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While you can use any sort of coin, it’s best to start with larger ones, such as a quarter, unless you’re making a ring for smaller fingers. The coin doesn’t need to be new—it can be spare change you already have around the house. Make sure the coin isn't valuable before deciding to turn it into a ring. Look for a flat piece of heavy metal, a concrete floor, or any other solid, hard surface. This will be your base for hammering, and you’ll hold the coin on top of the hard surface.  Choose a surface that won’t be damaged by a hammer. A bench vise is another option for a flat, hard surface. Hold the coin using one hand so that it’s sitting up straight with a sliver of an edge touching the hard surface. Use a hammer to gently hit the edges of the coin, rotating the coin slowly as you hammer so that all of the sides are hit evenly. Continue hammering until the coin reaches the thickness of a ring.  Make sure your fingers aren’t in the way of the hammer as it’s hitting the top of the coin’s edges. How thick you want your ring to be is entirely up to you—roughly 0.5 cm (0.20 in) wide is good. Thicker rings will take longer to hammer, so be patient during this step. Avoid hitting the coin with a lot of force, as this could turn it into an oval or damage the overall form. Make sure the piece of wood is thick enough that the drill can go down into the wood without messing up the surface underneath. Place the coin in the center of the piece of wood, and either hold the coin with your fingers or use an adjustable wrench to keep the coin steady. If you’re holding the coin, hold the very edges so that your fingers won’t be in the way of the drill. Hold the coin steady as you drill the hole. Start with a smaller gauge and drill all the way through the coin. Switch the drill bit to a larger size, then drill through the metal again. Once you’ve drilled the hole, the coin should be left with roughly 0.5 cm (0.20 in) of coin thickness around the entire edge. If you have a sanding wheel that attaches to your drill, this is great for sanding out the rest of the coin to create a smooth ring. Hold the coin steady using your fingers or some other sort of clamp as you sand the inside of the ring until it thins out.  A dremel tool with an attachable sanding wheel is also a good option. Continue sanding until the coin is the thickness you want your ring to be. Attach a polishing wheel to your drill or dremel, using a small amount of polishing compound to get the best shine. Run the polishing wheel along the inside and outside of the ring, smoothing out all of the edges so the ring is shiny and completed. To use polishing compound, dip a rag or polishing wheel into the container of polish, using a small amount to rub over the ring's surface.

Summary:
Find a coin you’re willing to make into a ring. Hold your coin on a flat, hard surface. Hammer the edges of the coin while rotating it slowly. Place the coin flat on a piece of wood. Drill a large hole in the center of the coin. Grind out the middle of the coin using a sanding wheel. Polish the ring using a polishing wheel or cloth.