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Coining - or - not coining

Coining - or - not coining

Coining - or - not coining

I'm a novice at engineering. As I understand it, coining requires high pressures and it induces plastic flow of the metal.

Okay, let's imagine that I want to produce a 2" diameter medalion and I am going to fill in fields I create with enamel. I want a 1/16" high raised lip say 1/16" wide near the edge, and a similar raised line, in the form of a circle, 1.5" diameter centred on the centre of the round blank.

Lets say I used 2 dies. One die would be cut with 2 circles, the other would have two raised up circles on it's surface.

Okay, when the die comes down on the blank it will merely deform the blank where the circles are on both dies.

In this scenario, the front of the medalion would have two raised up parts in the shape of circles, the back would have two recessed circles. You would see that deformation only takes place at the circles.

If however one die is used and has cut into it circles, then the medalion is flat on the back but two circles would be raised at the front.

Is only the second process coining? Thanks.

RE: Coining - or - not coining

You don't mention the material or coin thickness. If your first scenario only bends the material it may not be considered coining. If the process requires plastic flow in a closed die, that's coining. Around 150 Tons for a coin a little larger than a half dollar. Depends a lot on the material and coin size.


RE: Coining - or - not coining

So, need 150 Tons for something in the region of 35mm diameter to coin copper with nickel coating.

RE: Coining - or - not coining

I'm in UK, not sure who to talk to, perhaps a press manufacturer. But, the question I would pose is something similar to this: What press would I need to emboss an image (say something like what you see on a Kennedy half dollar)onto a 35mm copper blank that was say 2mm thick.

As I understand it embossing indents an image on one face, and produces a relief image on the other. Not coining obviously, and will take less pressure than coining. What pressure would be required for that embossing, or how to calculate it, I have no idea.

RE: Coining - or - not coining

richard4556 ,
Your first process describes embossing
The second process you mention of driving the metal into a shaped die with a flat driver is coining.
This requires forces above the shear strength of the metal.
In ancient times it was accomplished by shear brute force , a metal smith would take a blank, place it on a shaped anvil, place a punch over the top and smack it with a very large hammer. Nowadays we do the same thing with a hydraulic press or a drop hammer. The drop hammer is a slight improvement over the ancient metal smiths hammer.

You are judged not by what you know, but by what you can do.

RE: Coining - or - not coining

I may make a product that is often made by coining. I colud not do coining in a home business, becauae of the large press needed. So, I'm wondering if I can produce by embossing, because I would not need such a large press. I can emboss, in theory, but I'd have to be sure I could get the required finess/detail in the image produced.

RE: Coining - or - not coining

The Potter USA site that you linked to in your 4th post, has a press that will do what you require, or if you are handy at metalwork it is not too difficult to make a press of that type. You can also buy a somewhat less elegant press for less money at Harbor Freight USA.

You are judged not by what you know, but by what you can do.

RE: Coining - or - not coining

Let me clarify the mud a little.

Embossing uses a shaped punch to force an image into the obverse face of a relatively thin blank, and by means of a die with a surface that is complementary to that of the punch, or an elastic backing pad, produces a complementary image on the reverse face of the same blank. Think of the embossed foil used to form the surfaces of chocolate 'coins'. I don't know if the two cups are formed separately and then assembled around the chocolate or if both are embossed after/during assembly, with the chocolate acting as the backing pad, but each individual foil cup is embossed, not coined, somehow.

Coining uses a shaped punch to force an image into the obverse face of a relatively thick blank. The reverse face is similarly formed by another punch at the same time. Either punch may be flat faced and blank; either punch may bear an image. The images need not be complementary, or related in any way. Much more force is involved, but it's not beyond the capability of a home workshop at very low production volumes.

Almost five decades ago, according to rumor, a certain government facility elected to produce commemorative coins for a celebration. They were whoppers, about three inches in diameter and a quarter inch thick. The facility had a more than complete machine shop, but a whole lot of MBAs got involved, and they ended up contracting out production of the punches, production of the blanks, coining the blanks, knurling the edges, and color anodizing the finished (aluminum) coins. Certain insiders asserted that the original estimate was that they could homebrew the tooling and bang out a few thousand for a quarter apiece, but with all the contractors and overhead that agglomerated around the project, the unit cost ballooned to USD 3.50 or so. ... when that was a lot of money. ... and as planned, they gave them away, to everyone who showed up to see their operation. Those were the days.

Back to your problem.

First, the depth of your proposed impression is too great, even in copper. Greatest depth should be half a millimeter or less, else you will experience problems with cracking of coining dies and workpieces, and difficulty separating them after coining. The art in coins is suggesting visual depth of the images without using much actual physical depth.

Second, copper has some disadvantages, primarily that it is expensive to buy, and secondarily that its high scrap value makes it subject to, ahem, diversion, at every step of production. If you're going to cover it with nickel anyway, consider using aluminum instead of copper for the blank.

( Copper is even too expensive to use to make a penny. See The US Mint's process for producing precise copper plated zinc blanks and coining them after plating without exposing the zinc. )

Mike Halloran
Pembroke Pines, FL, USA

RE: Coining - or - not coining

A toggle press can give you extremely high tonnage without excessive physical size. These can be built in a homeshop, as well. Embossing will never give you the sharpness and clarity of detail that coining will, it's up to you to decide what the requirement is. Actually making the dies is going to be your most technically challenging undertaking, the press is the easy part.

Like Mike said, a 1/16 raised edge is going to give you problems. If you look at any coin minted within the last 2 to 300 hundred years, you will find that the "raised impression" is but a fraction of your proposed 1/16 inch. Visual effects such as mirror and frosted effect give much to the perception of depth of detail, as does incising the edges of detail for a shadow effect.

Good luck in your project.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

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