A rad radius gauge for measuring radii of around 40 mm and bigger. Use the free web app calculator to calculate the result, it's super easy!
UPDATE 2: Added versions without the shelf in the hole. Now it should be able to take calipers that has a vertical bracket across the depth probe rather than a screw. This is not really optimal, as the lack of the shelf will allow for some wobble, but you'll just have to hold it in place while measuring.
UPDATE 1: Added a 30 mm version for measuring smaller radii.
The companion app and instructions for use:
Now this is made to fit my digital calipers, which is pretty standard. The main shaft is around 16 mm x 4 mm, and it has a screw to keep the depth probe in place. To test if it fits, you can first print the small test piece, and then the complete model if it fits. A bit loose is ok, it's easy to keep in place while measuring.
If it's too tight vertically (z-axis), you could try to scale the z-axis in your slicer. Don't scale the whole thing, or the measuring distance will be off.
It's probably pretty easy for me to make another version for a different set of calipers, please let me know, and I'll see what I can do!
Please post a Make if you make this!
You might want to slow down you print speed for better accuracy.