Spin and tilt — follow this die’s odd journey

By Mike Diamond

Published: Oct 11, 2016, 8 AM

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In this 2014 India 2-rupee coin, the hammer (reverse) die was rotated 65 degrees counterclockwise relative to the obverse (anvil) die. The hammer die was also tilted up at one pole, leaving the area southeast of the statue’s base unstruck.

Original images courtesy of Mike Diamond

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Here's a full look at the obverse and reverse of the 2014 India 2-rupee coin struck by a reverse die that was rotated 65 degrees counterclockwise relative to the obverse die.

Original images courtesy of Mike Diamond

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In this 2014 India 2-rupee coin, the hammer die was rotated 132 degrees clockwise relative to the anvil die. The elevated pole of the hammer die left a broad swath beneath the statue’s base unstruck.

Original images courtesy of Mike Diamond

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In striking this 2014 India 2-rupee coin, the hammer die was rotated 128 degrees counterclockwise relative to the anvil die. The hammer die’s elevated pole left the area southeast of the statue’s base unstruck.

Original images courtesy of Mike Diamond

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.