Archive for the ‘Coin Presses’ Category

EVERY die-struck item – coin or medal – has two important hard and fast rules. I call these “undeniable truths.” They cannot be overlooked at any step of modeling, pattern-making or die-making in the medallic field.

Dies need to strike and withdraw. They must be made to insure that ability to withdraw from the struck piece. Otherwise the struck piece clings to the die. Pressmen call this a “hang-up.”

In a coining press a hang-up with a struck piece attached strikes the next blank that comes into position with two blanks between the two dies. The struck pieces have no design on one side and a mangled surface on the other, what mint error collectors call a “brockage.”

If it continues to hang on to the die and the coining press continues to feed blanks that first struck piece will wrap around the die. Mint error collectors call this “capping” or “cupping.” It is one of the worst situations for a coining press operator to experience.

Even if the die isn’t damaged by all this, it should be rejected anyway. It wasn’t made properly in the first place. It provides a devil of a time for the pressman. Reject that die. Its problem was an improper bevel.

The problem with the die started with the modeling of the design. Two rules govern here – two undeniable truths – no undercuts and proper bevel of all lettering and devices. The two rules are so closely related we discuss them here both at the same time.

An undercut is modeling of relief between the design and its background; the carving of overhang of design relief; a negative slope of relief. Metalworkers call it back draft. Relief sculptors call it under bevel. Everyone calls it undercutting and everyone connected with medal making attempts to avoid it right from the beginning for any die-struck or electroformed reproduction..

[Undercutting is a sculpture technique of full-round sculpture even though it can be attached to its background; it intensifies a contour line or relief by casting a shadow behind the relief. In the medallic field undercut designs can only be reproduced on bas-relief cast plaques, and then only made by rubber or flexible molds.]

For new artists who want to model coins and medals, I recommend hanging a sign above their workbench: “No Undercuts. Bevel All Relief.” Hopefully they would see it every day and burn it into their memory.

All relief requires a proper bevel. The sides of all relief and lettering must have a slight bevel. Each medal making process has its own requirement. It is ideal to model a bevel (also called draft or taper) to accommodate any process used.

Four boundaries must be considered here:

  • Vertical relief from 0° to 2½° is called holding taper. Not only is that taper impossible to cut into a die, or strike, it would be impossible for the die to withdraw from the struck piece after striking.
  • Hand engraved dies can accommodate a 5° to 10° bevel where the dies can strike and the struck piece release from the die.
  • Reduction on the die-engraving pantograph, as the Janvier, requires a minimum 15° bevel. This is required for the shape of the cutting point that mills the design into the face of the die.
  • Reduction by computer generated models, requires a minimum 20° to 25° bevel, draft or taper. This also is determined by the shape of the cutting point that mills the design into the face of the die.

Early in the modeling career of every medallic artist it would be wise to create the sides of all relief and lettering with a minimum 20° bevel and maintain this throughout their career. A 20° bevel on relief or lettering is about the slope of a sharpened wood pencil.

Here’s a tip for all medallic modelers: check the bevel of relief by holding a pencil upright next to your modeled relief. Light will show at the base of the relief if the relief is too steep.

The slope in which the relief rises from the background has the proper bevel of at least 20° it will carry forward in all the die-making steps. Anything less than 5° draft will cause a formed piece to “hang up” or freeze in the die or mold.

While steep vertical relief without any bevel is impossible to strike, relief with minimum bevel creates stress in the dies. The displacement of surface metal of the blank is greater at that point and the wear to the dies is at its maximum (which leads to diecracks and diebreaks).

Humans like the sharp, crisp detail in their medallic designs. Unfortunately they also like sharp rises and falls of the modulated relief to give emphasis to the design. So the designer and modeler must balance the need for a superior design with the requirements of the medallic technology.

As the artist shapes the sides of the relief in his design during modeling he must be aware of this angle or bevel at all time.

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The theory “If man can make it, man can made it better” doesn’t always hold true. Two cases in the history of manufacture of coins and medals – coins particularly – proves that point.

Ever since 1506 when Italian Donato Bramante modified a fruit press to blank sheets of lead, to make seals for Pope Julius II, we have had a mechanical way of striking coins, ultimately using this technique on a screw press.

The mechanical genius Leonardo da Vinci about the same time sketched such a screw press in his notebooks – whether it was ever build in his day is doubtful. Da Vinci theorized a single blow of a screw press could cut out a blank at the same time impressing an image on the front and back of a previously made blank.

It proved successful to do these in separate operations.

Thus coins struck in a press start from rolled out strips, which are then blanked to produce the circles of metal, then struck in a press. The  process is called milled coinage from the milling of the metal to a required thickness before blanking in a rolling mill.

The process is also called cold coining — the blanks are not heated, both blanking and striking is done at room temperature.

Rejected technique number one. Coins have been made since 640 BC. A lump of metal was placed on a lower die fixed to an anvil. The upper die was placed on top of this lump and it was hit with a heavy blow a number of times until it was flattened and took the image of both dies.

Called hammered coinage, the process continued for a long time. Since it was simple, elaborate factories were not needed to make the coins. It was more of a cottage industry where issuing authorities allowed individuals, called moneyers, to strike the coins. They could keep one coin for every 16 they made. The authorities controlled this by supplying the dies, but, obviously, false reporting ran rampant.

It was probably one of those moneyers in Rome who thought he could make his job easier, or perhaps make more coins in quicker time. Could he make a coin quicker by heating the blank?

The answer was NO.  The heated blank did not expand uniformly when impressed by the dies. Metal flows outward from the center of the blank’s surface to the edge. The heated blank flowed unevenly, some areas reaching the edge ahead of adjacent areas. The coins had ragged, saw tooth edges. This result is known as hot tears, unsatisfactory for a disk of metal made into a coin.

The first use of moneyers was in 104 BC. But even after the screw press and other devices for coining became available, the moneyers did not stop. They fought hard to keep their franchise.

By 1553 a gifted mechanic from Augsburg, Eugene Bergeron, brought his coining technology to the Paris Mint and attempted to establish a more modern method of coining, in effect creating the birth of milled  coining. Yet he was driven out of Paris in 1560 by the moneyers who learned his technology would replace their lucrative activity.

This occurred in England as well in 1561. Not until 1662 – a century later – did the moneyers become entirely replaced by a milled coinage with the use of the screw press. In 1641 the screw press was reintroduced, permanently, at the Paris Mint, and in 1662 at the London Mint.

Rejected technique number two.  Two years before Bereron’s trip to Paris, another Augusburg mechanic, Kaspar Goebels, came up with the idea of roller die coinage. Instead of blanking strips of metal first, then striking  the coins, Goebels idea was to impress the images on a strip of metal with a roller die first, then cut out the coins.

The German word for this process was taschenweke. The name of the roller mill specially engraved with the coin images on a roller was called a walzenwerke. Goebels attempts to use his process in Denmark, and later Spain. It had problems of registration, the obverse image had to match the reverse image, also the trimming or cutting out of the image exactly from the strip was also critical. The technique was a failure at both mints.

In 1637 no less than the chief engraver at the Paris Mint, Nicholas Briot, attempts roller die coinage again. It fails again at the Paris Mint. He is dismissed, travels to England and gets the job as chief engraver at the Tower Mint. There he tries a roller die coinage again without success.

Finally at the Edinburgh Mint in Scotland he is named mintmaster. He finally achieves somewhat of a satisfactory roller die coinage, but it only works for a large diameter coin. The process falls into obscurity, never to be attempted again.

Never? Would you believe it was tried again in the 20th century by no less an institution than General Motors!

In 1964 U.S. Treasury officials met with several top GM officials and discussed the cent shortage. Even with three-shift production of cent coins, the Mint could not meet the demand to end a cent shortage.

GM vice president Louis C. Goad – he was head of manufacturing – told Mint officials he could build a press which could produce 10,000 coins per minute.  The Mint took him up on the challenge.

He assembled a group of engineers and mechanics at GM’s training center in Michigan. They build a press and between 1964 and 1968 tried three times, three different ways.

Among several minor problems, the major problem was that the action generated a tremendous heat. In effect, it melted the dies. The process failed again and in 1969 they closed down the experiment.

Thus within the 2,652 years of coin-making technology we have 460 years of successful coin making by cold coining, milled technology. A heritage of blanking a strip of metal to create proper thickness blanks which are then struck in a press one at a time!

The technology works for creating coins. Forget roller die coining. Or heating the blanks.

Resource: Read more on that General Motors experiment: http://usrarecoininvestments.com/coin_articles/gm_roller_press_cent.htm

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This is the third of several reports on the basic information, the basic knowledge, of minting coins and medals. These facts are so important they should be embedded in the repertoire of  everyone associated with the medallic field and, certainly, everyone within the firms which make these. 

Ancient Coinmaking Illustration

Ancient Coinmaking Illustration

BEFORE coin presses, there was man and manpower. Ancient coins were struck by two men, one holding a die on top of a metal blank on top of  second die. The second man wielded a hammer, more like a sledge, for he needed to create a maximum amount of power to drive the top die into the blank and force it into all the cavities of the second die.

The dies even had names. The bottom die was called the anvil die and was usually the obverse of the coin. The upper die was called the pile die, usually the reverse. There were no set rules which should be obverse or reverse. But it was easier to reseat a partially struck piece back on the anvil die for a second blow if needed. Obverses usually had higher relief, with a larger cavity than a reverse, thus easier to place  back in position.

To lessen the need for such great striking force ancient coin makers experimented with heating and softening the metal blank. But this only caused uneven metal flow outward causing sawtooth edges of spikes and voids. The name for this was hot tears.

Hammered coinage.
For 2300 years coins were made by hammer blows on unheated blanks. Two men striking one coin at a time, one wielding the sledge, the other removing a struck piece and placing a fresh blank in place (and perhaps saying a prayer the sledge man hit the pile die and not his fingers!)

This manual process, known as hammered coinage, proceeded under the management of a moneyer; it was the major method of coinmaking from 640 bc until as late as 1662. A moneyer typically kept one of every16 pieces as his pay, as he enjoyed his monopoly. It is no wonder they rejected the introduction of any mechanical means of coin making.

Instead of a man wielding the sledge, why couldn’t the sledge be lifted by a pulley and dropped down a channel to affect the blow? This was the concept famed artist and inventor Lenardo da Vinci (1500) developed when he theorized how sheet metal could be blanked, and the blank be struck. His concept was brilliant; he visualized the same press doing both operations back-to-back. We have no record that da Vinci’s press was ever built, but he made sketches in his notebooks which have survived.

[In the 1950s IBM underwrote the building of da Vinci’s press from his notebook drawings. This press can now be viewed at the Smithsonian Institution in Washington.]

Primitive hammer presses (called klipwerk) were in operation for over a century in Germany and Sweden prior to 1763 when they were illustrated in a German encyclopedia. In Sweden the concept of the hammer was modified to a tilt hammer press. Powered by men or horses on a circular tread mill, the power was transferred by a capstan, this raised an arm with the hammer on the end; it would then trip and fall for the blow, the die slamming into sheet copper.

The copper plates were heated before this striking, and the plate was moved from a center strike to the four corners where the die struck there as well, all in quick time. This is the well documented method of manufacture of Swedish plate money from 1644 to 1776.

Screw presses for striking coins were “invented” in 1506. An Italian, Donato Bramante (1444-1514) modified an existing press (perhaps a fruit or olive press) that year for striking lead seals for Pope Julius II (1503-13). Other early screw presses where built by Nicolo Grosso and used at the Florence Mint for blanking at approximately the same time.  Renaissance artist Benvenuto Cellini employed such a press in 1530, again for Papal seals, but further, he illustrated it in his book on goldsmithing.

The screw press was further developed by Max (or Marx) Schwab in Augsburg, Germany, in 1550, who also improved on the rolling mill to draw plates for preparing metal for blanking. His rolling mill was immediately accepted by goldsmiths in Germany, but Schwab wanted his equipment used for striking coins. He was unsuccessful in selling his equipment to the Venice Mint, where he first offered it, but the French ambassador learned of his improvements and ordered a set of his “engines” for the Paris Mint. These were installed in 1751 and operating early in 1752.

Screw press operation. Forstriking coins, blanks had to be ready in quantity. Three or five men operated a screw press. One was the coin setter, he removed the struck piece and inserted the next blank. Two or four men were spinners, they operated the balance arm, swinging it back and forth. Lead weights were added to the balance arm to give it more strength about 1740.

Screw Press in Operation

Screw Press in Operation

Straps were attached to the ends of the balance arm enabling two spinners to pull on opposite ends sending the  spindle – the stem with large gears – crashing down with great force onto the blank. The die on the end of the spindle, called the pile, forced the blank into the stationary anvil die forming the design on both sides. The coin was struck.

The other two spinners would pull on their straps causing the spindle to rise. With the arm flying back and forth workers gave this press the nickname fly-press. The crew would change after 20 minutes, but they did this for 5 hours at a time, doing other work at the mint for a 10-hour day. The speed of swinging the balance arm was astounding: 20 to  30 times a  minute!  They had to develop a great rhythm!

While the screw press was a major improvement, it took more than a century to replace entrenched moneyers and hammered coinage. Moneyers fought the innovation despite the fact coins could be struck with a screw press in quicker time creating a far more uniform coin with a better rim  by cold coining. The screw press was introduced at the London Mint in 1551, the moneyers revolted, the screw press rejected, and it was not until 1662, 111 years later that it finally was in full use there.

The same thing happened in France. Schwab’s screw press was introduced to strike coins in 1552, but not until 1641 were coins struck in Paris on a production basis. The development of the screw press, delayed for over a century, was then widely accepted at mints around the world. It was first powered by men, and continued so, but some mints adapted it for water power, then for steam power.

Roller press.  Development of the roller mill led to the concept of impressing the rolled strip with the designs first, then blanking afterwards. The idea originated in Germany, but it was Nicholas Briot, who tried it first at the Paris Mint (1606-25), then at the London Mint (1633) and finally at the Edinburg Mint (1635-39).

Briot’s concept was unaccepted until Edinburg where he finally accomplished coinage by roller die (taschenwerke). Despite these early attempts in Germany, Scotland and Spain, this form of coining never surpassed the mill and screw, of rolling the metal, cutting out the blanks first, then striking individual coins.

[General Motors undertook an experiment in the 1960s in cooperation with the U.S. Mint. It built a modern version of such a roller press at one of its experimental laboratories in an attempt to revive this concept. But in operation the press creayed such high temperatures in the steel dies it melted the image on the face of the dies.]

Development of Coining Presses.  What the Industrial Revolution – and Matthew Boulton, Father of Modern Minting, with his Soho Mint – brought to coinmaking was the concept of how to do better repetitive steps, to improve the mechanization of striking coins. A blank had to be brought to a position where it could be impressed with both obverse and reverse dies, then the struck piece had to be ejected, the concept of cold coining. A German, with great mechanical insight, best solved this task.

In 1812 Deitrich Uhlhorn (1764-1837)   of Grevenbroich, Germany, began experimenting with striking. Instead of one die going up and down (as on the screw press) he employed a knuckle-joint to allow one die to retract enough for the piece to be ejected, the next blank inserted, and the continuous action controlled by a flywheel. This was brilliant and it worked.

By 1817 Uhlhorn had perfected this press; he patents it and begins building presses in a factory he establishes. His first sale was to the Berlin Mint, followed by other mints as they learned of his new press. Uhlhorn, and his sons after his death in 1837, were to build more than 200 presses over a span of sixty years.

Knuckle-joint press improvements.  With continued use, other improvements were adapted to Uhlhorn’s knuckle-joint press. The layer-on of placing the blank in position was one of these improve- ments, as was the feeding mechanism. A Frenchman in 1833 in Paris, Eugene Thonnelier (dates unknown) was to do more to improve Uhlhorn’s press than anyone. But he did not manufacture presses, he had no factory, he supplied drawings for presses to be built by local constructors.

The automatic feed eliminated exposure to loosing fingers as is present in all hand-fed presses. Prepared blanks were fed by hand into a tube that brought the blanks into position. Later improvements were made by Taylor & Challen which made Uhlhorn-style presses under license. Even in the 20th century, as late as 1961, Horden, Mason & Edwards placed the toggle mechanism beneath the feed table for a final improvement.

Modern coining presses have reduced the size of the flywheel, enclosed the mechanism with a covering (no moving parts exposed) and have changed the feed mechanism to an indexing plate. Some presses continue a horizontal feed with vertical die movement; but one German press has a vertical feed with horizontal die movement.

Modern presses are manufactured by Schuler and Grabener in Germany, by Reinhard & Fernau in Austria, by Heaton, Taylor & Challen, and Horden Mason & Edwards (now a division of America’s Cincinnati Milacron) in England, by Raskin in Belgium, and by Arboga in Sweden.

Medal Presses The first struck medals (Padua in 1390, Venice in 1393) were made on screw presses, but thediameter was limited, only medals smaller than 40mm could be struck with this type press. With the development of knuckle-joint presses the size was increased but limited by the pressure of the press, so larger presses were built. (Centuries later a 1000-ton press – pressure per square inch  – could strike a medal up to six inches in diameter.

The development of the hydraulic press in as early as 1852 led to greater versatility in medal striking. The action of a hydraulic press is best described as a squeeze, in contrast to the blow of a knuckle-joint press. Production of both presses must take into consideration work hardening. A knuckle-joint has greater production speeds but requires annealing more often. Hydraulic press production is slower but has greater surface movement. Both presses are in use in modern medal manufacturing.

Powering the press.  The source of power has changed in the last two hundred years. Early screw presses were powered only by man. The power was increased by longer balance beam, then by adding lead weights on the end. Power was also increased by more men pushing on the beam, up to four on each end.

Horsepower was used where the horses could walk in a circle deriving the power by a capstan. This was ideal for a trip hammer press, but not effective for a screw press. Then water power was employed, with power transferred by belts. This was ideal for the Uhlhorn and Thonnelier presses because the belts could be connected to the flywheels.

But steam engines, first developed by Boulton & Watt in 1775 and used in their mint as early as 1788, became the major source of mint power for over ninety years for most mints. It wasn’t until 1883 that electricity began replacing steam power, first at the Philadelphia Mint, then elsewhere; belting was eliminated and separate electric motors ran individual coin and medal presses.

Pressroom practice. The operator of any press employed to strike coins or medals, as a coining press or any type of medal pres, is a pressman.

A pressman reports to a pressroom foreman, who is responsible for all activity to produce the coins or medals. Operators of presses have always been called a pressman since 1819 (although Medallic Art Co has at times employed lady “pressmen”).

A pressman’s greatest responsibilities with automatic presses are:

  1. not to break a die
  2. to use the correct blanks for striking the order at hand
  3. to setup the press properly
  4. to insure the feed mechanism is delivering blanks to the press properly and continuously
  5. to frequently inspect the struck items during a production run.

The pressman must have a feeling for die clearance and die alignment during setup and that the dies are seated and locked in position correctly. He must know the correct gauge of blanked stock. He must be able to look at the relief and size of a die to determine the correct gauge to use in striking if it is not specified in the work order.

While presses are running the pressman must have a “sixth” sense of knowing his press is functioning properly by the sound it makes (and how fast he can hit the power button when he hears a clink or thud instead of a hum of satisfactory continuous striking).

He should know just about when a die is going to break and retire it before it can jam the press. (While obtaining the maximum use from a die is an admirable goal, it is less important than that of preventing a die from breaking on the press.) Also he must maintain the press or presses under his command in working order.

During inspection, a pressman must know what to look for. He must know the concept of highpoints (that the metal flow is filling every cavity in the dies by surface deformation). He examines these places on the struck piece under magnification on both sides.

He must be conscious of all the points of stress in a die (he must carefully examine the areas between lettering and the rim where stress is the greatest). He must also examine the rim/edge juncture in trying to meet (but not exceed!) this point with the most metal mass of the blanks. He should check the axis on both sides of the piece that they are properly aligned. His goal is to produce perfect struck pieces at all times.

Presses provide the means of producing multiple replicas of the image the artist created in the first place.

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