece geometries. The upper bound predictions for mean forging pressure ratio were compared with experimental results and good agreement was obtained. In addition, the change in the workpiece geometry during forging was noted and is commented upon.