Gravity drop hammer coining machine - Page 2
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  1. #21
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    Sep 2004
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    You're right. Nickle silver is extremely stiff compared to other copper bearing metals. I feel the best solution will be to strike the part in bronze or copper and nickel plate the finished part. Private mints can coin NS but they employ 350+ ton knuckle presses. Helps to have the right gear. Advice seems to be moving towards using a flypress in this applcation. Any opinions on this?

  2. #22
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    Sep 2005
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    wouldn't put my hands near one unless i had to to make a living

  3. #23
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    Feb 2003
    Eugene, Oregon, USA
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    When I first started forming tin and nickel silver I used an arbor press. I first pressed the die down and while still under pressure I hit the top of the ram with a dead blow hammer. This set the detail. I was forming .010 material so it would not be the same as a coin.

  4. #24
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    Jun 2006
    Rocklin, CA,USA
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    There is an outfit in San Francisco using screw presses for die forming jewelery. IIRC they are only doing precious metals-so doesn't help you with your current dilemma.

    I saw (some years ago) a drop press used to upset the end of a piece of steel for use as the back end of a muzzle-loading rifle barrel. It was quite effective. It employed an over-center cam(toggle) to catch the weight on the rebound. It was tripped as the head went past it on the first downward pass. Would break the toggles off if not set properly before releasing the weight. (Or so I was told...Wouldn't want to be around if that happened.)
    Seems like that might be the sort of thing you are looking to use in your application.

  5. #25
    art_deco_machine Guest



    For practical cases, however, with a free-flow relief all around, it
    is well to figure, for steel, 100 tons per square inch and higher on the
    total final area squeezed, and for copper around 75 tons per square inch.

    Under the third classification in the squeezing group of operations,
    including coining, stamping and embossing, comes a variety of work in
    which the metal is required to flow comparatively, little but is subjected
    to extremely high pressures to bring out sharp designs or lines or to
    obtain a very accurate surface. On most of this work the metal is
    either completely contained in a closed die, or practically so, with the
    result that there is no outlet or relief for the flow of surplus metal.
    Under such conditions, if care is lacking so that oversized stock is used
    or adjustments are carelessly made, the rigidity of the presses may

    Greece 500 B.C. Egypt 300 H.C. Corinth 300 Tt.C. Rome 27 B.C.

    FIG. 210.—Upper row: Four ancient coins contribute to the story of early craftsman-
    ship and mechanical progress. Lower row: Four excellent examples of commemo-
    rative medals from the seventeenth and eighteenth centuries. (Courtesy of Dr.
    Frederic R. Sanborn)

    build the pressure up to several hundred tons per square inch with
    serious consequences to the equipment.

    Coins.—In the upper row of Fig. 210 are shown four ancient coins
    from the collection of Dr. Frederic R. Sanborn. These were " struck "
    two thousand years and more ago. In many cases the designs are
    excellent and the tools were well made. The rough outline, however,
    reflects the crudeness of the methods employed. The lower die was
    secured on an anvil. A pellet of precious metal was placed upon it.
    The upper die, hinged to it or held over it by one man, was struck with a
    sledge by another man. The results obtained would seem to confirm
    this story. Coins became thinner as time went by, but the irregular
    outline remained until about the seventeenth century.

    The lower row (Fig. 210) shows an interesting group of commemora-

    COINS 241

    tive medals, the Cromwell medal, 1655, and the Charles II medal, 1670,
    both struck in England; the American Liberty medal, 1783, and the
    Napoleonic Egyptian Campaign medal, 1798, both struck at the French
    Mint. These all have the thorough finish and fine detail of modern
    coins but were undoubtedly made in crude presses, for the first knuckle-
    joint coining press, Fig. 211, was not invented, to our knowledge, until
    1833. It was brought out by a Frenchman, M. Thonnclier. One of
    these machines was included in the original installation of equipment
    for the United States Mint in 1836.

    The mint at Philadelphia still has in its possession a simple hand-
    operated screw press antedating this installation and said to be the

    FIG. 211.—The original knuckle-joint
    press invented in 1833 for the French

    Fro. 212.—One of the early power
    blanking presses developed for cutting
    coin blanks.

    original machine used at the time coins were first made in this country.
    It is a small machine mounted on a massive wooden table and was called
    upon to do both the blanking and coining operations. With power
    equipment came the Grecian-columned Planchett Cutting Press shown
    in Fig. 212. It was superseded later in the last century by small,
    solid-frame, straight-sided presses of the more modern over-drive type.

    An even more recent change has been the addition of the Planchett
    Upsetting Machine, Fig. 213. This machine automatically rolls the
    coin around edgewise, upsetting and thickening the edge to reduce a
    certain amount of the strain on the coining dies.

    The coining operation itself is now performed in knuckle-joint

    presses of the type illustrated in Fig. 214. The coin feed is a develop-
    ment of that on M. Thonnelier's machine and is probably the earliest
    type of mechanical feed. As now constructed, it includes the tube
    filled with blanks and a pair of fingers to take the blanks to the die and
    the finished coins out of the die. A crank-actuated bottom knockoutis timed to
    lift the coin to the surface of the die as the fingers close.

    A report issued sometime ago by the Mint, Table
    XX, showed that the ac-tual pressures required to
    bring up clear impressions on United States gold, silver
    and copper coins varied between 85 and 125 tons
    per square inch. These fig-ures are considerably above
    the freeflow yield points of the metals as rolled, yet the
    presses actually used have rated capacities two or three
    times what the experimental figures would indicate. This
    is good conservative practice, for in using closed dies a
    double blank or a careless set-up will raise the pressure
    tremendously in a rigid machine. And an extra-rigid machine contrib-
    utes a great deal to the tool life.
    Plastic Working of Metals and Power-Press Operations

    E.V. Crane


  6. #26
    Join Date
    Sep 2004
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    There is in your post several new bits of info that I have not known. Significant among them is the chart of forces. In ideal circumstances I would get a proper press of adequate tonage to do the work I plan. I have only two ways to go at this time. Either I have my medallions made for me or I make them myself. I only need a handful a year as they are intended to ship with my custom fly reels. I currently cut an engraved design for my use. It would be nice to have the sculptured coined alternative. The artist in me likes that idea. Buying a lot medallions may be cost effective but the same artist in me likes to change things. Looks like it'l be better to strike what I need as I need them. (If I can)

    The chart brings a couple things into clear focus. If my math is correct. I can drop a 55 pound weight 48 inches and develop the following. The kinetic energy just before impact would be 301 Joules. If the die moves .01 inches after impact, the impact force would be 1,186,624 Newtons or 266,763 pounds force. Given a 3/4 inch diameter medallion, the force in psi is 603,828 or 300 tons. It's been a while since college physics but I think this is correct. That being the case, I can strike my coins on the occasions that I need them with a gravity drop hammer of the above specifcation. Once I get some time I'll setup some controlled experiments to confirm this. At least it'l let me know if it is practical before I start to build it.

    Thanks for taking the time to put it up.


    [ 02-26-2007, 10:23 AM: Message edited by: garyd ]

  7. #27
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    Sep 2004
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    There are other options for you I believe.
    Certainly, lost wax casting may be something you should look at and another may be PMC or precious metal clay. I have never tried the PMC but I belive it can be molded or worked just like clay and then it is fired in a kiln or I think even with a propane torch. It is sterling silver when it is fired - as in solid metal. If I recall the stuff shrinks significantly when fired so any detail you have gets better - as in sharper. In other words, you can work larger than your finished product and the shrinking part "cleans" up your work. I do not think it is very cheap - maybe several times the cost of sterling silver. You can buy it at Rio Grande, the jewelry making supply house in Albuquerque and probably many other similar places.


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