Errors in the rolling mill: Collector's Clearinghouse
- Published: Dec 11, 2015, 4 AM
A tapered planchet is one that thins out at one pole. Appearing in all denominations, this type of planchet error is most commonly found among copper-alloy Lincoln cents.
Struck sometime in the 1950s and weighing 2.4 grams, the illustrated Lincoln cent displays a typical tapered planchet error in the northeast quadrant. For reasons that are unclear, the 1950s produced an unprecedented number of such errors.
A number of hypotheses have been floated to explain the presence of tapered planchets. All take place in the rolling mill, where strips of coin metal are squeezed between a pair of rollers until they reach the proper thickness for blanking.
One common scenario suggests that tapered planchets are produced by a tilted roller. While possible, this scenario runs up against a significant problem. If the roller is tilted down at one end, it must also be tilted up at the other end. Thus, a tilted roller should produce as many overweight planchets as underweight planchets. However, coins struck on overweight planchets are much rarer than tapered planchet errors. This is certainly true of cents struck in the 1950s, where there is no uptick in the number of heavy specimens.
It has also been suggested that some tapered planchets might reflect a temporary and unintended reduction in the spacing between the rollers. This would leave a “dip” in the strip. Tapered planchets would be punched out from the sloping sides of the dip, while conventional rolled-thin planchets would be derived from the floor of the dip.
Some evidence, described below, suggests that tapered planchets are derived from the leading or trailing end of the strip. It’s possible that the ends of the strip are sometimes thinned for easier insertion into the rolling mill. It’s also possible that the gap between the rollers shrinks as resistance to their pressure drops at the end of the strip. Why the gap should diminish is uncertain, but our final hypothesis incorporates a possible mechanism.
The thickness of planchet strip is presumably determined by a fixed gap between the rollers. However, should the spacing mechanism become loose or disabled, then strip thickness will be determined by a combination of roller pressure, strip malleability, and the speed of progression through the rolling mill. Under these conditions, any slowdown in progression would result in an abnormally thin segment. Similarly any reduction in resistance to roller pressure (as at the freely extruding ends of the strip) will also result in a thinner segment.
Clues to the origin of tapered planchets may reside in certain co-occurring of planchet errors.
Tapered planchet errors often co-occur with rolled-thin planchet errors. In other words, all parts of the coin are thinner than normal, not just the taper. Shown here is a 1953 Lincoln cent with a combination rolled-thin/tapered planchet error. The coin only weighs 2.18 grams, an excessive weight loss for a simple tapered planchet error. Predictably, most of the coin’s edge appears thinner than normal. This error combination isn’t particularly enlightening, as it’s consistent with a tilted roller, a dip in the middle of the strip, or decreased roller spacing at the end of the strip.
Among copper-alloy cents, tapered planchet errors also occur rather frequently with straight clips. A 2.12-gram 1955 cent presents such a combination. This error combination places the origin of at least some tapered planchet errors at the leading edge, the trailing edge, or the side of the strip. But once again, the evidence is not sufficient to tease apart many of the competing scenarios described above. It’s also unclear why the pairing of a straight clip with a tapered planchet is so seldom seen in other denominations.
Among copper-nickel clad dimes and quarter dollars, tapered planchet errors are almost always associated with partial or full missing clad errors on examples that are heavier than predicted. For reasons discussed in the April 27, 2015, column, this association places the origin of these tapered planchets at the leading or trailing end of the strip.
Our last specimen is a 1.84-gram 1955-D cent struck on a rolled-thin planchet with a taper that terminates in a straight clip. This triple-combination error is likely to have come from either end of the strip.
While the compound tapered planchet errors described above are most likely derived from the ends of the strip, I can’t rule out other explanations for simple cases.
I wouldn’t be surprised if there were several causes for these.
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