Dies for Coins and Medals

Q: How are the dies for coins and medals made? Is this something a jeweler can do without a lot of expensive equipment? Can etching techniques be used?

A: In ancient times, this sort of die was engraved directly, with ordinary engraving and grinding tools. An enhancement to this, which is still sometimes used today, was the use of custom punches. Thus a punch in the shape of a star can lay out a whole little semicircle of stars around the periphery of a medal rather than having to engrave each one. More elaborate punches, called hobs, may include anything from small details to the entire surface of a medal, if the design is such that it’s easier to carve into the steel in positive relief instead of in reverse. Hardened and pressed into the annealed die, it yields a reversed image ready for additional work or hardening. Often, more than one technique or sequence of punches might be used in the development of a design. Some parts may be directly engraved, others punched, or punched and then modified. In cases where large quantities are needed, usually the finished master die is actually a hob, from which many working dies can be made. Etching is a rather imprecise process, and is not much used in 3D die work, other than perhaps as a means to achieve a surface matting. But there are other, more easily controlled methods even for this, such as abrasive blasting or simply leaving a ground surface on the hob or die instead of polishing it.

The technique outlined above was the one used historically. In more modern times (mostly in the last few hundred years) the use of pantograph engraving has made extreme details possible. Factories such as the Franklin Mint usually work up a design in clay or plasticine, which is then cast in plaster. This master model is usually about 10 times the size of the finished die. A dinner plate sized model is easy to detail in clay so that the finished coins measuring an inch across will look perfect. The pantograph that transfers this plaster positive to a metal die rotates the model under a feeler on a long arm, like a record player. Near the other end of the arm, only a tenth of the distance from the hinge compared to the distance to the feeler, is a cutter working on the steel, which rotates in sync with the model. The result is a high-precision reduction. This type of pantograph has been in use for several hundred years, but in the last few decades the use of EDM machines has made possible the conversion of soft models directly into hardened steel dies with great accuracy. The original can even start as a wax model.

For your own experiments, try first working on a simple punch, such as carving the end of a tool-steel rod into a simple positive design or, if you are ambitious and talented, a recognizable face. Keep the degree of relief low. When you like it, harden it, and with a sledge-hammer, if you don’t have a decent hobbing press (grin) punch it into an annealed tool-steel blank, which then may be hardened and used as a die. You can also experiment with direct engraving into the blank like the Greeks did. With some inventiveness, you’ll be surprised at how easy steel is to engrave and work, and what kinds of effects you can achieve.

by Peter W. Rowe M.F.A., G.G.