Die making

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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 stationary 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 deign. The steel rule shape and height above the substrate can result in a cut, a crease-mark or a perforation.

[edit] Die Making in Coin Production

One interesting area in which dies are commonly used is die for pressing coins. A long and complicated process, the die begins with the engraver. He or she creates the initial image in plastilene, 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 tremendous amount of 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. Professionals call this work hardening, and it is necessary to heat 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.

However, this stage in the process is where two errors can occur, resulting in less than perfect dies. It is impossible to tell without expensive and lengthy tests whether these errors occurred, but they will lead to significantly weaker dies and a higher tendency to deteriorate quickly. The first thing which can happen to a die in this process is called by metallurgists decarburization. Decarburization is the result of an improper mix of metals and chemicals in the annealing furnace where the dies are sent to soften. When steel is made, it is necessary to add carbon to the iron being used. Carbon makes the iron strong, and it is important to get exactly the right mix of carbon and iron to make good steel. An improper mix would result in either soft steel or in steel which is too brittle. In the annealing furnace, the same conditions apply. If, when the steel is heated, there is too much carbon in the furnace, the steel will absorb more and will become hard and brittle. If there is not enough ambient carbon in the furnace, the steel will give up some of it’s carbon and will become soft. This process is known as decarburization and results in a thin surface of inadequate metal whose chemical composition has been altered and is below standard. Either of these alternatives would produce a die that would wear too quickly and create Die Deterioration Doubling.

Another less obvious error can also occur when heating the dies. After being taken out of the annealing furnace, the dies must cool for 24 to 36 hours. They are usually given an oil bath to cool them more quickly, but the dies must still cool for a period of several hours. When the dies are cooling, if they are not stored properly or not allowed to fully cool before being used, the result is a much softer die. When the die is subjected to the hubbing press again and work hardened again, it will be softer and more susceptible to wear. If the die is softer to begin with, for example, if it is not fully cooled, it will not harden to the same standard as a die which has received proper treatment. However, there will not be a noticeable difference between a good die and a bad die; the change in the structure of the die does not appear until it is used for a period of time.

[edit] Use of a die

The second factor which contributes to Die Deterioration Doubling is excessive wear while being used. In modern presses, a die strikes approximately 120 coins a minute. This is a tremendous amount of coins produced in a short time, and obviously the die cannot last forever. However, the process of wearing the die is only hastened by the metals used in coins. Nickel, one of the main metals used in today’s coins, is exceedingly hard and causes wear quickly. Copper has been used for centuries because of its malleability and the ease with which it makes coins. However, it too wears the dies when they are used for too long. An infamous example is the 1955 "poorman’s double die." This coin is sold as a replacement for the 1955 doubled die, but it is no more than Die Deterioration Doubling, caused by wear on the dies.

When a coin is struck, the planchet is not heated. Although the planchet would be softer and more malleable, the extra time and expense would prove too great for the mint. The planchet is therefore struck at room temperature, and the only thing which makes the coin form is the tremendous pressure used to strike it. With a metal such as nickel, which is harder than a normal coin metal like silver, gold or copper, the pressure must be greater. When a nickel coin, or any coin, is struck, the metal must "flow" into the hills and valleys of the dies. It is through the molecules of the metal flowing into the dies that flow lines are created. However, when metal flows over a sharp corner in the die, like the edges of a mintmark or words, it tends to roll the detail out. It wears on the die, and a little detail is lost with every strike. Die Deterioration Doubling is most prevalent on the date and mintmark because these fine details are alone in the middle of the field, and the metal must flow into these without the help of other valleys nearby. When the metal rolls into the mintmark or date, it wears away the corner of the die, and after long enough will appear on the coin as Die Deterioration Doubling.

Combined with the wear the dies suffer through the striking of large numers of coins is the practice of polishing the dies and using them longer, leading to increased wear. When a die begins to show evidence of wear, a mint technician will polish the die and reuse it. However, the die is still rapidly deteriorating, and polishing it cannot avoid the problem of wear. So, after many thousand coins in rapid succession, the wear on the die has gotten so bad that it appears as Die Deterioration Doubling.

[edit] Types of die deterioration doubling

There really are two different kinds of DDD: Inside and Outside Abraded Die Doubling. This is because Die Deterioration Doubling manifests itself in two different ways. The two classes look different, and are caused by wear in two different places. Inside Abraded Die Doubling is caused by wear around the edges and inside the actual detail affected. It appears as an enlarged, irregular outline of the element overlapping the original design element. Outside Abraded Die Doubling is caused by actual wear in the field of the die. The field of a die is the highest point, so when a mint technician polishes the die or the metal flow of a coin wears it away, the field is the first thing to be worn down. The difference here is that Outside doubling does not overlap any of the original design, but rather appears like an irregular shelf around the object.