By Mike Diamond for CoinWeek…
Weak strikes on planchets of normal thickness are primarily the result of two proximate causes. The most frequent is an increase in minimum die clearance, which is the distance that separates the dies at the lowest point of the hammer die’s downstroke when no planchet is present. When set correctly, this clearance is considerably less than the thickness of a planchet, allowing the coin’s design to strike up fully. The absence of a planchet and the presence of a die gap prevent the dies from clashing. When the minimum die clearance approximates the thickness of a planchet, the design will be weakly struck. If it exceeds the thickness of the planchet, no design will be left on the planchet, and it will be ejected from the press as an unstruck planchet, indistinguishable from one that bypassed the press.
The other proximate cause of a weak strike is inadequate ram pressure. This is the amount of tonnage applied to a planchet of normal thickness when die clearance is also normal.
In the case of a simple, centered weak strike, it is often impossible to distinguish between these two causes. However, when a weak strike is combined with another striking error, or when the weakness develops within a series of coins struck by the same die pair, it is often possible to separate the two.
Establishing the ultimate cause behind a weak strike is essentially impossible unless you were there to witness the striking. It can be due to a cracked, bent, or weakened press frame, a fragmented die shaft, a pile-up in an adjacent striking chamber, dies falling out of adjustment, a jam in the press’s internal mechanisms, a mis-timed or broken cam, a mis-timed hammer die stroke, a loose, broken, or bent mechanical linkage, or an electrical power interruption. There is little evidence to suggest that weak strikes that enter the marketplace are the result of press adjustments made before or during a press run, or that they occur when a press is starting up or shutting down. Therefore, the use of terms like “die adjustment strike,” “die trial,” “test strike,” and “set-up piece” is unwarranted.
The image provided shows a simple, centered weak strike on a Roosevelt Dime. Despite having much lower mintages than Lincoln Cents, I’ve seen more weakly-struck dimes than any other denomination. This makes perfect sense from the standpoint of weak strikes as spontaneous mechanical errors. With such a thin planchet, there is less room for error, as so little vertical distance separates normal minimum die clearance from excessive minimum die clearance. Thicker denominations, like Jefferson Nickels, have a more generous margin of error, which partly accounts for the smaller number of weak strikes in that denomination.
Our next specimen, a 5-cent coin, also appears to show a simple, centered weak strike. However, a closer look reveals clash marks on both faces. This indicates that at one point, minimum die clearance was zero (or less than zero). Later, the minimum die clearance expanded to approximate the thickness of the planchet. The change in die clearance may have been gradual or instantaneous and was presumably spontaneous.
Our next specimen is a double-struck dime with one very weak strike and one strong strike. I do not believe it is a tandem (saddle) strike because the distance between the two strikes is too small, and the head-to-head orientation of the two obverse designs is not quite correct. The larger (and weaker) of the two strikes consists of a faint outline of Roosevelt’s head on the obverse and the letters “PLU” of PLURIBUS on the coin’s proto-rim on the reverse. The obverse design of the smaller strike is well-struck. The reverse face is a first-strike brockage with the top of Roosevelt’s head and the “Y” of LIBERTY. For the larger strike, it’s clear that the minimum die clearance matched the thickness of the dime planchet. We can’t be sure that die clearance was normal during the smaller strike, because any excess space would have been taken up by the intrusive dime responsible for the brockage. The presence of that dime, and the resulting double thickness, would have ensured a well-struck obverse design even if die clearance had been excessive.
Our next specimen, a Lincoln Cent from the early 2000s, has an unquestionable saddle strike. It was struck at least four times. The first strike is a centered weak strike. Afterward, a faulty ejection left the cent straddling the gap between two adjacent striking chambers. Two closely-spaced, very weak strikes left portions of the word “TRUST” at 7 o’clock on the obverse face. Although the reverse face is also die-struck, only one set of letters (“NT”) is present. The outermost obverse strike may be the result of a misaligned third downstroke by that striking chamber’s hammer die. A larger off-center strike at 11 o’clock is uniface (struck against an underlying planchet). I suspect die clearance was also excessive here, since adjacent die pairs in a dual or quad press are connected to the same mechanical linkage and thus operate in synchrony. As with the previous coin, the intrusive disc took up the excess space between the dies, allowing a strong design to form on the opposite face. Although a third downstroke should have taken place in this striking chamber, there’s no evidence of an additional strike at this pole. It’s possible that the third downstroke erased the initial design or that the strikes are too closely spaced to be distinguished. It’s also possible that the hammer die failed to contact this area on the third downstroke. This could be the result of the underlying coin being ejected before the third downstroke. The remaining struck tab would have been too thin for the dies to make contact, given the likely persistence of an excessive die gap.
Rapid changes in minimum die clearance are well-documented by the presence of double-struck coins in which all strikes are unobstructed and a weak first strike is followed by a strong second strike, or vice versa. Fluctuations in minimum die clearance are beautifully illustrated by the triple-struck Washington Quarter shown here. The first strike was normal in all respects. The second strike was about 40% off-center and very weak. The third strike was about 70% off-center and exceptionally strong. All strikes were die-struck on both faces.
Fluctuations in die spacing can be much more dramatic, as when normal spacing increases to nearly twice the thickness of a planchet in the interval between one strike and the next. The illustrated 1969-D Jefferson Nickel received a slightly weak first strike, indicating that the minimum die clearance was a bit smaller than the thickness of the planchet. Die clearance vastly increased during a second strike that was delivered about 30% off-center. This strike was exceptionally weak, despite the presence of a previously-struck 5-cent coin obstructing part of the reverse face. This underlying coin generated a weak brockage of the obverse design on the reverse face. Die-struck, second-strike obverse details appear only on the obverse face and only where the obverse die and the intrusive coin lie across from each other in vertical space. Elsewhere, there was no die contact during the second strike. In this area, we have a so-called “invisible strike.” It’s clear that, during the second strike, the dies were barely closer than the thickness of two stacked Jefferson Nickels.
Invisible first and second strikes are well-documented among all denominations. In an invisible strike, the obstructing element can be a planchet, a coin, or a foreign object. Shown here is a Jefferson Nickel with a properly centered but largely invisible strike in association with a struck-through error on the obverse. A roughly circular or spherical object was interposed between the obverse die and the planchet when this nickel was struck. Die spacing was only slightly less than the combined thickness of the 5-cent planchet and the overlying object. As a result, the small island of die-struck design on the reverse face, which lies opposite the struck-through error, is weakly struck. I have seen another 5-cent coin identical to this one. It suggests that the foreign object attached itself to the hammer die for at least two strikes and that the excessive die clearance remained stable during this time.
Multiple, independent lines of evidence suggest that spontaneous equipment malfunctions cause the vast majority of weak strikes. These malfunctions are often unstable, short-lived, and self-correcting. Weak strikes have been found in association with nearly every type of striking error—off-center strikes, double and multi-strikes, saddle strikes, uniface strikes, horizontal and vertical misalignments, struck-through errors, brockages, indents, capped die strikes, and so forth. Even so, I don’t expect any amount of evidence will kill the zombie idea that weak strikes are rare escapees from the Mint and that they represent test strikes or coins generated during the start-up or shut-down of a press. Grading services, auction houses, and collectors have too much invested in this disproven notion to abandon it.
Really interesting, I never knew about this!