Put a quarter on your finger for style and for use at those .25c candy dispensers, remember those?
In this example I use Rhino for CAD and a quarter for my coin. This could easily be done with other coins of any size and any program as the steps are fairly simple.
Step 1: Choose Your Coin and Find the Right Dimensions
Using a US quarter for my coin, I looked up the correct dimensions at the US Mint website and created a 3D model equivalent to these dimensions in Rhino. I used millimeters, which helps when working with smaller objects, and also because most 3D printers print in mm.
I then offset this shape by .2 mm, to account for small discrepancies from the digital world to the real world. This new shape can now be booleaned into the part of the ring that will hold the coin.
Step 2: Create the Coin Holder
The coin holder in this example was meant to keep the coin in place but allow for it to be seen. The attached model has the exact shape but any shape is possible at this point as long as it holds the coin in at two ends. This is because the design with eventually be cut in half to allow for the coin to be removed.
I added an additional 1.5 mm for the base. I made the base thicker to have a stronger connection to the ring, which will experience the most torque.
Step 3: Find Your Ring Size
If you dont already know it then you need to do a bit of work. A good way to test is measure the finger you would like to wear the ring on by warping a string around it to find the circumference. Using simple math [ C=d*pi ] you can find the diameter and create a few 3d models using a mm below and above that number. Doing some test prints you can hone in on which size fits best. This is your ring size.
Step 4: Put It All Together
The next thing to do is make it easier for yourself to print the object. In order to do this the object needs a flat side. Boolean the ring to the existing coin holder and split the object down the middle, making two symmetrical pieces.
The entire process could have been based off of one shape printed twice but visualizing the entire object allows for modification after a successful print.