Coining, sizing and straightening are commonly used to improve the dimensional accuracy of forging. When tolerances closer than those that can be economically produced in the forging die are specified, coining and sizing operations are often employed. These processes cause plastic deformation of the forging, either by striking or squeezing a defined area. Coining may be performed either hot or cold, and can be accomplished either before, during or after treatment.
Cold coining is preferred because material shrinkage is not a factor and the process can produce excellent surface finishes. Special dies can be used to coin irregular surfaces, but flat surfaces, such as bosses are most suitable and can be sized to very close tolerances.