Minting Press Tonnage Calculation Question
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    Default Minting Press Tonnage Calculation Question

    Hi,

    I'm a layman wanting to press my own coins. I researched a basic formula for how much tonnage is required to press coins of gold and silver, and the calculation came out to be like this:

    Diameter = 50 mm = 1.97 inches.
    Area = 1.97 x 3.14 = 6.1858 sq inches
    Silver Density = 10.5 grams / cm cube = 172 grams / inch cube
    Thickness of a 60 gram silver Coin = 0.057 inches
    Ultimate Tensile Strength of silver = 20300 pounds per sq inches = 9.2 tons per sq inch
    Force required = 6.1858 x 0.057 x 9.2 = 3.2 tons.

    However, this 3.2 tons is far less than what professionals in the industry tell me. E.g. Kempler guy suggested that I would need this
    63 ton Maypress to be able to mint coins.

    USED 63 TON MAYPRES COINING PRESS - Kempler Industries

    So my question is, why is the "theoretical" calculation coming to be so low, and why in real life a much higher tonnage is needed?

    Will I totally not be able to press coins from a 4 ton press? Or would it make a difference on the quality etc.?

    Thanks
    Asif

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    WHY are you making your own coins?... starting a new country???

    as for your calculations.... ever consider that your "theory" might be wrong?

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    force required= strength of material (in psi) x square inches.

    leave out the thickness

    Redo your math

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    Default Pay attention to your units

    Quote Originally Posted by asifshiraz View Post
    Hi,

    I'm a layman wanting to press my own coins. I researched a basic formula for how much tonnage is required to press coins of gold and silver, and the calculation came out to be like this:

    Diameter = 50 mm = 1.97 inches.
    Area = 1.97 x 3.14 = 6.1858 sq inches
    Silver Density = 10.5 grams / cm cube = 172 grams / inch cube
    Thickness of a 60 gram silver Coin = 0.057 inches
    Ultimate Tensile Strength of silver = 20300 pounds per sq inches = 9.2 tons per sq inch
    Force required = 6.1858 x 0.057 x 9.2 = 3.2 tons.


    Asif
    I continually remind my children when I help them with math, physics, chemistry, etc, and the bright young engineers I work with that they MUST keep track of the units. I am an Electrical Engineer and know little about mechanical strengths and nothing about coining. But I know that your calculation is wrong because the unit of your answer is not "tons", but "inch tons"

    6.1858 in sq X 0.057 in x 9.2 tons/in sq = 3.2 inch tons

    I don't think tensile strength is the property you want to use, but if you take the thickness out of your equation you get 57 tons, at least the units are correct and it's close to what the man selling presses told you that you needed.

    CarlBoyd

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    One of the things that must be taken into consideration when coining is what is called "oil canning". Consider this, if you take a one inch diameter blank of any malleable material .5 thick and reduce it's thickness to .25 you will require one tonnage. If you take the same blank and try to reduce its thickness to .06 you will have an entirely different tonnage due to the fact that the area that your pressing upon has increased by quite a bit ( I am not doing the math here). Years ago when I worked in the forging industry I learned that lesson the hard way. The thinner you want to go the harder it is. If you want to do the numbers try to calculate the tonnage required to reduce the thickness of a 1 inch diameter blank .01 thick and reduce it to .005 thick in a press.

    Ernie

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    Here is a website of a guy who does, indeed, mint his own coins. He has been doing it for years, in a slightly larger than home shop style- certainly no deep corporate or government pockets here- and he has found out what actually works. If he could be doing it with 3.2 ton presses, he would be, thats for sure. But his main two coining presses are 150 tons and 320 tons.
    Work Shop Tour

    Coining presses are usually pretty specialized beasts- they have a precisely adjustable BDC so that the exert full force at exactly the right height, and they tend to hit hard and fast. I have a buddy who has a 150 ton Italian jewelry industry coining press, and the thing is scary- it can be adjusted to hit with the desired tonnage, at the exact die height you want, and when it hits, you know it. Its fierce. Works really well, though.

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    @Gary: I'm making my own coins because I believe that everyone should be owning gold and silver... just like Ron Paul says... but irrespective of why I'm making my own coins.... you should not be surprised even if the answer to that question were to be what you said: starting my own country. Are there any rules on who can and who cannot start a country? What if I really am. Do you have any objections? And on what grounds? But that is out of scope for this forum.

    Now, the very reason why I came to this forum, and asked a question, was because I considered that my theory might be wrong... Hope that makes you happy.

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    I have several coining presses for sale. Hydraulic presses from 50 to 500 tons and even a Z&H percussion press like the one mentioned in the link above.

    Best of all, I am located in Austin, Texas as well.

    Send me a PM.

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    I have done sone coining on a 50 ton press I made. I think you are going to need AT LEAST 100 tons to coin something that large. To coin an annealed sterling silver heart pendant about the size of a quarter,I have to press it,anneal it,and press it again. You may have to do the same thing,even with 100 tons.

    I think the U.S. mint uses 75 tons to coin a quarter(25 cent) coin. There has been a TV documentary about the mint on a few times.

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    Hundreds of years ago when they began "coining" ( I know, coins have been used longer than that, BUT...), they did not use coining presses. There were no 150 ton presses. They used drop hammers. Maybe 50 pound weight hammer, with the upper die, and fixed lower die. This was at a Renaissance type thing, Medieval re-enactment type thing. They would strike you a beautiful commemorative for a buck or so for a slug of brass.

    . Always wiped both dies with lint free wipes between strikes.

    I am no physicist, but I think it is mass X velocity X distance. That 50 or so pound weight falling 2 feet or so, with a one inch die is going to impact with MUCH tonnage. Instantaneous, not goint to strike lots per minute, but, depending on your care with the dies, beautiful.

    I would be more concerned, to the OP, what is the value of your own minted coins? I can trust that a US Mint coin is .9 Silver. What makes you think people will accept your own mintage at face? You may be trying to foist lead alloy as "legal tender".

    2 inch coin, 60 gram, close enough to 2 ounces, with no panic, 60 buck coins. We may NEED 60 buck coins, in a panic. Mebbe I should hoard rather than sell my silver and gold. 60 buck coin might be small change, if gas costs 60 per gallon.

    George

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    OP go back and read your original post, you state over 6 sq. inches, times over 9 tons force and you come up with almost 4 tons, thickness has nothing to do with it.

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    Quote Originally Posted by asifshiraz View Post
    why I'm making my own coins.... you should not be surprised even if the answer to that question were to be what you said: starting my own country. Are there any rules on who can and who cannot start a country?
    You already live in a country separate from the union...it is called "TEXAS" How much more can you ask for?

    Years ago, Dad would test out the new hydraulic presses used to remove "kinks" from 11" OD solid steel bars. He would take a half-dollar and "squesh" it as flat as it would go at 500 tons of force!

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    Thought you (especially Ries) might enjoy the story of the Lasqueti Island Mint.
    http://www.lqmint.com/images/MonetaNew.pdf

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    Forging has nothing to do with tensile strength. The main factor is shear strength. There is a book "Cold And Hot Forging: Fundamentals And Applications" which you can get on Amazon and will tell you how to do the calculations.

    You need a press of 50 to 100 tons to coin silver and gold.

    You may want to start small and do hammer forging instead. Once you have the right kind of die you can whack it with a sledge hammer to make the coin. Practice with aluminum. Once you have pounded out a few hundred coins by hand you will have a better appreciation for the capabilities and requirements of a press.

    The main challenge in coining, by the way, is not the forging, it is the making of the die, the die block and the header.

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    Interesting thread. I'm wanting to make something like a badge, but much bigger, maybe up to 160mm x 60mm.
    Badges are made by feeding thinnish copper strip into some kind of press. I believe the blank in effectively coined. So, it takes quite some pressure to cold coin a blank to make a badge. Something like a 100 to 150 ton press I believe. But, then you see some demonstration of a guy taking a large hammer and giving the die a huge whack, and he coins the metal. What is the difference between a drop hammer and a press? Sorry, I'm not much of a mechanical engineer.

    I've got a feeling it's blacksmithing that it is what I need to look into. Open die drop forging- maybe.

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    Another type of press used for this application is called a "fly press". They have a vertical screw driven by a flywheel that is spun manually. You get it spinning, it drives the screw down, and the impact is the inertia of the flywheel multiplied by the screw. Of course, the operator supplies the energy, but it is controllable by the amount he puts in. I have never actually made coins, but I have formed parts and have known people who did. For example, the now long gone Koste brothers, who knew their business, used a four post 100 ton hydraulic press.

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    What is tonnage figure about? One says, I want to coin a piece of copper sheet, x length by y length or x diameter. You might get back, that someone did a calculation and says you need 50 ton press. Do you really need a 50 ton press or do you need a impact pressure of 50 tons? What is the formula for pressure required for coining saying?

    I could do to read a book.:-)

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    Depends...if you want to do it out of copper at say 30 gauge for the detail relief, then put a back on it out of like 18 gauge, you could do the whole thing on a shop press. Make a negative, use around 30 tons or so (guess) and a urethane "punch".

    Moving metal is about force, faster the weight decelerates, smaller the force can be. F=MA. Accelleration is going to be huge (in a negative way) as you are going from a high rate of speed to zero in a very small distance (minting distance) and a lot of the kinetic energy is going to just get recoiled.

    Best bet...make the die, use hydraulic cylinders until you get what you want.

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    New member here! I make fantasy coins. I joined in order to try to find out more about Janvier and Augenstein reducing lathes (just bought one I'm trying to get running)... but noticed that Ries referenced my website in this discussion of presses and force required.(quoted below) The main thing I have learned is that you always want a bigger press. I don't care what you've got... as soon as you start using it you start to lust after something bigger. If you check out my workshop tour page you can see some of my presses. The blue manual screw press is supposed to be about a 40 ton. You can strike pennies all day easily, but trying to strike a quarter size piece is just about all a body can do. If you actually NEED 40 tons of force for a job, you WANT an 80 ton press to do the job. You never want to run a 40 ton press AT 40 tons. They give press tonnage as a maximum limit, not a safe working range. The Waterbury Farrell knuckle-press shown on the page has become my favorite press for almost everything. It's supposed to be a 150 ton press... but I do not run it on jobs anywhere near that pressure. At pressures of more like 70 tons it will just chug along all day long. I have struck more than 50,000 coins on it so far, and as long as I run it well below that maximum capacity it stays happy.

    I receive queries almost daily from people wanting to learn how to make coins, and the questions are always about the press. But the actual pressing is just the least and last of the processes involved. Blank-making is actually the most time-consuming and critical part of the process. Melting metal, casting bar, rolling to gauge, punching blanks, deburring and polishing blanks.... you have to do all that just right in order for the coin to come out right. And then of course there are the dies. After doing this for fourteen years, die-making is still the bottleneck. The striking of the coin in the press is simply the moment at which all the preparation of the blank-making and die-making literally come together.

    As far as WHY someone would want to make their own coins... there is a groups called the Unrecognized States Numusmatic Society which is devoted to making coins from... you guessed it... unrecognized states! My own Shire Post Mint specializes in making fantasy coins... that is, coins from fantasy places. Our main current project is making coins under license for the popular A GAME OF THRONES series by George R.R. Martin. So these are "coins" that are made so as to be FROM the fictional world, coins you can hold in your hand to help immerse in the fantasy world.

    Here's a pic of our most recent one... just started making these last week... dies are still up on the knuckle press.
    hand-06.jpg
    hand-07.jpg

    I'm thinking about writing a book. There are only a couple books out there that are of any use at all, most of them focus on the massive-scale processes used in big national mints. People doing hobby-scale coining need some guidance on what works and what doesn't. I've busted a lot of dies and ruined a lot of coins to figure it out... would be a shame to waste all that.
    Tom Maringer
    Shire Post Mint
    Home : shirepost.com


    Quote Originally Posted by Ries View Post
    Here is a website of a guy who does, indeed, mint his own coins. He has been doing it for years, in a slightly larger than home shop style- certainly no deep corporate or government pockets here- and he has found out what actually works. If he could be doing it with 3.2 ton presses, he would be, thats for sure. But his main two coining presses are 150 tons and 320 tons.
    Work Shop Tour

    Coining presses are usually pretty specialized beasts- they have a precisely adjustable BDC so that the exert full force at exactly the right height, and they tend to hit hard and fast. I have a buddy who has a 150 ton Italian jewelry industry coining press, and the thing is scary- it can be adjusted to hit with the desired tonnage, at the exact die height you want, and when it hits, you know it. Its fierce. Works really well, though.

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    And of course.... making coins is just a heckuva lot of fun!
    Last edited by tmaring; 06-30-2014 at 03:56 AM. Reason: oops... double post


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