How could you know a struck fake made with a CNC die?

Discussion in 'Ancient Coins' started by Insider, Sep 2, 2020.

  1. Insider

    Insider Talent on loan from...

    A member of CT recently asked me that question. Without giving away any trade secrets, I believe the answer is fairly easy. In fact, members of this forum seem to do it often. Can anyone here enlighten him of one of the most basic "naked-eye" methods of counterfeit detection used for decades that still works on excellent fakes even today. Don't worry, the guys making the fakes already know about it.
     
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  3. Kentucky

    Kentucky Supporter! Supporter

    What is a CNC die?
     
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  4. finny

    finny Well-Known Member

    I don't follow - why keep "trade secrets" and not just teach us?
     
  5. Kentucky

    Kentucky Supporter! Supporter

    Cause he's a sadist... :)
     
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  6. Iepto

    Iepto Active Member

    CNC - Numerical Control, a manufacturing method used for machining pieces. It's an automated tool process which you program to make pieces in a replicable manner (/not by hand).
     
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  7. Kentucky

    Kentucky Supporter! Supporter

    Thanks
     
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  8. Inspector43

    Inspector43 Celebrating 75 Years Active Collecting Supporter

    A CNC cutting tool would leave minute marks that might well be detected. The part is being turned rather than struck.
     
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  9. finny

    finny Well-Known Member

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  10. Suarez

    Suarez Well-Known Member

    I agree with this. There is no current tech that can replicate the level of detail from a coin simply by copying the contours of another. A high end CNC has a precision of about 0.3mm (I could be wrong here but probably not by THAT much). There is another process that I think is even more accurate, EDM, which I believe has a maximum accuracy of 0.1mm. Maybe we could talk about electroplating which yields a foil negative that is fairly accurate but I think it's in the order of 0.1mm too. None of the electroplated samples I remember seeing stood out as especially convincing - and never mind that this process has a very limited "palette" of materials.

    In any case, on a coin a 0.1mm margin of error stands out like a sore thumb. The unaided eye can make out details just 0.04mm apart. A coin sculpted at 0.1mm would look like a cartoon of the original. The "troughs and peaks" of flow metal are probably on the order of 10μm - much smaller than the diameter of a hair but still easily distinguishable when you rotate a coin in light. There is no metallurgical process that can tackle this. None, at least, that are used commercially.

    Maybe if they adapted the stylus of a tunneling electron microscope the data could be encoded to make a near-perfect reproduction of a coin's surface but that's still only one half the puzzle. How do you reproduce that level of detail? Maybe vapor deposition? Molten metal inkjet printers??

    Please someone fact-check me. It's late and I'm probably missing something important :-D

    Rasiel
     
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  11. Ed Snible

    Ed Snible Well-Known Member

    My local coin club recently had a custom medal struck by https://www.medalcraft.com/ . We had an artist create plaster models, 12” in diameter, which were laser scanned to produce a digital model that drove a CNC *milling* machine to produce 2.5” dies. The resulting dies, although they were of sublime beauty, were not perfect fidelity copies of the plasters.

    I have heard that electrical discharge machining (spark eroding) can do a very deceptive job. I have been unable to locate photographs of coins or medals produced with CNC EDM machines as part of the process. I would love to see examples, if such work has ever been attempted, either by artists or forgers.

    If you have ever seen electrotype copies of Greek coins they are sublime and the quality is excellent. I would be especially curious to see a comparison of electrotype copies and copies with EDM as part of the process.

    Dies made from genuine coins are called “transfer dies”.

    Dies can be cast from genuine coins. The resulting dies produce coins about 1-2% smaller. The quality is higher than CNC milling.

    Here is an extreme enlargement of one of the side figures on the reverse of my club to show the difference. In hand, the medal looks great, but careful side-by-side inspection, especially with magnification, shows differences.
    cnc-detail.jpg
     
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  12. dougsmit

    dougsmit Member

    I somehow believe that someone 'inventing' a technique that would replicate the required level of accuracy would be able to find enough money in legitimate uses that they would not have to make coins. For most of us, CNC will be a way of setting free the eagle captured inside a piece of wood:


    I would expect that laser technology might come closer by evaporating parts not required for the coin. Nothing is impossible but miracles do take longer. Will fakes kill the hobby? Will we have more and more items where 'experts' can not agree whether the item is good or bad? Our hobby will continue to change well after any of us need to worry about it.

    I feel a great question is whether we will learn from our past mistakes or continue to make the same ones over and over again. Who remembers the Black Sea Hoard?
    http://coinproject.com/jan/volume1/issue4/volume1-4-6.html
     
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  13. ernstk

    ernstk Active Member

    Thanks for detail answer but curious. 1-2% smaller size for a coin that has diameter of 25mm would be immeasurable. isn't it??
     
  14. Ed Snible

    Ed Snible Well-Known Member

    In Wayne Sayles' book _Classical Deception_, page 55, he talks about a disagreement between authenticators was resolved in 1963 when the fakes were shown to be 2% smaller than other coins of the same die.

    In other words, highly deceptive in the 1950s

    Sayles' book doesn't say how they were compared. I would guess photographs printed on transparencies but even that is hard -- the photographs must be taken precisely. Using a comparison microscope?

    Usually fake detection doesn't reach this point because the forgers take shortcuts and do things like incorrectly touching up dies created from off-center coins.
     
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  15. ernstk

    ernstk Active Member

    The novice ones yes but there are very good ones out there who create a whole class of professional works that even big auction houses get caught by them.
     
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  16. Suarez

    Suarez Well-Known Member

    438863.image0.jpg
    Scale is the key. A 1% difference is unnoticeable or a gaping error depending on your vantage point. Six inches away a ±.25mm difference against a 25mm reference piece is obvious at first glance but much harder to tell apart at arm's length.

    Rasiel
     
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  17. ernstk

    ernstk Active Member

    I doubt any human even with magnifier can detect 1% difference in size or weight
     
  18. Suarez

    Suarez Well-Known Member

    I was wrong about this. A CNC can easily do better than 0.1mm with the right stepper motor. The functional limit is the stylus. The thinnest router bit I'm aware of is a 1/64" (which works out to 0.3mm). Your CNC therefore is limited to this "resolution".

    You can make a thinner bit of course but so does the maximum pressure you can apply before the bit hits the breaking point. I don't think there is a material that has sufficiently high tensile strength AND hardness to be able to carve detail into steel at <0.1mm.

    However, there is another method I didn't think about. You can just use a fiber laser machine. With a resolution of just 0.01mm(!) I think that could do a convincing job of replicating fine detail at least to pass an unaided eye test. Just one problem though.... the laser does the carving by burning away material. Good detail but scorched black. So not so good for forging coins after all!

    Rasiel
     
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  19. Suarez

    Suarez Well-Known Member

    Well, call me superhuman then but I work in this field and I can tell when something is off by 1%. You can too. Try putting a 10.1mm peg in a 10mm hole. No magnifier needed.

    Rasiel
     
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  20. Alegandron

    Alegandron "ΤΩΙ ΚΡΑΤΙΣΤΩΙ..." ΜΕΓΑΣ ΑΛΕΞΑΝΔΡΟΣ, June 323 BCE

    LOL, nailed it.
     
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  21. Ryro

    Ryro Trying to remove supporter status

    Why are we sharing trade secrets on how to make fake coins on a public forum or any forum for that matter?
    Keep honest people honest. And our coins authentic.
     
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