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Old 04-07-2009, 01:16 PM
qwasty qwasty is offline
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Join Date: Apr 2009
Posts: 18
Default Re: The Coin Collet! Interested?

I'm a machinist, and I've got a cheap and unbelievably easy solution that will work for many needs:

Use a low melting point bismuth alloy to basically mold a custom vise for each coin. It's often called "Field's metal", and make sure you get the non-toxic variety. It will melt in hot water, and you can literally hold it in your hand while it's molten. The bismuth expands when it cools, so if you do it right, it'll get a tight grip on the edge of your coin.

It'll take some experimentation to get this right, but you can remelt and start over if it isn't working. Here's how I'd try it first:

1. Place your coin engraving side down on a very soft silicone mat.

2. Place a short (1 inch?) piece of square pipe centered over the coin that's at least 3 times the diameter of your coin. Bigger means less flex and more clamping power on your coin edge.

3. Place a piece of scrap metal on top of the coin to weigh it down and press it slightly into the soft mat. The heavier the weight, the higher the coin will lift above the molten "vise", and the better the rim sealing to keep the molten metal off the face of the coin. The trick here is to keep from tilting the coin too much (a big deal for machining, but not so much for engraving), and to not cover up too much of the coin with the weight so that the molten metal can flow into and grip the design details of the coin for extra holding power.

4. It might be a good idea to weight down the square pipe a tiny bit only if your molten metal leaks out.

5. Melt your metal in some hot water (or whatever way is convenient) and pour it into the square mold you just made!

6. Once it's cool, you'll have a square metal block with a coin face sticking out the top of it. The square shape makes it easy to clamp into your engraving vise.

7. You'll have to get creative to make this setup cost more than $100, and you can do it with any kind of part you want. You'll be able to engrave things that were too small, delicate, and oddly shaped before you had this technique.

8. Have fun, and answer all my newbie engraving questions!

9. Here's some links:

http://en.wikipedia.org/wiki/Field's_metal
http://www.google.com/search?q=fixturing+alloy

Buy it here:

http://www.mscdirect.com MSC #: 00263590

10. For edge engraving, try wrapping the coin in a silicone mat tube so that if you look in the ends, you see the coin faces, then pour your molten metal in each end, and stuff something flat in there so when it cools, it'll have a nice flat face on the metal block. When you remove the silicone wrapping, be careful not to seperate the two halves from the coin faces, and now you can clamp it into a kant twist clamp or a vise, or whatever withoight damaging the coin face, and you'll have access to the entire edge of the coin. This method requires more thought and experimentation, but you could end up with better results than any other method that requires you to put clamp pressure on the faces of the coin. For that I'd clamp with sandpaper, but that'll damage the coin. Try cotton cloth, hard rubber, or plastic in a C clamp if it gives you rigid enough holding for engraving purposes, with no marks, and full access to the edge in one setup.

That #10 was an after thought, so I have no idea how well it'll work for engraving. But, if you engrave the edge first, the molten fixture will grip the coin very tightly, for when you engrave the face.

I bet some of the machinists here could put together an excellent coin engraving fixture kit with the info I have here, along with a pre-fixtured coin! Wouldn't that be cool?

Last edited by qwasty; 04-07-2009 at 01:30 PM. Reason: messy #10
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