Ken Potter's Educational Series ...
Ken Potter's Educational Series ...
Selected
Causes of Weak
Strikes On Coins
Guest Article by Sean Moffatt of the Hoffman Mint
June 19, 2001
Photo by Ken Potter
There are several reasons for what are known as weak strikes. The first, and most common cause, is the obvious die adjustment strike (DAS). At Hoffman Mint, we have a standard procedure when setting up all of our presses to gradually increase pressure making only one coin per pressure setting until we have the desired piece.
In contrast, older presses in the 1800's did not have very good cycle control. When the press set up people gradually increased the pressure they could not engage the press for one cycle only, the press would strike 2-3 times before it stopped. Many times the set up person would adjust the pressure in-between strikes while the press was running.
In regard to some of the modern Schuler presses, there are times that they never adjust the pressure settings. The presses were designated for a certain coin size and since every set of dies is exactly the same, dimensionally speaking, there is no need to adjust the pressure settings. One of my vendors who also services equipment at the US Mints said that on occasion the ram pressure adjustment wedges would become stuck from not being moved. So die adjustment strikes are probably the main cause for what are known as weak strikes (on these particular presses).
Second, there is a "phenomenon" which is kind of the opposite of a DAS. Almost all modern coin presses have a ram pressure locking device. This is most commonly a large screw that pulls the ram pressure adjustment assemblies together and locks them in a set position. There are times that an operator will forget to tighten these ram locks. Due to the forces involved I have seen coining pressure settings "un-adjust" themselves. Most times this happens during the first several hundred pieces of a run. I have seen a coin go from fully struck to having nothing but traces of denticles around the outside diameter in about 50 strikes.
Other times an operator fails to lock the ram down tight enough to last through a run and the lock comes loose at some random point in the run and the pressure eventually backs off until there is only traces of an image left.
A mechanical failure in some locking devices can cause the ram settings to come loose also, which will have the same effect as previously described.
There is no way to differentiate between these weak strikes and DAS pieces.
Third, weak strikes can be the result of "gutless" presses or trying to run too large a coin on too small a press. I have inspected several presses that somebody thought they could strike 1.5" coins on with 110 - 150 tons, only to break the crank shaft or crack the press frame in the process.
A prime numismatic example of "gutless" presses can be seen on the Colombian coinage of the 1950's and 1960's. Show me a fully struck 50 centavos from this era and you will have a real keeper. These coins can all be called weak strikes, even though none of them may be DAS coins. The Colombians did not have enough presses with adequate tonnage available for these larger size coins, so instead of killing their presses they ran the coins with weak strikes.
There have been several occasions at Hoffman Mint that we have tried to fully strike a coin on a 200 ton press only to find that the press did not have enough guts, so we had to move the run to a slower 360 ton press of which the coin was able to be fully struck.
A fourth reason for weak strikes is not enough material in the coin blank to fill the designs. There are limits as to how much movement you can get out of a coin blank’s metal when striking. Coins run on blanks that are too thin will not fill the images in the dies. We have certain coin sizes that we offer in two thicknesses just for this reason. Some of our standard size tokens that are used in coin mechanisms are also popular sizes for advertising specialties, game pieces, commemoratives, etc. To keep costs down and to comply with token industry standards for tokens intended for mechanical devices, the tokens are struck on thin material with lower relief designs. These thinner blanks when used for some of the nicer design "specialty" tokens do not have enough material to completely fill the designs and if struck will appear as a weak strike. You could hit a piece like this with five times the tonnage and still not move enough material to fill the dies. By using thicker material we can reduce our tonnage AND get a fully struck up piece.
For legal tender coinage most times you can not just go to a thicker blank. Their coinage sizes and weights are defined by law, and the designs are picked by artisans and the likes. A certain coin size will run fine until there is a design change then all of a sudden they can not get the entire image to fill in. If there is time they can rework the sculpts and cut other sets of dies (Cuba 40 centavos KM-14.1, .2, &.3) or they can just run them missing some detail. I recently purchased a BU Egyptian 1 pound coin (Suez Canal) that exhibits this.
Another thing to consider regarding this is that the people who pick the coin designs do not have a clue on what it takes to get some of their images fully struck. This is especially true when the deepest part of one side of a coin’s design is directly opposite from the deepest part of the design on the other side of the coin.
I recently purchased a Peruvian 1 Peseta in XF (KM-200.1) that exhibits this to an extent. The word Una on the reverse is almost nonexistent while the rest of the reverse is fully struck. Exactly opposite the word Una on the obverse is the highest point of detail in the woman’s head which is also missing detail because these are rather thin coins and the relief was too deep for the metal to fill.
A fifth cause of weak strikes are sunken dies. Pile-ups (brockages) can cause the center of the coin dies to "sink". When the dies sink it is like adding extra relief to fill in. Imagine a set of coin dies. the fields are flat (not domed). Each coin die has an image depth of say .005". The coin blank only has to fill a total of .010" in the design only, the rest of the blank is compressed by flat field. Now say there is a pile-up and the dies "sink". Typically if the pile-up was reasonably centered the dies could exhibit a concave design like a cereal bowl. For sake of argument lets say the dies sunk .005" per die, which is not uncommon. Now imagine this same set of dies facing each other. Not only is there .005" worth of image to fill on each die, now there is an extra .005" per die to fill before the fields are even touched. This increases the total depth of the reliefs to .020". Many times the centers (or highest relief point) of coins that are run with sunken dies will not fill in, looking almost exactly like a weak strike. I have seen people’s entire faces disappear from a coin after a pileup that resulted in sunken dies. I have a beautiful Indian 1 rupee from the 30's that shows this effect (as well as some matching die cracks from the pile up). There are ways sometimes to repair sunken dies, but most times a new set of dies needs to be made.
A sixth cause of weak strikes is soft dies. This was a major problem in the early 1900's Chinese coinages, especially in some of the more remote provincial mints. Soft dies will at best resemble sunk dies as described before. At worst the image will wear almost entirely off (This does give a different look than the weak strikes we are discussing and will exhibit full edge markings or reeding on applicable coins). There are many of the dollar sized base metal coins from China that were never struck up all the way and exhibit weak strike characteristics. Many times it was because of a combination of soft dies, gutless presses, and not thick enough blanks. Try to find a fully struck 20 cash coin and you will see what I mean.
The seventh and final cause of weak strikes that I can think of is a loss of power due to blown breakers, transformers, etc. In this event the dies may just kiss the blank and a weak strike will result.
Sean Moffatt has installed coinage presses, maintained them, worked as a die setter and trained personnel in coining operations. He has been in the minting business for 31 years with the past 14 at Hoffman Mint where he is currently the operations manager.
Ken Potter
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