Dynamic die rotation errors sometimes 'freeze' in place
- Published: Jun 29, 2012, 8 PM
Ever since the U.S. Mint started producing coins in 1792, our domestic issues have been struck with “coin rotation.” In other words, the obverse face points north while the reverse face points south.
Significant deviations from coin rotation are considered errors. A perfect 180-degree rotation is called “medal rotation” since medals are minted with both faces pointing north. Medal rotation is normal for some foreign issues, especially those produced by the United Kingdom and its former colonies.
Rotated die errors can affect the hammer die (the die that delivers the impact) or the anvil die (the die that absorbs the impact). In most cases, it is impossible to determine which die rotated. However, if combined with a second error (e.g., a horizontal misalignment, a vertical misalignment, or a “one-sided” double strike), the malpositioned die can sometimes be identified, and it is almost always the hammer die.
Rotated die errors arise in three basic ways:
(1) A die is incorrectly installed.
(2) An improperly positioned “flat” is ground into the die shank. A flat is a guide mark that helps position the die. If the flat is ground in the wrong spot relative to the design on the die face, it may lead to incorrect installation.
(3) A die can work its way loose and spin on its vertical axis.
The first two scenarios will result in a stable (fixed, static) rotated die error. The third scenario will result in a dynamic rotated die.
A stable rotated die error is one in which the die (and each coin it strikes) maintains the same degree of rotation throughout the entire press run. A dynamic rotated die error is one in which the die slowly spins or oscillates on its vertical axis so that most of the coins it strikes show a different amount of rotation.
The situation is actually a bit more complex than what I’ve portrayed. Some dynamic rotated die errors show an unusually narrow range of positions or are dominated by a single position. It seems that sometimes a rotating die will “freeze.”
Occupying this nebulous category of semi-stable rotations is an extensive series of Jefferson 5-cent coins struck in 1966 (see photos and the June 21, 2010, “Collectors’ Clearinghouse”). The vast majority of these coins exhibit a 90-degree clockwise rotation, as shown. However, a small number of coins struck by this die pair show lesser degrees of rotation. Two members of the latter cohort are shown here, one with a 20-degree clockwise rotation and one with a 40-degree clockwise rotation.
It is unclear why the die froze in a 90-degree position for so many strikes. Perhaps it was installed in this position and then worked its way loose. Perhaps it spun for awhile and then, when the press operator noticed the problem, it was tightened down in the wrong position. Or perhaps it simply became jammed in this position. It’s also unclear why the full range of positions only occupies a quarter circle.
Some rotations long thought to be stable have instead been found to be semi-stable once a larger sample was accumulated. The dies that struck the two illustrated 1994 Lincoln cents produced a large number of examples with a consistent 165-degree counterclockwise rotation (shown in the second column). This is the only position listed for this date in the Rotated Die Coin Census (www.rotateddies.com). I had assumed it was a stable die rotation error and reported it as such in the Collectors’ Clearinghouse column cited earlier. However, I’ve since become aware of a smaller subpopulation with a 170-degree clockwise rotation (shown in the third column). There also seems to be a third, even smaller subpopulation, one characterized by a 180-degree rotation. This third position was reported in a March 10, 2011, thread that may be found on the Lincoln Cent Resource forum (www.lincolncentresource
Once again it is unclear why the die should rotate or oscillate through such a narrow range of positions or why it should settle in two primary positions.
It would seem that any claim of a stable rotated die error should be tempered by the realization that a larger sample might reveal the presence of other positions.
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