This article tries to cover the steps involved in the coin
minting process so that readers understand and appreciate the various steps and
stages involved in the minting process. The process described is what is
followed in current modern mints and does not discuss the older method of
minting which although similar has its own intricacies.
Although the article focuses on coin manufacturing, the
process is essentially same to manufacture Medals [Army/others], Silver coins
with Jwellers, Metallic Keychains with shop name etc
Design Process
The coin minting process begins with the process of creating
a design. First a sketch is created on a paper often by an artist. The sketch
may or may not have all the routine details that are typically present on the
coin. From here Sculptors and engravers take over. They convert the 2-D design
into a 3-D design. Often the material employed plaster or such materials that
are no different from the ones that are used for creating a work of art. Typically
the size of the plaster model is 10” to 16”. The design is always in relief [or
positive] just like the one that appears on the coin. This plaster sculpture after
finishing is then coated with epoxy resin. This acts as a hardener as well as a
preservative. This is coated epoxy plaster sculpture is called “galvano”. This
completes the design process.
There are 2 different models, one for the obverse and one
for the reverse.
The next step is the Die making process.
Die making Process
As the Galvano is quite large, the first step is to reduce
to the desired coin size.
Master Hub:
The galvano of 10” to 15” dia is placed in a Transfer
Reducing Machine on one side. The other side has a steel rod [circular bar] of
the diameter of coin that would be minted. The machine based on fulcrum
principal will carve out the steel rod with the design. This is also called
reducing stage of die production. This steel piece is then passed through a
tempering or annealing process[1].
This hardens the steel. At the end of this we have a “Master Hub”. Typically
there is only one single Master Hub for the entire set of coins that are minted
for the particular design. It is to be noted that the master hub has same
relief as the galvano, i.e. the image is like what is seen on the coin.
Master Die:
The next step is to create master die from the Master Hub by
a process called hubbing. The Master Hub is placed in a Hydraulic hubbing press
at one end. The other end has a steel rod of same diameter as of the coin and
typically 4 inches long for grip. The two are then pressed together under
pressure. This result in the master die having the impression of the Master
Hub. However the image is Incuse [Mirror Image, Negative image]. As the Master
die itself is of steel, a single press would not result in sufficient depth of
design on the Master die. It would require the above step to be repeated
several times as required to have the incuse of required depth. At every step,
the Master Die is annealed so that it retains the required strength. If the
entire process is attempted in one press by putting more pressure, it can
weaken the Master Die and it could develop small cracks that would wear it out
sooner.
Typically depending on the number of coins to be minted, the
Master Dies created are 2 to 4. At times it can even be a single Master Die.
Working Hubs:
The next step is to create what are called as Working Hubs.
The process followed to create the working hubs is same as that of the Master
Die; except that one end of the hubbing press has Master Die. The design on Working
Hubs will be in relief [Image as on Coin, positive].
The number of working hubs created is more than the Master
Die, typically 4 to 10.
Working Dies:
The next step is to create working dies. These are the dies
that are actually used to strike the coins in the coin press. The process
followed for created is same as that of Master Di; except that one end of the
hubbing press has the Working Hubs. Just like the Master Die, the design on the
Working Dies is of Incuse [Mirror image, Negative].
The number of working dies created is as per die wear and
the number coins being stuck. On an average a working die can strike around
500,000 coins before wearing and needing replacement.
Mint Marks & Dates:
If there are multiple mints that are minting the coins, typically
the Original plaster sculpture does not have mint marks or date on it. These
are added on the working dies typically by hand punches. This is where some of
errors like date doubling, double mint marks, and errors happen.
However if the number of coins being stuck are quite high,
the mint marks and the dates are added to the Master Die.
At times if every mint is doing their own design [which is
not often, as it’s expensive and results in small difference in design], the
mint mark and date are also added to the plaster sculpture itself.
Planchet Making
These correct sized blanks then undergo annealing and
cleaning process. After this the blanks are fed into an upsetting machine. This
machine creates the raised edges around the blank.
There are 2 purposes for having raised edges around the
coin. During the striking process the edges help the metal flow into the die
design rather than flow off. Plus the edges allow the coins to be stacked
properly.
As a final step, these blanks are now passed through a
furnace and given a chemical bath. This gives the coins a distinctive color or
tone based on the metal used.
After this entire process is completed the blanks are now
called as Planchets that are ready to be stuck.
Striking the Coins
The process actually produces the coins that are put in
circulation. The striking chamber has 2 working dies. The reverse working die
is placed in what is called as “Anvil die” and is stationary. The “Hammer die”
has the Obverse working die. The striking chamber has a funnel through with
coins are fed at the top. At the bottom of the funnel has feeder tubes at the
end of which are 2 feeder fingers. These place the planchet on the anvil and
eject the stuck coins. The fingers keep sliding over a smooth steel surface and
keep moving forward and backward.
Once a coin is placed [it’s actually slide] on the anvil, a
collar [metal ring] holds the coin in its position. The hammer now comes down
with pressure and strikes the planchet with the desired force. This strike at
high pressure causes the metal to flow and get the desired imprint of the coin.
The collar also prevents the planchet to expand beyond the desired diameter of
the coin. As a special feature, some collar will have reeds so that the
planchets stuck will have a reeded edge. After the strike, the hammer retreats,
the anvil movers slightly upwards vertically on the cam shaft. The feeding fingers
then slide this coin out of the striking chamber, move still further and
deposit another planchet. The anvil retracts back to its original position, the
fingers slide back, and the hammer comes down again strikes another coin. This
process continues.
Current machines are capable of minting around 120 coins per
minute. There are some machines that can strike coins at even faster rates.
There are striking machines that use one pair of dies or 2 pairs or even 4
pairs. IE in one strike, 4 hammer dies will strike 4 coins; the feeding fingers
feed 4 planchets.
Once the coins are stuck, these are then passed through
quality check. Typically whenever a new die is fitted in the striking chamber,
the initial run is visually inspected to rule out any errors on dies.
Counting and Bagging
The coins that come out of the striking chamber move on a conveyor
belt into the counting and bagging area. The coins are then counted by machines
and bagged accordingly. The bagged coins are also weighed to verify that the
coins are of correct quantity and weight. As a quality control, the bagged
coins are put into gunny bags with the official of the mint signing the bag.
The bags are then dispatched to the central bank for further distribution to
public.
Proof Coins Manufacturing
The manufacturing of the Proof Coins is similar to the
process described above. There are some additional steps:
Die Metal
Extremely high grade die blanks are used to create dies for
Proof coins.
Die Selection & Preparation:
The dies are carefully chosen from the multiple working dies
that are created; more so for lower denomination coins. Each die is then hand
polished to give it perfect mirror finish. At times 3-4 rounds of polishing
with different diamond paste is done so that the coin struck with this gets a
highly mirror finish. The mirror finish was earlier followed, and today the
finish is Frost finish, here after polishing the die, the entire die is covered
with soft tape, only the key design is then exposed. The exposed area is sand
blasted, so that when the coin is struck, the background gives a mirror finish
and the design gives a frost finish.
Blanks Selection & prepration:
The blanks used for striking proof are polished so that they
give a smooth and good finish.
Strikes:
The proof coins are hand struck, at times more than 3-4
times so as to give a sharp edge to the design. The pair of dies used is fresh
pair of dies and hence the wear is not shown on the coins. After every 20-30
strikes, the dies are cleaned. Further the dies are continuously inspected for
wear, minor imperfections are removed by polishing the die again as required.
Inspection:
Each coin is manually inspected for imperfections before
packaging in the desired package and shipping.
Typically the rate of coin production for proof is less than
500 per day compared to the around 100 coins per minute in a fully automated circulation
coin strike.
Notes:
Typically each die lasts for around 500,000 strikes. The
capacity of the press with a single die set is more than 120 coins per minute.
So even if only one set of dies are used to strike a coin and the press is
operated for just a day, we have around 60,000 coins. The number of
commemorative coin collectors in India are estimated to be around 15,000 to
20,000 serious collectors and around 15,000 to 20,000 occasional collectors.
Logically it should then follow that every collector should be able to lay his
hands on any of the coins that he / she wish to collect. But we all know at
some point in time the serious collectors has to buy them from dealers.
[1]
Tempering is a process where by steel is heat to extreme temperatures and
cooled quickly in oil or coolant.
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