Avoiding Flash in Closed Die Operation

Hi,

I have a newbie question, since I have very little background in this.

I came across a paper discussing a schematic for coining die. The author mentioned that in order to reduce flash, the size of the enclosure in the closed die operation should be the same as the desired finished coin. If the coin material is more, then it will try to escape through the edges and cause flash formation.

In a stamping operation, I believe the press will inadvertently keep pushing down, unless there is something to stop it from pushing past the point where desired formation has already happened. In such a case, I suspect that the author is proposing that the die cavity cannot be compressed past that point, so there is something in the design which ensures that. But it is not described, and I cannot pinpoint what that is.

So I my naive understanding is that it would work in the manner in which I have marked the diagram below. The upper die will protrude just enough that its protrusion + finished coin height both exactly equal the cavity in the lower die. Past that point, if the press keeps on pushing, the two edges of the die set will come in contact with each other not let further press be put on the medallion. Is my reading correct in this matter? Or is this not the way it is done? I noticed that the author has shown considerably bigger protrusion, so not sure if that is just to illustrate, or the upper die actually comes down way more, and there is some totally different mechanism to limit continued pressure on the medallion?

Thanks in advance.



PS. The attachment turned out to be too small to be read, so here is another link: (Google drive needs url to be refreshed twice to load the picture)

https://drive.google.com/file/d/0BzE4ZyIj-ZpCU1VNbnRDVmo3ak0/edit?usp=sharing

I've never been able to examine a coining press and what I've seen in videos was very limited, but it looked to me like the presses had a fixed stroke. And the idea of fixed stroke seems to be supported by images of partial strikes of undersized blanks. As long as the blanks are properly sized a fixed stroke would work fine. Though I could see some sort of pressure limiting mechanism as possibly being useful to prevent damage in the case of an over sized blank.

I would guess that there are die springs that take up the tolerances.

Tom

If this die is mounted in a die set, then stop blocks are usually used to control depth when setting up in a press with a adjustable ram screw. Basically when setting the die the ram is adjusted down until the impression is correct which should also coincide with the stop blocks just touching. All of this timing is done in the tool room when the die is built. The exact volume (diameter and thickness) of the metal required for the impression can be determined experimentally by making the forming blocks first,mounting them, and trying different size blanks. Also, there is more than one of doing all of this.

You can't mount enough springs to take the kind of tonnage that you generate with a coining press. The newer presses are hydraulic. Set the tonnage and away you go. The blank size is critical for a coining operation. We had some 50mm brass coins made for our 125 aniversary. Those were run in a 1200 ton hydraulic press.

Years ago, in trade school, we made a blanking die and a coining die to make a quarter sized aluminum coin. The 120 ton OBI we had was barely big enough for that. The parts would come out warm. It all depends on the material, the features that you are imparting, and the size. I suppose that if you made a very accurate 3D drawing of the coin and determined the volume that way would be fairly accurate. Then load up the FEA software and determine the tonnage necessary to get "flow".
JR

Let's step back into history. Coining was done long before hydraulic presses. How did they do it then?

Tom

Just like a cider press, a big ass screw. Before that, I would assume with a BFH.

With the 1,200 ton I first mentioned, the "coin" was almost 2" in diameter and brass. For the aluminum coin, the problem with the OBI was that there is no fine adjustment.
JR