Die alignment errors can involve all three axes and planes that
define our three spatial dimensions. In other words, dies can rotate,
tilt, pivot, or slide.
Significant die tilt is a problem largely confined to the hammer
die. After all, any tendency for the anvil die to tilt will be
resisted by the surrounding collar.
A loose hammer die could conceivably tilt within its recess, but
that would presumably be a very unstable situation, with the direction
and severity of the tilt varying considerably from one strike to
another. Consistency in the direction and severity of die tilt is, on
the other hand, indicative of a tilted die assembly.
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Dynamic die rotation errors involving significant rotation are
undoubtedly caused by a die spinning in its recess. It’s hard to
conceive of an entire die assembly rotating to this degree. Among
dynamic rotated die errors in which the identity of the spinning die
can be established, it’s almost always the hammer die. It’s not clear
why this is the case. Perhaps the hammer die isn’t mounted as securely
as the anvil die.
Should a situation develop in which a spinning hammer die is housed
within a tilted die assembly, one would predict that the two movements
(tilt and spin) would be independent. This prediction is borne out in
a sample of nine 2014 India 2-rupee coins struck by the same die pair.
The abnormal strike is most easily interpreted as having been caused
by a rotating, tilted hammer (reverse) die. This production run is
easily identified by the presence of a massive die crack on the
reverse face. All of the coins are broadstruck (struck out-of-collar),
with no trace of collar contact. In this series (2011 to present), the
obverse die or the reverse die can serve as the hammer die, although
the former setup is more common.
Normally struck in medal rotation, all but one of the coins show a
rotated die error. The extent of rotation varies widely. Degrees of
rotation are as follows: zero, 180 degrees, 90 degrees clockwise, 120
degrees CW, 132 degrees CW, 142 degrees CW, 65 degrees
counterclockwise, 128 degrees CCW, 123 degrees CCW. I cannot determine
if the die rotated in a consistent direction or periodically switched directions.
In contrast to the highly variable extent of reverse rotation, the
direction of die tilt is relatively constrained, as documented by
patterns of weakness observable on the obverse face. On the obverse
face, the weakly-struck pole (which lay opposite the elevated pole of
the hammer die) is located roughly between 4:00 and 8:00. More
specifically, it coincides with the Ashoka lion’s base and the area
beneath it. In some examples the center of the zone of weakness lies
directly below the statue’s base; in other examples it’s located to
the left or right of the statue’s base. This relative consistency in
the location of the weakly struck area suggests that the anvil
(obverse) die was fixed, while the spinning hammer (reverse) die was
loosely held by a hammer die assembly that was tilted in the same
general direction during each strike. The modest variability seen in
the location of the weakly-struck pole suggests that there were some
fluctuations in the hammer die assembly’s direction of tilt.
The degree of die tilt (and possibly the striking pressure) varies
slightly between examples, so that design elements at the
weakly-struck pole range from faintly visible to completely absent.