Bimetallic Coin – The Making

Singapore has just launched its third series of coins last month, with the $1 coin stealing the limelight with its bimetallic composition. A golden brown outer ring encased a chrome inner core, which in fact are respectively brass plated and nickel plated metal. Much as a coin calls to mind the buying and selling process, there are also engineering involved in the metallic item – the manufacturing of it.

The array of new Singaporean coins launched in June 2013, with the $1 bimetallic coin taking center stage in this article's discussion (images taken from Monetary Authority of Singapore website).

Coins are generally manufactured through a metal stamping process known in the circle with the uncreative name of ‘coining’. Basically a strip of metal, usually plated metal, clad metal or metal alloys, is fed into the stamping machine which holds 2 pieces of dies. Die, has nothing to do with death – that is the term for the specialized tool used for shaping or cutting metal. The 2 dies hold an image on each of them, and when pressed together onto the metal strip, will form the images on it as well as cutting the formed piece, usually round in the matter of coins, from the strip.

Despite the obvious different outlook, bimetallic coins are made through the same process, with some extra steps. The process is broken down via the following general steps:

1. The metal strip that will provide the outer ring is punched out of the strip, with a hole punched out of it at the same time. This leaves a ring of metal.
2. The metal strip meant to be the inner core will be punched out of its strip as well, leaving a simple round piece of metal.
3. Milling is performed on the side of the inner core to create ridges.
4. Both inner core and outer ring is placed together and stamped with the 2 dies which carry the images.

The pressure of forming the coins will force the metal the flow. In this case, the outer ring’s inner diameter wall will flow into the ridges of the inner core, locking them together through the forming process. 

A cross-section illustration of the 2 pieces forming the bimetallic coin (image taken from Fleur-de-coin).

The extra process of forming the ridges is important. Despite the high pressure involved in pressing the 2 pieces of metal together, the fact that the parting between them is vertical, or perpendicular to the plane of the coin, means that a slightly high force such as simply striking a coin on the corner of a table, may dislodge the inner core from the outer ring, essentially destroying the coin despite the fact that the metals themselves are intact. 

Both pieces may also twist from each other, creating an image that is not in sync with each other, thus a misaligned image. The ridges and the flowing of metal from one piece thus become an anti-twisting mechanism as well as locking mechanism to prevent them being dislodged too easily. 

The 1 and 2 Euro coins are also bimetallic coins (image taken from WBCN).

The process of manufacturing a bimetallic coin may seem relatively straightforward, but an engineer will appreciate the difficulty of design which belies such simple process. The simpler a process, the higher the constraint it places on a design. The process itself can be found in patent offices, such as US5094922, EP0529349B1 and EP0868315B1.

Sources: 

1. Monetary Authority of Singapore - Singapore’s New Coins
2. Wikipedia: Coining (metalworking)
3. Wikipedia: Coining (coin making)
4. Wikipedia: Die (manufacturing)
5. Fluer-de-coin: Bimetallic coin
6. Core 77 - How They Make Bimetallic Coin