Modeling the Influence of Design Geometry on the Coining Process

Abstract

A number of aspects of the coining process are investigated, both through experimentation using several types of tooling using blanks made of copper 110 or brass 260, and by developing and using a FEA model. Several relationships have been found which describe the effects of changing the type of coin blank or the geometry of the coining tooling on how much volume of the coin is formed at different forces.

The open-die bulk upsetting test was used to find the true stress and strain curves of both materials, and the ring test was used to determine the coefficient of friction. Coins were made over a large range of forces in order to test the general nature of how the diameter and design of a coin are formed. While the diameter begins to increase, the thickness of the coin reduces and material is pushed into the punch cavity, filling the design’s volume up rather linearly.

Tests on the effects of changes in the wall angle were inconclusive. As the punch design depth increased the force requirement went down in a manner roughly inverse to the ratio of the increase in depth. Effects of coining with a punch on one side versus two sides were tested. Effects of the perimeter of the punch design showed that a longer perimeter actually reduced the forces required for thinner coins, a difference that got smaller as the coin blanks got thicker.

Blanks required 1.4 times the force to form than a coin half its thickness. A direct correlation of forming force to the yield stress of the material was expected but rather appeared to be related to the full nature of the true stress-strain curves. The FEA model was able to match experimental results relatively closely, but only up to about 333.3 kN, the lowest force used for the bulk of the experimental samples. The FEA model provided a good look into what happens to the coin while it is under load and the mysteries of ghost coining were unveiled.