For the mechanical engineers in the house, is it possible to have die flow lines on the edge of a co

Longacre

I was taking a look at those super-sized images of the new $20 UHR Saints that the mint is producing. I could not help but notice what looked like die flow lines on the edge of the coin.

For the mechanical engineers in the house (or anyone else for that matter, since I know we have a bumper crop of docs on these boards, but a dearth of mech engineers ), do coins typically get die flow lines on their edges? Does it make a difference if the coin has a lettered edge or not? If the coin has a smooth edge, does it, by the law of physics, avoid any coherent pattern of die flow lines because there is no design to "fill" and "flow to"?

I would also love to hear from someone like Denga, who might have a historical/minting technology perspective on this.

Eagleguy

As an ME, I'm embarrassed that I cannot provide a good answer to this question. Sorry!

JH

Coinosaurus

I'll be very interested to hear what the experts have to say as well.

Here's another way of framing it - if you had a blank die and struck a blank planchet, would there be die flow lines?

My guess is that you would have them, just a lot less than with an engraved die, because the metal flow is much less. By this reasoning I'll suggest that the same thing is true for the edge - there can be flow lines, but one would expect to see less than on the surface.

Frankcoins

Flow lines would still radiate outward from the letters and stars. What shows in the pictures are likely finishing marks from preparation of the blanks.

notwilight

Flowlines are caused by repeated metal movement in a consistent direction. When you squish (technical ME term) a planchet between the dies it moves out infinitesimally toward the collar. doing this a million times puts flow lines in the dies so when you see flowlines on a coin it is because they have developed in the dies from the million coins that came before.

Now you should never see flow lines on a coin made from new dies such as the UHR sample pieces.

As for the original question, I guess it is possible to get flow lines on the edge since the entire coin moves down as it is pressed between the dies. however, the collar would need to be very tight before the movement ended. So I guess it would depend on how tight the collar fit. If the collar became tight halfway through the downtravel I guess flowlines would be theoretically possible.

Now on less expensive coins like the presidential dollars and most (maybe all) circulating coinage the plancheds are sheared out of sheets of planchet material and this results in smeared metal on the edges that could resemble flowlines.

I'm educated as a physicist and nuclear engineer but hold a Professional Mechanical Engineer License in CA (all you have to do is take the test and pass it). --Jerry

19Lyds

As a layman, I think it depends upon whether or not the edge lettering is incused.

If its incused then metal is being displaced outward from the letters but if the lettering is in relief, then metal must flow into the incused portions of the die.

I suppose that both situations would have metal flow but in examining the edges of the Prewsidential Dollars, I haven't seen anything of any significance with regard to die flow lines on the lettered edges.

GrumpyEd

Not an "expert" but since I look at a lot of variety coins I use higher mag to see details, I say flow lines are there.

Look at at any later die state coin and you will see abrasion doubling starting on any letters or design near the rim that goes to the rim. The abrasion is from metal flow and it goes from the center all the way to the rim. It also goes away from any design element (not only to the rim). The detail I'm not sure of is if the lines near elements are to or from the element, it seems it would be metal going to the element (to fill it). Incuse designs would be a different direction but still causes flow lines.

In areas without design you still see flow lines but less dramatic.

Even on an early die state you can see very faint flow lines, that's why the coins have cartwheel even when the die is EDS.
If there were no flow lines the coins would not have cartwheel, the coins with the least might be EDS prooflike ones but they still have some.

If you're thinking of the actual edge of the coin what you might see are cut lines from when the planchet was cut. It makes lines across the edge so when struck they may still show.

notwilight

I just saw the photos from the USMint and I'd say most of what you are seeing are faint die polish lines. --Jerry

RWB

If the poster is referring to the EHR $20 (small diameter) that were struck yesterday, the surface is that of the new die. They are not polished like a mirror. This surface is slightly different than that on the original pieces because of changes in technology. With use, the dies will develop metal flow lines and “luster” unless resurfaced frequently. Modern chromium plated dies are much more durable then the originals.

dcarr

Smooth-edge and reeded-edge coins struck in a collar will typically have striations on the edge, oriented perpendicular to the coin's obverse.
When a coin is stamped in a collar, the metal flows up against the collar and the coin will become lodged in the collar. One of the dies (usually the lower die) is used to push the coin out of the collar. Sometimes, consideable force is needed to do this. As the coin slides out of the collar, any existing surface texture on the edge is obliterated by this sliding of metal on metal.

A coin with edge lettering that is applied at time of striking is different. It can not be ejected in the same manner as a smooth-edge or reeded-edge coin because, of course, the raised lettering would get sheared off. So a 3-piece collar is used. After striking, the three collar segments each retract away from the coin, leaving the coin free for manual extraction. This type of collar could develop flow lines after a sufficient number of coins is struck.

messydesk

You're most likely seeing machining artifacts on the collar die. I doubt these are polished the same way the obverse and reverse dies are. When a coin is struck, the metal expands radially and erodes the dies. The collar comes into contact with the planchet, but isn't contributing to the expansion of the planchet parallel to the collar die in the same way that the obverse and reverse dies are. Since the collar is imparting its mark on the edge of the coin, technically there is some erosion of the collar die occuring, but nowhere as fast as on the obverse and reverse.

To address the question as to what would happen if a flat planchet were struck by flat, featureless dies in a smooth collar, something has to happen with the kinetic energy in the hammer die. With normal dies, this energy is used to push the metal of the planchet into the features of the die. It's also converted to heat and sound. With a featureless die, if the planchet used had absolutely nowhere to expand radially by virtue of a closed collar, there would still be the same amount of energy that needs to do something. Absent empty spaces for the planchet to expand, there would be more stress put on the collar or the press in general. The heat generated would probably cause enough die fatigue over time that the fatigue would take on a pattern that would eventually look like flow lines if the die didn't break at first from having to absorb the excess energy expended by the high striking pressure.

Longacre

This thread is turning excessively interesting. I want to thank everyone so far for the replies (before someone complains that Longacre never thanks responders to his threads). Let's continue the discussion.

dorkkarl

flowlines can appear anywhere on a coin, & are actually more likely nearer the edge

at the instant the dies hammer the planchet, the metal behaves essentially like a liquid being compressed under high force. it actually flows like a liquid! for an instant, it is compressed under pressure, then expands back, & this in turn abrades the dies ever so slightly, which is of course what advances the die state

K S

Nysoto

Pre-1837 lettered edge coins would not have flow lines imparted by striking because the open collar is thought by most to be a centering device which does not control the diameter. However, the lettered edge coins can still have flow lines on the edge from the Castiang style edge lettering and rimming machine, which upsets the edge (physically, not emotionally), although the operator could become upset also. The thickening and work hardening of the edge provides meat for the dentils, and controls the diameter. The opposing edge dies had die stages and internal cuds, hundreds were used on bust half dollars

Yes, the smooth edge coins can have flow lines from the edge die, but not as much as on the obverse and reverse as the metal does not move as much. I would suspect reeded edge coins can have flow lines from the reeded edge collar die.

SanctionII

I will join in and pose my own related question (just because I thirst for knowledge).

What happens if a planchet is placed into a striking chamber under circumstances where there is no room (assuming this is possible) for expansion of the metal (radially or otherwise) during striking? The collar, dies, planchet and striking chamber are milled to the highest possible degree so that when the dies strike the planchet there is no place at all for the metal to flow. Assume also the dies are completely flat, that the planchet is completely flat and that the collar is completely flat. Assume also that the planchet is made of a metal which is less,..... tempered? (strong), than the dies, collar and striking chamber

Does this scenario result, post striking, in a planchet of certain size and weight becoming denser (molecules and the atoms contained therein being forced closer together) with the post striking planchet having the same weight and the same diameter, only thinner? If so will this denser planchet retain the increased density post striking or will it morph back to its pre striking denisty?

I am not a science guy, so please humor me with what may be an ignorant question

tmot99

Can't these lines develop strictly from the punching of the blanks out of the roll stock? I would think they could still be present even after minting and pushing against the collar. We don't typically see them becasue of the reeded edge.

cladking

Planchets are upset (rolled on end to increase the amount of metal at the edge) so
that there is sufficient metal to form the edge and rims. Metal flow tends to be greatest
near the rim despite this and flow lines first show up in this area on most designs. The
edge itself is formed by the collar and these "dies" are changed at less regular inter-
vals apparently.

Coinosaurus

<< <i>

Does this scenario result, post striking, in a planchet of certain size and weight becoming denser (molecules and the atoms contained therein being forced closer together) with the post striking planchet having the same weight and the same diameter, only thinner? If so will this denser planchet retain the increased density post striking or will it morph back to its pre striking denisty?

>>



I am way out of my league here, but I think if you sufficiently compress any element it will turn into a denser element. Probably it changes crystalline structure first (isn't this how carbon is changed into diamonds??) but if the pressure gets high enough I suspect that the electrons would rearrange themselves into larger groups (the characteristic of denser materials). Whether the change is permanent I am not sure. Hopefully a chemist will be along shortly to tell me how I am all wrong

cladking

<< <i>

What happens if a planchet is placed into a striking chamber under circumstances where there is no room (assuming this is possible) for expansion of the metal (radially or otherwise) during striking? The collar, dies, planchet and striking chamber are milled to the highest possible degree so that when the dies strike the planchet there is no place at all for the metal to flow. Assume also the dies are completely flat, that the planchet is completely flat and that the collar is completely flat. Assume also that the planchet is made of a metal which is less,..... tempered? (strong), than the dies, collar and striking chamber

Does this scenario result, post striking, in a planchet of certain size and weight becoming denser (molecules and the atoms contained therein being forced closer together) with the post striking planchet having the same weight and the same diameter, only thinner? If so will this denser planchet retain the increased density post striking or will it morph back to its pre striking denisty?

>>



There is a little compression of the metal anytime it is struck by the die. At
these pressures there wouldn't be a lot of difference in the inelastic defor-
mation dependent on tonnage. If the planchet fills the entire die and there's
still press movement (increasing pressure) then in you scenario the press
would just have to stop and the coin would have nearly the same density as
any other normal coin.

In real life however the press won't be brought to a stop by the back press-
ure of the coin and metal will flow up between the collar and the die creat-
ing "finning" which gives the mint lots of headaches some years.

sinin1

dies for both sides and edge

it depends on edge and how coin is ejected

also if edge lettering/design is done as a separate procedure or part of the main strike

very early coins had 3 or 4 piece edge dies that were held in collar and removed manually (if I remember correctly)

Nysoto

Density changes very little from cold working metal. Work-hardened aluminum has only about a 0.2% density change. Thus, you cannot "compress" metal very much. There is spring-back of metals, but this is from elasticity unloading after forming, and only involves micro density changes. We have software to calculate spring-back of various alloys at work.

Longacre

This thread is going way beyond my pay grade.

cladking

<< <i>This thread is going way beyond my pay grade. >>




No, it's not.

There is little density increase from work hardening.

When the metal is compressed there is also a little elastic deformity; it springs back and doesn't affect the density of the coin after the strike. This elasticity is nearly insignificant in the range at which a stamping press operates.

While the die does effectively bounce off a coin it's also more true to say the coin won't stop the die. The coin is quite fluid during the strike and isn't easily contained if the press is still moving.

SanctionII

One of my worst days as a lawyer was when I worked for another lawyer who had a general practice. he had a PI defense case where a trucking company employee was injured when a metal step into a delivery truck he was standing on failed and he fell backwards to the ground, injuring his back.

My boss had me handle the case. I had to take the deposition of the Plaintiff's expert (a metallurgist) concerning various issues, including the his expert opinion on why the metal step failed.

I knew nothing about metals science and my inept questioning during the deposition showed up clearly when reading the transcript. My real estate law experience did not save me from looking like an idiot that day.

One of my best days as a lawyer was when I deposed a Plaintiff's expert regarding his expert opinion about the dollar amount of damages my client [a bank who had taken title to a large commercial property back in foreclosure, in which the Plaintiffs rented one restaurant space in the entire property]. The "expert" was an economist teaching at San Jose State University. To supplement his income he hired himself out as an expert in court cases testifying about economic damages [lost revenue streams]. He professed to be an expert in his field and opined that my client caused the Plaintiff [who hated the original landlord and transferred his hate to the bank who took over the property] hundreds of thousands of dollars in lost revenue because my client did not, promptly after taking title to the property, respond to the Plaintiff's complaints about a clogged drain, slippery steps, sign size limitations and rough parking lot surfaces.

Since I knew real estate and real estate loan foreclosure matters extremely well and since the "expert" did not, I conducted a masterful deposition of the expert. After two days of asking him questions, his "expert opinion" was revealed to be based upon a foundation of cr*p. The case settled quickly thereafter.

RWB

Metal flow lines are an artifact of fatigue of the steel die. The only role of the planchet is to receive the impression from the die. As an earlier poster noted it is repeated striking against thousands of planchets that gradually introduces metal flow lines in the die.

I saw the situation SanctionII mentioned in an engineering lab long ago. The result was that energy was transferred to the supporting apparatus via a shock wave. You could see objects on the equipment ripple. The grad students like to joke that this was how they stirred their coffee.

dorkkarl

<< <i>Metal flow lines are an artifact of fatigue of the steel die. The only role of the planchet is to receive the impression from the die. As an earlier poster noted it is repeated striking against thousands of planchets that gradually introduces metal flow lines in the die. >>

gotta remember though, a planchet also acts like a die, hammering on the coin die! ie. over time, as thousands & thousands of planchet meet up w/ the die, it causes the die to become eroded, metal to stretch & deform, which is what results in either radial die lines, or amorphous distortion. these are what influences the luster on subsequent coins, whether it be carwheel luster, or orange peel luster

so, if you look at a brand new die, say a proof die, w/ mirrored fields & texture on the devices, & watch it progress as planchets are struck, you will find the mirrors fade, though since they also polish the blanks, it takes a LONG time for the die mirrors to fade. but eventually they will, taking on a more typical unreflective surface w/ radial flowlines. in addition, the frosted depressed areas ,which produce the cameo frost on the planchets, becomes distorted & smoothed down also, to where the microscopic changes in relief (which is what makes the frost) become distorted & equalized to where they are merely reflective.

but to the point of the opening post, the MAIN factor in flow line luster is from the distortion of the planchet, not what is impacted by the dies. the blanks are a little smaller then the dies, the die impact stretches them out, & the friction between the die & the planchet creates radial striations that are what creates the cartwheel effect. so where the metal distorts/stretches most is where the most luster is - which is at the edges.

K S