The Denver Mint is in the middle of automating components of its
die manufacturing operations to improve overall die quality, extend
die life and cut die production costs.
The facility has already automated most of the production of its
circulating coin denominations, and has plans to turn a production
line previously dedicated to dollar production to cent production.
Randy Johnson, division chief for die manufacturing at the Denver
Mint, told Coin World during a May 9 floor tour of the
facility that installation of a robotically controlled lathe operation
for use in die production will trim the time for one stage of the
process by 75 percent.
Currently, under the older system, a three-lathe cell operation
takes 2.86 days to complete three grinding and polishing operations in
the initial phases of die production. Converting to a robotic
operation will drop the time to 0.61 day.
The switch will also reduce the number of Mint employees needed to
complete the three tasks to just one person, who would use a
computer-controlled program to operate the centrally positioned
robotic arm to rotate a working hub — a right-reading, positive matrix
— between three pieces of machinery. One of those pieces of machinery
has already been installed, with the other two soon to follow.
Employees currently involved in die production who will no longer
be needed there after the robotics operation is in place will be
reassigned within the Denver Mint, according to Barbara E. Hurtgam,
Denver Mint deputy plant manager.
The general die production process will continue to be followed
when the robotic system is in operation — each step now performed will
continue to be performed. What will change is how the die steel is
handled at certain stages.
Production of hubs and dies begins with selection of a rod of die
steel having the proper diameter for the denomination (hubs bear
right-facing, positive designs, with dies bearing mirror image,
incused designs; the production techniques described are identical
whether a die or a hub is being prepared). A length of the rod is cut
slightly longer than needed to serve as the die body. The shortened
piece of die steel is placed into a hydraulic press and, under
pressure, one end is formed into a mirror-finish cone (the cone shape
facilitates metal flow when the die is hubbed, the step where it is
given its design). From there, the die steel with its conical face is
moved to a hubbing press, where the conical face is impressed with the
coin design by a hub or a die (hubs are impressed into the conical die
steel to create dies, while dies are used to create hubs).
For the next stage of the process, currently, the partially
completed die is manually moved to each of three separate stations to
(1) grind the body of the working hub to the proper dimensions to fit
within a coining press, (2) remove extra burrs or protruding metal,
and (3) clean up the tool produced. This three-part stage of the
process will be affected by the installation of the robotic system;
the partially completed hub or die will be transferred from one
station to the next robotically, while currently an individual human
operates each station.
After these three steps are completed, the die steel undergoes
(and will continue to undergo) separate additional steps for light
sandblasting and heat treatment.
Johnson said the efficiencies in the time saved by the robotic
system will allow dies to be made to order when needed, rather than
made for inventory, and will also save money. The savings will include
a nearly 50 percent reduction in die scrap, Johnson said.
Johnson said the Denver Mint has also been experimenting with die
steels to help improve die life. A 52/100 die steel, standard for the
industry, had historically been used on obverse and reverse dies for
circulating coins from cent through half dollar, Johnson said. The
Denver Mint now uses an A-9 die steel for circulating and Uncirculated
Mint set coin dies, resulting in a 40 percent improvement in die life,
Johnson said die failure is often the result of metal fatigue,
resulting in cracks and chips.
Current die life is:
➤ For obverse and reverse cent dies, 300,000 strikes each.
➤ For 5-cent coins, 800,000 coins per obverse or reverse die.
➤ For dimes, 400,000 coins per obverse or reverse die.
➤ For quarter dollars, an average of 300,000 coins per obverse and
reverse die. Die life varies for each America the Beautiful quarter
➤ For Kennedy half dollars, approximately 700,000 coins per die
Johnson said some production problems have been experienced with
the production of Lincoln, Union Shield cents. Any problems
encountered with metal flow or striking problems with a design are
reported to the engraving staff at the Philadelphia Mint so those
problems can be rectified for the next year’s production, he said.
Gary Hall, chief of the Denver Mint’s quality assurance division,
said the Denver Mint is currently having production problems for
several coins that will be produced only in 2012, including the 2012
Acadia National Park quarter dollar for Maine. Average die life for
that coin is at only 100,000 pieces per die.
The facility also encounters striking problems with Presidential
dollars where the portrait depicts the chief executive with a full
head of hair and a full beard, Hall said.
The Denver Mint has also incorporated improvements to its edge
lettering operations for Presidential and Native American dollars.
Instead of just one arced groove in the edge segment containing
the edge lettering (E PLURIBUS UNUM, the date, Mint mark and 13
stars), the segments now have two arced grooves, only one of which is
placed into operation at any one time.
If any of the raised lettering in the edge segment groove wears or
is otherwise damaged, that area is Xed out to identify the problem
area, and the edge segment is flipped over so the second arced groove
can be used.
Struck coins are transported to the edge-lettering equipment
through a conveyor system integrated with the coinage presses.
The conveyor drops the struck coins into a centrifugal bowl, which
then feeds the coins into the edge-lettering equipment at the rate of
a 1,000 coins or more per minute. The coins are rotating
counterclockwise as they are fed into the edge-lettering equipment,
which contains a vertical impeller wheel, also rotating
counterclockwise, on one side and the edge segment with the edge
lettering on the other. The impeller wheel grips the plain edge of
each struck coin and propels it along the grooved segment to form the
inscription, then out the end of the edge segment into a steel trough.
The edge segment is positioned so that enough space exists between
the segment and impeller wheel to allow for the proper imparting of
the incuse edge lettering and any expansion of the dollar during the
process. Periodic checks are made to ensure the dollar coins remain
their proper thickness.
Hall said the edge-lettering application is checked every 10 to 15
minutes with inspection of coins to assure the edge lettering is
imparted to the proper depth (is not too deep or too shallow) and is
not missing letters. If movement occurs in the edge segment and the
spacing becomes too loose or too tight, a die setter will readjust the
edge segment for proper alignment, Hall said.
Should any dollar coins being fed into the edge-lettering
equipment jam, the jam can be cleared by reversing the spin direction
of the centrifugal bowl on the equipment before trying to run any more
coins through the system.
Hall said edge segments are capable of placing the edge lettering
on more than 1 million coins before being retired. ■