US Coins

How the U.S. Mint makes its 3-inch-wide dies

The U.S. Mint’s production of dies, at least for its America the Beautiful 5-ounce silver coins, has come nearly full circle and harkens back to its founding nearly 225 years ago, though using machinery based in 21st century technology.

The Mint does not use hubs to create dies for the 5-ounce silver coins, but instead cuts the designs directly into the die steel via a CNC (computer numerical control) milling machine, and then the U.S. Mint’s engraving staff finishes by hand each die by cleaning up rough details through a series of polishings and related techniques.

The Mint’s engraving staff uses ceramic stones, natural stones, sanding pads, and a brass bristle brush with compound to remove all CNC tooling lines.

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That hands-on finishing treat­ment is not dissimilar, at least in concept, to die production methods used in the late 18th century at the Philadelphia Mint, when all dies featured elements created by hand (through punching or engraving or cutting), although the technology and tools used in 2016 are far different than what was used more than two centuries ago.

Step by step

The dies for the America the Beautiful 5-ounce silver coins are too large to hub, according to U.S. Mint officials. The 3-inch coins are the largest the U.S. Mint has ever struck by far, nearly twice the diameter of both the second-largest U.S. coin struck today, the American Eagle silver dollar at 1.58 inches, and the silver dollars of 1794 to 1803, which are just a shade smaller than the American Eagles. Therefore, every die for one of the 3-inch coins is individually CNC-milled.

Generally, four pairs of dies are created for each America the Beautiful 3-inch bullion coin and five to 10 pairs of dies for the Uncirculated version, depending on demand.

U.S. Mint officials provided a step-by-step breakdown of how the 3-inch dies are made.

??Obverse and reverse die blanks, referred to as “chess pieces,” are turned on CNC lathes and stored for use.

??The original designs used for the standard America the Beautiful quarter dollars for circulation and collector set versions (with a diameter of 0.96 inch) are then increased to the 3-inch size by computer, in a four-hour process that is closely monitored by a Mint employee. 

??Designs are sent to CNC Engraving where computer programming is generated that will direct the cutting tools in creating three-dimensional designs on the die. It takes about five to six hours to develop all of the necessary programming for each 3-inch design.

??A die blank is loaded in a CNC milling machine that is outfitted with the proper carbide cutting tools. 

??The die is CNC milled follow­ing the generated programming.

??The die is removed and is first inspected under a microscope by the project manager of engraving for approval of overall machined finish and detail, before being released for cleaning by a steel die engraving specialist. The project manager then inspects the die again, as does the Mint’s quality control inspector.

??The die is hand finished by a member of the Mint’s engraving staff  to remove all cutter marks, which are lines created by the engraving tools used to cut the die during CNC milling.

??The die is then sent to be hardened by heat treating.

??Once hardened, the die is then cylindrically ground and ready for striking in the press.

??The finished dies are sent to the Numismatics division in preparation for striking.

According to U.S. Mint officials, it takes about 16 to 20 hours to CNC mill each die. It takes about four to six hours to hand finish the die artwork to remove cutter marks for production readiness.

It takes about one hour to turn each die on the lathe to achieve the proper taper and another hour to grind each die once hardened.

Die life

Die life ranges depend on the complexity of the design, according to U.S. Mint officials.

On average, the bureau strikes about 12,000 coins per pair of bullion dies and 5,000 to 8,000 coins per pair of Uncirculated dies. 

It requires three strikes at 560 to 610 metric tons per strike, depending on design detail, to produce each coin.

Back to the past

While clearly techniques used in 2016 are quite different from those used in the 1790s, both involve a degree of individualized results. In the past, placement of a punch for a letter, number, star, or other design device could differ from die to die, making die varieties possible. Today as well, a potential for individual differences from die to die would seem to exist, given potential human variations in applying the techniques used to finish each die.

Each die made at the early U.S. Mint was unique, the result of manufacturing techniques that included hand-engraving and -punching details, with maybe only central design details hubbed into a die. The expansion of hubbed details (not fully realized until the late 20th century) helped standardize dies.

A hub bears a coin’s design in relief and right-facing. When impressed into blank die steel, the hub creates an incused mirror image version of the design. Some hubs and dies are used only for making other dies and hubs, with working dies used in striking coins.

Since no hubbing is involved in creating the dies for the America the Beautiful 3-inch coins, what would normally be considered a master die is actually the working die — the die placed into production for striking finished coins.

Other modern changes

Before the introduction of digital sculpturing in 2006 and the retirement of the Janvier reduction lathe (introduced at the start of the 20th century), improvements, corrections or enhancements to a design had to be executed through the time-consuming process of altering and revising plaster models.

The Janvier reduction lathe would then trace a finished model (generally larger than the finished coin) with a pantograph on one side of the machine, while on the other side of the machine a blade would cut a coin-sized version of the design into the steel of a master hub. The reduction process could take days.

Changes to a design can now be made in a fraction of that time, through digital operations that negate the need for physical models.

A two-dimensional coin sketch or 3-D clay model can be scanned into digital sculpting software for subsequent manipulation and refinement.

Once the digitally sculptured design is translated to a die, the unfinished die is sent to the U.S. Mint’s engraving staff for cleaning up rough details through a series of polishing and related techniques.

Once cleaned up, the dies can be put into production.

“The digital working die is done for the American the Beautiful 3-inch program along with all the 3-inch congressional [gold] medals dies and bronze duplicates,” according to Mint officials. “Also, we do research on direct milling dies on the CNC for all new programs that may come up.” 

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