I’m curious to see if I can make this work: a coining die so I can create small copper challenge coins in my little 20 ton press. Every year, I try to create an art object as a fundraising opportunity for a local knapping group I attend. August is our annual “knap-in” at Knapadonia, located on the North end of Whidbey Island, near Seattle. Hopefully these little challenge coins will raise funds for “Krapadonia” – the porta-potties we rent for the attendees.
Here’s the start of creating a CNC’d coin die out of 12L14 Ledloy steel. I’ve chosen this particular steel because it will machine well in my tiny little Nomad 883 desktop CNC mill, and because I have it on hand in the correct diameter. As so many have pointed out in my Facebook and Instagram posts, yes, harden-able tool steel would make a better, longer lasting die. Also, as several have pointed out with the authority of experts, that my tiny 20 ton press won’t work, that a 100 ton press would be the smallest that would be capable of such a feat. Now, with the hindsight of having done the “impossible,” I’ve rediscovered what I learned so many times before. It seems I’ve spent my life accomplishing the impossible, only to find out that I’m not particularly extraordinary, just that “impossible” is simply the state of someone else’s mind. I’ve also found that a closed mind and an open mouth seem to be natural partners…
The blue painter’s tape creates a little well for containing a puddle of coolant. I wrap the copper wire around the base of the painter’s tape in case the coolant I’ll use (WD-40™) decides to attack the glue of the tape. Have I mentioned I’m a belt and suspenders kind of guy?
Here’s the die blank installed in the CNC mill, with a small puddle of coolant. Below are the Feeds and Speeds I used for milling the steel die.
Nomad 883 Feeds and Speeds (mild steel)
RPM Depth of Cut Feed Plunge
8800 0.002 inches 3.0 inches/min 1.0 inches/min
Of course, the die isn’t the only bits I’ll need to pull this off. I’ll also need a flat faced steel pusher and a large tube to keep the pusher, copper coin blank and die all aligned while I apply 20 tons (40,000 pounds per square inch) of pressure.
Above is the first pressing from the challenge coin die. I had a 3/4″ circle of 1/4″ thick annealed tellurium copper for the first attempt. I’m going to call this one a failure…while it looks good, it took 7 or 8 annealings and 20 ton pressings to get a complete impression. Too much work, so I’ll make another shallower die and try again.
Above is the new die with a shallower arrowhead milled into the surface, hopefully to take fewer pressings to obtain a good impression. I’ve also engraved the lettering around the edges. BUT, oops…do you see the problem? Talked myself into it, out of it, and into it again. That’s what I get for doing critical work when I’m tired…
Above, I’m using my Lindsay Palm Control Airgraver to hand engrave the lettering for the coin.
Above, the resulting copper coin after just 2 pressings and annealings. I’ll take that – time to make the lettering for the back and start production… It kind of looks like an ancient Greek coin with the backward lettering…
So, I lathe-turned away the lettering, at the same time making the arrowhead a little shallower – probably a good thing! Above, I’m engraving the new lettering, properly mirror-imaged on the coin die this time.
Here’s the production version of the Knapadonia Challenge Coin die system, with all the bits and bobs of the die system. In order to get by with the smaller press and multiple pressings, I needed two different steel pusher cylinders. There was no problem re-indexing the front face (arrowhead) of the coin after the first pressing, but I didn’t have much faith that I would be able to index the reverse side die, so I made the first pressing with the blank faced steel pusher, then re-annealed the coin and used the engraved “not all who wander” steel pusher for the second pressing. I also made the first pressing while the copper coin was red hot with the smooth pusher.
So, now it’s time to begin coin production. I weighed out multiple bits of clean scrap copper, approximately 0.4 ounces each. I determined the quantity by weighing the original tellurium copper scrap blank that just happened to turn out about perfect. A blind-squirrel moment for me (even a blind squirrel finds a nut once in a while)!
Above I’m using an oxyacetylene torch to melt the scrap copper into a (mostly) round blob, then into the pickle for quench and copper skin oxide removal. Don’t forget to use your proper light filter googles – the acetylene flame is dangerously bright and can burn your eyes.
Here they are after hammering flat. I test each one agains the aluminum alignment tube to make certain they aren’t to large in any dimension to fit inside the tube. Finding out they’re too big while glowing hot isn’t the best of times to try and adjust their diameter…
The Knapadonia Challenge Coins are complete. Most copper, a few in silver. It took two pressings each in a 20 ton press to get a full impression. They kind of look like I just dug up a hoard of ancient Roman coins. Turns out the impossible isn’t impossible if you understand a little metallurgy and use a bit of ingenuity…
Thanks for Looking!