Honest Question: What causes these deep flow lines? (UNC 1976 P Ike T2)

While we should all agree that the flow lines are created by furrowing of the surface of the dies by displaced planchet metal taking the path of least resistance, the direction in which planchet metal flows to create the observed effect is not universally agreed upon . . .

It is true that some metal flows toward the recesses in the die, and some toward the rim, but my observation has been that in the majority of the noticeable cases the flow is predominantly toward the rim, and not away from it.

The recesses in the dies, and the annulus between the planchet and the collar define the volumes receiving displaced metal. Both are significant, and the ratio of one to the other varies from one coin design to another. But that ratio by itself does not determine how much metal flows inward and how much outward, as variations in planchet dimensions (both thickness and diameter) and striking pressure also influence the outcome.

Much like a river starts inland as a stream, and grows in size as it picks up more water downstream and carves a larger channel as it heads out to sea, so too do the metal flows that furrow the coining dies. This is easily seen when comparing the fineness of the flow lines near the central devices with their coarseness near the rim.

This phenomenon is made even more pronounced by the fact that coins, being round, require that material flowing toward the center must crowd into an ever smaller flow area, while that flowing outward meets less resistance because the flow area increases with an increase in radius. Think of it this way . . . dividing the coin into pie slices, the metal within each slice of pie must remain within the bounds of its own slice as it moves under striking pressure . . . it much more easily moves toward the rim than toward the center.

The striking of the Ultra High Relief Proof Saint Gaudens Double Eagle supports this notion. The resistance of flow toward the central devices was so great, and so frustrated efforts to execute the design to its fullest, that the coins had to be struck nine times, with full annealing of the coin between each strike.

In the case of your coin, the obverse die obviously saw a lot more service than most others, or had not been properly hardened, or both.

Again, I'm not disputing that metal flows to the center, but I do believe most of it flows toward the rim. Hopefully, this answers your question.

 

When a coin is struck without a collar the metal actually both inwards and outwards. The evidence that is true are the coins themselves. For if it were not true and the metal only flowed outwards then the central portion of the dies would not fill and there would be no design in the center. The fact that the design is there confirms that it flows both directions.

 

There's a very simple way to realize that simply isn't true and cannot even be true. And all one has to do is ask yourself a question. If the majority of the metal flowed outwards, then why are blanks run through the upset mill to make planchets ?

The answer of course is because that's the one and only way there is to get enough metal at the outer edge to form the rim.

Rather obviously, if the majority of the metal flowed outwards when a coin is struck, then upsetting the blanks would never be necessary. The fact that it is necessary tells us the vast majority of the metal does indeed flow inwards.

 

Blanks are run through the upsetting mill primarily to reduce the diameter of the blanks enough to reliably feed into the collar. Pre-forming and pre-hardening the rim are beneficial by-products of this operation.

The whole reason a collar is employed in coining is to confine the planchet which tends to expand outward, and to impart a regular geometry at the edge of the coin, thus preventing what was known as "splash striking" in days long past.

The confinement of the coin by the collar is what facilitates bringing up the central details. Since significant diametral clearance exists between the ID of the collar and the OD of the planchet, metal must flow outward to fill that void before metal begins to rise into the recesses of the dies.

The amount of volume that must be filled in the annular gap between planchet and collar can be surprisingly significant. This is because of the perimeter of the annulus, and the thickness of the rim to be formed. I'm at lunch at work, and don't have coin and collar specs at hand, but if the planchet is .5mm smaller than the collar, and the rim is 2mm thick on a coin 20mm in diameter, the volume to be filled is 31 cubic mm, which is a pretty meaningful number.

 

C'mon now Mike, that isn't why planchets are run through the upset mill. If that was the reason all they'd have to do is cut the blanks to whatever diameter they wanted, and skip the upset mill entirely.

Planchets are run though the upset mill for one reason and one reason only - it is because that extra metal is needed to form the rim. That's why they invented the upset process to begin with. If you strike a planchet without it having been run through the upset mill, no rim is ever formed.

Now ask yourself a different question. Where does the majority of the metal end up once a coin has been struck ? It's a really simple question and all you have to do is look at a coin to see the answer. The majority of the metal ends up in the central devices. And that means the majority of the metal flows inwards, not outwards.

It's a combination of things really. The edge of the blank is upset so there is enough metal already there to form the rim. And yes the collar, and the dies, stop and contain the metal at the outer edge so the metal simply cannot flow any further outwards than that. And because the metal is stopped at the outer edge, it is forced to flow inwards towards the central devices. And it has to be forced to flow inwards because if it isn't the central devices will not strike up.

They developed this process because you're right about one thing, when something is submitted to great pressure the natural tendency is for the material to spread outwards. But to strike up coins they absolutely have to counter that tendency, they have to force the majority of the metal to flow inwards. And that's exactly what they do with the process that is used.

 

Dies are not convex or concave, they are flat. Now there was some experimentation with Morgans where they made some slightly convex dies, but that was about it. All other dies are and always have been flat.

And you can prove that to yourself simply by using a micrometer and measuring the fields of a coin. You'll find they the the same thickness everywhere.