240 COLD OPERATIONS OF THE SQUEEZING GROUP
For practical cases, however, with a free-flow relief all around, it
is well to figure, for steel, 100 tons per square inch and higher on the
total final area squeezed, and for copper around 75 tons per square inch.
Under the third classification in the squeezing group of operations,
including coining, stamping and embossing, comes a variety of work in
which the metal is required to flow comparatively, little but is subjected
to extremely high pressures to bring out sharp designs or lines or to
obtain a very accurate surface. On most of this work the metal is
either completely contained in a closed die, or practically so, with the
result that there is no outlet or relief for the flow of surplus metal.
Under such conditions, if care is lacking so that oversized stock is used
or adjustments are carelessly made, the rigidity of the presses may
Greece 500 B.C. Egypt 300 H.C. Corinth 300 Tt.C. Rome 27 B.C.
FIG. 210.—Upper row: Four ancient coins contribute to the story of early craftsman-
ship and mechanical progress. Lower row: Four excellent examples of commemo-
rative medals from the seventeenth and eighteenth centuries. (Courtesy of Dr.
Frederic R. Sanborn)
build the pressure up to several hundred tons per square inch with
serious consequences to the equipment.
Coins.—In the upper row of Fig. 210 are shown four ancient coins
from the collection of Dr. Frederic R. Sanborn. These were " struck "
two thousand years and more ago. In many cases the designs are
excellent and the tools were well made. The rough outline, however,
reflects the crudeness of the methods employed. The lower die was
secured on an anvil. A pellet of precious metal was placed upon it.
The upper die, hinged to it or held over it by one man, was struck with a
sledge by another man. The results obtained would seem to confirm
this story. Coins became thinner as time went by, but the irregular
outline remained until about the seventeenth century.
The lower row (Fig. 210) shows an interesting group of commemora-
tive medals, the Cromwell medal, 1655, and the Charles II medal, 1670,
both struck in England; the American Liberty medal, 1783, and the
Napoleonic Egyptian Campaign medal, 1798, both struck at the French
Mint. These all have the thorough finish and fine detail of modern
coins but were undoubtedly made in crude presses, for the first knuckle-
joint coining press, Fig. 211, was not invented, to our knowledge, until
1833. It was brought out by a Frenchman, M. Thonnclier. One of
these machines was included in the original installation of equipment
for the United States Mint in 1836.
The mint at Philadelphia still has in its possession a simple hand-
operated screw press antedating this installation and said to be the
FIG. 211.—The original knuckle-joint
press invented in 1833 for the French
Fro. 212.—One of the early power
blanking presses developed for cutting
original machine used at the time coins were first made in this country.
It is a small machine mounted on a massive wooden table and was called
upon to do both the blanking and coining operations. With power
equipment came the Grecian-columned Planchett Cutting Press shown
in Fig. 212. It was superseded later in the last century by small,
solid-frame, straight-sided presses of the more modern over-drive type.
An even more recent change has been the addition of the Planchett
Upsetting Machine, Fig. 213. This machine automatically rolls the
coin around edgewise, upsetting and thickening the edge to reduce a
certain amount of the strain on the coining dies.
The coining operation itself is now performed in knuckle-joint
242 COLD OPERATIONS OF THE SQUEEZING GROUP
presses of the type illustrated in Fig. 214. The coin feed is a develop-
ment of that on M. Thonnelier's machine and is probably the earliest
type of mechanical feed. As now constructed, it includes the tube
filled with blanks and a pair of fingers to take the blanks to the die and
the finished coins out of the die. A crank-actuated bottom knockoutis timed to
lift the coin to the surface of the die as the fingers close.
A report issued sometime ago by the Mint, Table
XX, showed that the ac-tual pressures required to
bring up clear impressions on United States gold, silver
and copper coins varied between 85 and 125 tons
per square inch. These fig-ures are considerably above
the freeflow yield points of the metals as rolled, yet the
presses actually used have rated capacities two or three
times what the experimental figures would indicate. This
is good conservative practice, for in using closed dies a
double blank or a careless set-up will raise the pressure
tremendously in a rigid machine. And an extra-rigid machine contrib-
utes a great deal to the tool life.