myblogs: Die making

Die making is the process of creating a tool for the manufacturing of precisely shaped objects from a stock of workable material. Dies are typically made from steel, and are applied to a medium under pressure to cut out parts that are used in finished manufactured goods. For example, a die might be used to make many small metal parts for a mechanical assembly, to cut rubber parts in shoemaking, or to cut paper parts for stationery or box-board products. Die makers are skilled craftspeople who typically learn their trade through a combination of academic course-work, hands-on instruction and a substantial apprentice period.

In the common die cutting of materials like paper, cardboard and the like, the tool of choice is the steel rule die. The die maker creates a shape from a thin, blade-like steel strip with a sharpened edge which is attached to a high-density plywood base. The steel rule acts like a cookie-cutter to stamp out parts from the material of choice. Computerized machines can cut and form the steel rule to the specification of the design. The steel rule shape and height above the substrate can result in a cut, a crease-mark or a perforation.

One area in which dies are commonly used is die for stamping coins. A long and complicated process, the die begins with the engraver. He or she creates the initial image in plasticine, similar to modeling clay, which is then covered with graphite and immersed in a copper bath, where it receives a thin shell. This shell is then carefully removed and filled with plaster or epoxy. A reducing lathe, called the Janvier Transfer Engraving Machine, reduces the size of the image by tracing the plaster model and engraving it into steel. Known as the Master Hub, this steel image is used to make all other dies and hubs.

Because making the Master Hub takes a lot of time and work, it is used very few times. When needed, it is put into a special hubbing press, which exerts a pressure of approximately 1500 short tons-force per square inch (21 GPa), forcing the image of the Master Hub into the Master Die. The Master Die is then used to form as many Working Hubs as needed through the same process, and then the Working Hubs are put through the same process to form the Working Dies. These Working Dies are the actual dies which will strike coins. The process of transferring the Hub to the Die can be repeated as many times as necessary in order to form the number of dies needed to make the amount of coins required. The difference between a Hub and a Die is that the Hub has a raised image and a Die has an incuse image, so one forms the other. When making Working Dies, the Mint has found that by using a lower amount of pressure in the hubbing press, they can prolong the life of the Hubs and Dies used. In between each hubbing, however, the die being made must be subjected to an annealing furnace to soften the steel, making it easier to push the image into the Die. As the Die is compressed in the hubbing press, the molecular structure of the steel changes. The large amount of pressure exerted on the steel forces the molecules of the steel to be compacted, making this hubbed die much stronger and denser. In the field of metallurgy this is called work hardening, and it is necessary to anneal the steel in order to get it malleable again. If, when the die is subjected to another hubbing, it is not lined up exactly with the hub, the result is a secondary image, or doubling. This is called hub doubling, and results in such spectacular coins as the famous 1955 doubled die cent.

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