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Coin Rings

Makezine and Hacked Gadgets are pretty cool websites (once you wade past the stupid sh*t people believe to be clever). I saw a video on Make showing Bre Pettis hammer a silver half-dollar into a ring. Interesting. Further research led me to this guy’s work. Judging from the accompanying link, Coin Cutting, I believe it safe to assume he’s a jeweler – if not professional, then at the very least a highly accomplished amateur. Very, very nice work, that.

I decided to try it myself by first using newer coins, then, once confident, moving to 90% silver coins (minted in 1964 and earlier). I found a dolly, a hand held anvil used for auto body work, for just a few bucks at the local Harbor Freight Tools. I also bought an el-cheapo 16 ounce ball pein hammer. My advice on tools in general: do not buy cheap stuff! For the most part you really do get what you pay for; quality tools will last multiple life times. I knew this, but did it anyway. The el-cheapo was a mistake for two reasons: it’s too heavy and after less than an hour’s work the head began rattling on the handle. The dolly works pretty well and while I do like the fact that the dolly’s rounded surface almost forces a rounded surface on the ring band, it, combined with the rattling headed hammer, makes avoiding off-center strikes difficult: I now have a Sacagawea(1) dollar coin with 9 flats. I can probably work it back into round, but I’m not holding out too much hope. The key here is to pay strict attention. If you need to look away from the work, by all means stop hammering! This seems like common sense, but you’re barely tap, tap, tapping and you get into a groove. I not going to test it, but I’m fairly certain if I struck my finger using the same force I’m applying to the coin, it wouldn’t much hurt.

As per the video, I drilled out the center mass, but found this more difficult and more hazardous to the ring’s finish than I’d expected. Whilst clamping it in a pair of vice grip pliers, I discovered the fine line between applying just enough force to hold securely and bending the ring out of shape(2). Also, I horribly gouged another ring’s surface when the drill bit jammed in the work and spun it out of the protective layer of tape. Two strikes for drilling. Time to find A Better Way.

I saw an old coping saw in one of my tool chests and decided this would be the better way: simply punch a hole, feed a blade, and saw away the center. I knew full well wood blades wouldn’t work. Do they make super fine toothed coping blades? More research led me to this discussion about using a jeweler’s saw. It seems that these tiny, tiny blades have a tendency to snap. Even more research found that there should be at least three teeth in contact with the work, so I calculated that I needed at least a #5 blade. This gal likes a #2 blade, but she’s working with fine wire while I’ll be cutting quarter and half-dollar and the occasional dollar coins, so a #3 should do the trick(3). Even though most swear by Herculese brand blades, I read that Pike’s were more forgiving. Now, Swiss watch movements are the standard by which all others are compared – they’ve for centuries cut minuscule gears using minuscule saw blades – so who better to know such things? Pike brand blades and a Swiss made frame it shall be. As for the hammer, I put the ball pein away and picked up a 16 ounce carpenter’s claw hammer. It’s still four-fold too heavy, but that’s what I have for now. Perhaps later I’ll look into a jeweler’s planishing or chasing hammer.

I ordered a 4″ x 4″ x 0.75″ steel bench block to replace the dolly, saw frame, #3 saw blades, and lubricant from FDJTool. I’d narrowed my vendor selection down to FDJTool and an outfit in Colorado, but went with FDJ (only because they’re located in the south; I like to keep the money as close to home as is possible). When I received the package I thought I’d made a mistake in vendors: they botched the order by sending a second frame instead of the blades. I had to send two emails and wait a full week for them to respond. That time, plus the weekend and several days for shipping, set me back nearly two weeks.

All’s well now, however. I got the blades and return-shipped the extra frame. Thus far I’ve sawn only one coin, a bicentennial quarter – the same one that was marred when the stuck bit spun it out of the tape. I believe this will work well as I didn’t need much pressure to keep the coin in the vice grip jaws. Instead of tape, I’m now using a strip of leather cut from a worn pair of work gloves. My problem now is that I don’t have a dremel sanding drum small enough to fit inside of the quarter. Need to stop by the local Ace Hardware, see if they have anything.

  1. If you have a gripe about the spelling, I got it directly from the US Mint web site. Send them your complaint.
  2. Yes, yes, there’s little problem with compression before drilling the center, but once a hole is drilled, there’s no longer any support. As you learned in physics class, the mass in a solid cylinder or beam adds little to the strength, so hollow cylinders and i-beams are used in construction due to tremendous weight savings. However, this leads to a different problem: catastrophic failure. While a solid bar will bend and bend, at least partially supporting the structure, a cylinder will bend until the compressed side eventually crimps. Once crimped, there’s little resistance to the force and it fails completely.
  3. The thinnest coin I expect to cut is a quarter. Quarters are in the neighborhood of 0.0685 inches thick. I need 3 teeth in contact with that cross section, so divide by three to get 0.0228. Divide 1 by 0.0228 to get a TPI calculation of 43.7956 (round to 44). This is a #5, so a #3 at 48 TPI will more than suffice.
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