The various coining facilities of the United States Mint are factories, whose products happen to be coinage of the realm. Like any other metal-working factory, the U.S. Mint has a variety of presses, engraving and reducing machines and metal-working equipment.
Like any metal product, coins don't "just happen." A number of intricate steps must be taken, from the preparation of the raw metal used in the coins to the striking of the coins. And before the coins can be struck, dies must be produced.
Modern United States coins have their beginnings in the private sector, where a number of companies produce some coinageblanks and planchets and all coils of strip metal the Mint purchases. Blanks and planchets represent the same product at different stages of production: the unstruck, circular pieces of metal that become coins when struck between the dies. The Mint produced its own strip metal as late as Fiscal Year 1982 at the Philadelphia Mint, but the operations were closed officially in Fiscal 1983. The Mint still produces some of its coin blanks and planchets.
In preparing the raw metals used in coining, the coinage metals are assayed, melted and formed into slabs which are then rolled to the proper thickness. For clad coinage, bonding operations are required to bond the two layers of copper-nickel to the core of pure copper. The strip is then coiled and shipped to the Mint for blanking.
Blanks are unfinished planchets that haven't been through all of the processing steps necessary before they can be struck into coins. Once a blank has been through all of the processing steps, it becomes a planchet and is ready to be struck.
Blanks are produced on blanking presses, which are simply punch presses similar to those found in any machine shop. They have a bank of punches (or rams) which travel downward through the strip of coinage metal and into a steel bedplate which has holes corresponding to the punches. The presses punch out blanks each time the punches make their downward cycle. The blanks made at this stage are slightly larger than the finished coins. Because of the shearing action of the punches, the blanks have rough edges. Most of the rough edges (or burrs) are removed during succeeding operations.
The blanks are next passed over sorting screens which are supposed to eliminate all of the defective pieces. Thin and incomplete blanks will fall through the screens. Rejected blanks are remelted.
During the finish rolling and blanking press operations the blanks have again been hardened and must now be softened (heated) to controlled temperatures, approximately 1400 degrees Fahrenheit, changing their crystal structure to a softer state. Planchets are "frozen" into that state by a water quench bath. The annealing process prolongs the life of the coining dies by ensuring well-struck coins with lower striking pressures.
Despite a protective atmosphere, annealing causes some discoloration on the surfaces of the blanks which must be removed. The blanks are tumbled against each other and passed through a chemical bath. Then they are dried by forced hot air. Many of the blanks' next stop is an upsetting mill. (The Mint no longer finds it necessary to upset the rims of 5-cent blanks.)
The upsetting mill consists of a rotating wheel with a groove on its edge. The grooved edge of the wheel fits into a curved section (or shoe) which has a corresponding groove. The distance between the wheel and the shoe gets progressively narrower so that, as the blank is rolled along the groove, a raised rim is formed on both sides of the blank. This raised rim serves several purposes. It sizes and shapes the blank for better feed at the press and it work-hardens the edge to prevent escape of metal between the obverse die and the collar.
The blanks are now planchets and are ready to be struck into coins on the Mint's coining presses.
A new prototype inspection system underwent testing at the Denver Mint in 1996. Planchets are carried along a conveyor belt beneath electronic sensors that reject substandard pieces. Both sides of the planchets are examined. A Coin World staff member examining planchets rejected by the prototype was amazed at how well the machine caught even minor imperfections.
Coining presses are designed for any denomination of coin. Dies and collars are interchangeable and striking pressures are adjustable for the various denominations and metals. A circular piece of hardened steel forms the collar, which acts as the wall of the coining chamber, and one die forms the bottom. The dies impress the various designs and devices on the obverse andreverse for the coin while the collar forms the edge of the coin, flat and smooth on cents and 5-cent coins and reeded on the larger denominations. The collar, which is five-thousandths of an inch larger in diameter than the dies, is mounted on springs which allow slight vertical movement.
Generally, the reverse die is the lower (or anvil) die while the obverse die is the upper (or hammer) die; however, there are exceptions, and on some presses, the dies are mounted horizontally so that they move parallel to the floor. Still, the terms anvil die and hammer die are appropriate.
Planchets are fed by gravity from a basin attached to the press through a cylindrical tube. This tube stacks 20 or so planchets. From this stack the bottom planchet is fed into the press by one of several feed devices.
One device is called the feed fingers: two parallel pieces of metal joined in such a way that they can open and close; on one end of the two pieces is a covered recessed slot and in the center is a hole. A second device is a dial feeder: a circular piece slotted with holes which transport the planchets to the coining chamber and then transport the newly struck coin from the dies.
No matter which feed device is used, the coining process is the same. The anvil die at this point is stationary. As the hammer die moves toward the anvil die it impresses the designs of both sides into the planchet and then completes its cycle by moving upward to its original position. On presses using the dial feeder, the dial remains stationary so that the hole transporting the planchet remains centered over the anvil die, with the hammer die passing through the hole to strike the coin. Now the anvil die starts to move above the collar, actuated by an eccentric cam, raising the struck coin out of the coining chamber. Depending on the feeder system used, one of two things happens.
As the anvil die moves, about the same time the feeder fingers, in a closed position, start to move forward with a planchet lying in the center hole. At this time the anvil die reaches the top of its cycle, the recessed slot (ejection slot) slides over the struck coin, and pushes the coin away from the coining chamber. The feed fingers have completed their forward movement and now the center hole is moving towards the coining chamber. Having imparted movement to the struck coin, that coin continues onward until it hits a shield which deflects it into the struck coin hopper. The feed fingers open, allowing the planchet to fall into the coining chamber. Then the feed fingers reverse direction to return to their original position.
On presses using a dial feeder, the struck coin is pushed back up into the hole that had carried the planchet; the dial rotates, moving the coin away from the coining chamber while the next hole drops a new planchet onto the lower die. The cam action now causes the lower die to move to its stationary position.
Presses fed by dial feeders have sensors that automatically stop the press if a planchet is mispositioned, of the wrong size or incomplete, or is completely missing. The Denver Mint especially has good use of this feature to largely eliminate many of theerror coins that entice collectors.
Frequently, while a press is in operation, the press attendant will pick up a finished coin for inspection to catch some of the remaining varieties and errors that are still produced. The inspector examines the coin under a magnifier and it reveals any defects made in the die during operation. Another attempt is made to prevent improperly shaped coins from reaching circulation by passing them through a second riddler.
Throughout the minting process, computers track such statistics as the productivity of each press operator, any repairs to a coining press, quantities of coins struck per press, plus installation, movement and destruction of the dies.
After the coins have been struck they are ready for a final inspection. After passing the inspection, they are counted automatically by machines, weighed and bagged or boxed. The bags are sewn shut and the boxes shut. The coins now are ready for shipment from the Mint to the Federal Reserve Banks or private money-handling firms like Brinks for distribution.