Thread: 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

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

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

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

leave out the thickness

Redo your math

Originally Posted by asifshiraz

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

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

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.

@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.

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.

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.

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

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.

Originally Posted by asifshiraz

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!

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

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.