Parametric sizing ring for US ring sizes

Yet another ring sizer for US ring sizes. Why another? I needed to not only find my ring size, but I also wanted to know if I would be okay with wearing a ring on the finger in question, so I wanted it wearable. Also figured that spending some time with it on my finger would be more telling than just sticking my finger in a ring and taking it out. This turned out to be true as I thought an 8.5 was fine, but in wearing it, I realized it was a little too big.

This one is wearable and has the sizes marked on it in both US ring size and inner diameter in millimeters. Plus, with the Fusion 360 file you can key in any US ring size and generate a ring with the appropriate markings. (I've documented how to do that below)

How to use

I've made all the sizes from US 4 to 13.5 available, which should cover a broad range. That's at least the range covered by most online ring size charts. If one of those is what you're looking for, then just grab the STL and print that, or, print a couple in the range around that size. Print settings are pretty much whatever you want, but they're so small there's not much you can change beyond layer height. It's designed to work with at least a 0.2mm layer height, but smaller would work as well.

Note that the text is very shallow so won't be very prominent, and that's by design. I find it can be ready fairly easily depending on the filament, with good lighting making it easier. Of course if you disagree you can 

Size chart

Here's the chart showing the diameter for each size. Note that if you check for these charts online, they'll usually be within a tenth of a mm or so, but rarely agree across the board. This doesn't appear to be a well standardized thing.

Custom sizes in Fusion 360

If you need a custom size, download the F3D file and import it into Fusion 360. Before you open it, make sure you download and install the Parametric Text plugin from Thomas A, as it's key to updating the text size and diameter markings.

Once you have it imported and the Parametric Text plugin is working, open the imported file. 

• In the solid modeling tab, choose Modify, Change Parameters:
  
   
• This will open the parameters window:
  
   
• Change “RingSize” as necessary. You can also change the ring width from the default of 5mm.
   
• The design will resize immediately, but the text markings will show the previous size. Click “OK” to close the Parameters window, and after a moment you will see the text update as the Parametric Text plugin does its thing.
   
• Export the file for printing!

Design background

Warning, this gets unexpectedly nerdy! You've been warned!

I wanted this ring because I'm a member of the Order of the Engineer, and had long ago lost my ring. I wanted to get the replacement ring, but I also wanted to make sure that wearing a ring on that finger wouldn't bother me. I used to wear watches but now don't as it feels strange, so no point getting a replacement ring if I wouldn't tolerate wearing it. So, I wanted something that would allow me to check the size, but I could wear like a normal ring, which precluded most of the gauges out there. So, I found a chart with the sizes, took a quick measurement with my calipers, and printed out a couple rings in those sizes. This turned out to be good because the ring that I thought fit well turned out to be too loose when wearing it all day, and the one that felt too tight was actually what I wanted.

Playing around with ring sizes and diameters, I got curious what the relationship was between the ring size and the actual diameter - if there was one. So, I copied an online ring size chart into a spreadsheet, and graphed it, and discovered that it was pretty much a straight line:

If it's a line, that means there's a formula for the conversion. I did a bit of searching and found nothing, so then looked at the spreadsheet, and discovered there is a “LINEST()” function that takes an X series, a Y series, and gives you the gradient and intercept of a best fit straight line, such as the equation: Y = G * X + I

Now that I could have an equation, this meant that I could drive the ring diameter directly by entering the US ring size number. Feeding in several more ring sizing charts, I settled on a G of 0.825 and an intercept of 11.55. Once you've got an equation, making the ring fully parametric then became really easy. Dig into the Fusion 360 file if you're curious how I did it.

I did run into some challenges on the original design, whenever I updated past the original size 8, I ran the very real chance that Fusion would crash. It seemed that either Fusion or Parametric Text (or the combination of both) didn't like updating the debossed text on the torodial surface of the ring. To deal with that, I first made a square ring with text on the ends, but that looked dumb. The next attempt was a ring with the torodial surface in between two cylindrical surfaces. I managed to get that working and had it updating from size 3 to size 13 with no issues, and almost posted that design, but I didn't like how it felt as an actual ring, so I gave the torodial design another try, starting from scratch. This time I kept everything as simple as possible and wound up with a nice torodial surface ring that updates without crashing Fusion.

Note that the two cylindrical ones in the photo were very early prototypes that I ran off on an older printer with older filament, which is why they look like junk.