Double-Strike Issue on Coin Stamping Drop Hammer
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  1. #1
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    Question Double-Strike Issue on Coin Stamping Drop Hammer

    Hello,

    This past winter, I set about restoring a 100lb drop hammer to be used at a business to strike coins from scratch. We have been testing with some pieces of aluminum, but have been getting a double-strike on the coins. As soon as the ram contacts the top of the die shoe, the first impression comes out great. However, it jumps a bit, making the coin rotate/move ever so slightly, and then falls back down, putting another image/half image on the surface of the blank.

    - I have tried adding a spring to the assembly, but then I have to raise the ram higher to overcome spring pressure. However, with the higher height, it still bounces and restrikes.

    - I've also thought about a bushing to fit over the die to match the size of the round coin blanks we will be using.. When the metal expands into the bushing, it'll get stuck a bit and prevent it from hopping out or rotating. That's the thought at least.

    - We've been toying around with the idea of a piece of leather/gasket material with a center hole to hold the blanks in the middle and keep them from hopping out. Maybe secure the piece of leather on four posts around the bottom die.

    I'm looking for any and all ideas on eliminating a secondary strike from the ram bouncing. I appreciate anything you guys can suggest.

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  2. #2
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    How high is the rebound distance of the hammer?

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    Quote Originally Posted by HuFlungDung View Post
    How high is the rebound distance of the hammer?
    Maybe a half inch or so.

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    Any dampening might help, perhaps.

    A thick leather base for the coin ?
    A rubber band on the ram ?

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    The solution is simple. You need a dead-blow action. The most effective and simple method is to add a cannister of lead shot to the moving part, preferably directly in line with the stamping die. If you already have max weight, you'll need to remove something to add the lead. More is usually better.

    If you've never used a (good) dead-blow hammer, try one and you'll be convinced.

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    I don't know how fast you want to crank these things out, but maybe a mechanical preload pressure could be applied to the die sandwich so that there is no intra die bounce clearance when the hammer hits. It may then double strike but the strike will be in the same place. I'm thinking of a long lever mounted on an eccentric to open the die against some die springs, which would be used to apply the pre-strike clamping action.

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    Since the rebound is so high, you have room to install a mechanical latch. Initial strike releases the latch, which rides on the side of the upper die (spring-loaded). When the upper die rises high enough, the latch moves inward and stops. When the upper die comes down from its rebound, the latch prevents it from falling all the way down.

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    The lead shot idea sounds like an easy low tech solution. I have seen similar applications where a weight is allowed to fall, strike, rebound, then at top of rebound when it starts to fall again it is caught, pretty sure it was by friction, not gearing, no idea what that type of mechanism would be called. Lifting the hammer back to top of stroke re-set the mechanism.

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    Quote Originally Posted by sfriedberg View Post
    Since the rebound is so high, you have room to install a mechanical latch. Initial strike releases the latch, which rides on the side of the upper die (spring-loaded). When the upper die rises high enough, the latch moves inward and stops. When the upper die comes down from its rebound, the latch prevents it from falling all the way down.
    The ram itself weighs 100 lbs. I don't quite understand where this would be mounted on the shoe. I understand the concept but can't picture it fully.

    @HuFlungDung - I'll try this first with C-Clamps and see if the concept works. We should be able to mount an arm on some sort of an eccentric. We are looking to crank these out at a reasonable rate, maybe 3 per minute to start with.

    @Gordon Heaton - There should be a cavity we can fill with lead shot. I'll give this a try as well.

    Couple points of clarification:

    -There are 2 dies, that are both engraved with a different image. We are striking both sides at once. They are the pieces of 1.625" round with a piece of gasket in between.
    -Ram weight is 100lbs
    -Inside rail to inside rail dimension is around 7.5" (parts that ram rides on)

    Thanks for the help so far. Looking forward to some further insight.

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    Maybe add a pawl to the arm so that on first travel down it passes smoothly, but the bounce up ratchets it so that on the way down the second time it stops. Going up from this position would then reset for the next full blow.

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    With a moving mass of 100lbs, you'll need a significant amount of shot (don't be tempted to use steel or other types, you need both the weight and softness of lead). There are many factors which influence bounce, including 'ringing' or 'springing' of the frame, the hammer and even the floor or stand its all mounted to. If possible, I'd start with 50% of the total hammer mass in shot. A wider, shorter column of lead will probably work better than a tall slim one.

    If your existing cavity is long and thin, perhaps there is room to add a short, wide 'annular' tub around the lowest part of that column, then add shot to both areas.

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    I though the dead-blow hammers had the lead in some sort of oil or other suspension, so that they would hit slightly after the initial strike, and hold down the bounce. Have I been wrong? It seems that just putting a bucket of lead shot on top of the ram would, in effect, just make a heavier hammer.

    I’m not arguing; I think the concept is right. I just wonder about the approach.

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    As a professional that has pounded on lots of stuff with steel hammers, lead hammers and dead blow hammers, I'd be surprised if the shot filled hammer will have the oomph to do the job without perhaps doubling the weight of the hammer to try to get an equivalent effect. The upsetting energy that is wanted for the die is going into deforming the shot instead.

    I have a 5 or 10 lb dead blow sledge hammer and never found it comparable to a steel sledge. I do like and use 1,2 and 3 lb dead blow hammers all the time.

    If a guy could latch the drop hammer somehow, that would be great, but I couldn't figure out exactly what sort of latching system would work.

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    Two things come to mind:

    One, I was given to understand that the "milling", or ridges around the circumference of a coin was in no small part to help hold the blank in place as it was being struck. IE, the initial blow distorts the slug into the closely-fit grooved outer ring, preventing it from moving. You might consider something like that.

    And two, I understand the drop-hammer setup already exists and is in place, but have you considered the possibility of a simple hydraulic press? I have no idea how one might translate X weight falling from Y distance into press tonnage required, but a cheap surplus 40-ton press can likely be had relatively inexpensively.

    Doc.

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    What about the gasket in the middle of the blanks? Isn't that acting like a spring? I would try an experiment with no gasket and see what happens with the rebound. It may not be something you can do in production, but it might give you some more insight into what is happening. If it helps, perhaps something less resilent could be used.

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    Is it possible the use of Aluminum is creating a vacuum, at strike point. Might try a material with less Plasticity.

    R

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    spring load the die so it doesn't care if the hammer bounces

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  24. #18
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    I wonder if a thick oil or light grease would hold the coin in place, seems to me it should have a little lube anyhow.

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    Quote Originally Posted by HuFlungDung View Post
    The upsetting energy that is wanted for the die is going into deforming the shot instead. . .
    Exactly! And this is why lead shot works so well. It is very slow to 'dust up' even after a great number of deformations, and that very deformation is what allows for the damping effect. It is true that a bit of velocity is lost as the shot compacts, but ultimately most of the force is delivered to the target.

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    I'm no expert on coining but isn't that a job for a coining press?

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