Summary

Stackable outer radius measurement tool, 0.5mm to 20.5mm in 0.5mm increments.

Radii can be hard to measure with calipers. This tool makes it easy to accurately measure outer radii by observing the gap between an object and the measurement tool.
The disks are kept small, to make the tool fit in almost any situation. The disks are stackable, the smallest disk has an M8 thread, the largest disk functions as nut. The second largest disk has a larger hole, to accommodate a two-disk thick nut.

For more accurate measurement, a straight surface is included at each end of the 90 degree radius, and the start and end of the radius are marked.

The used font was designed to fit a Sharpie, but filling the numbers is a lot of work. I recommend printing in a dark color, and when about half the embossing depth is printed, pause the print and switch to a lighter color.

Printing scale is obviously important. Thin parts stick to the build-plate and typically shrink a bit less than larger parts.
Only the .stl file already has 0.4% scaling applied, to compensate shrinkage. I used ABS, for PLA you may need to scale it down 0.1% (scale by 0.999 or scale the original files up by 1.003). The expected length of the largest part (from the end with the notch) is 55.63mm.

The 0.5mm increments yield ~2.5% accuracy at 20mm radius. Shrinkage (~0.2%) or extrusion variations should not affect the usability of the tool. Excessive over-extruding or under-extruding may affect the radius, and any burr remaining on the print-bed side must be removed.

I recommend a small layer height for a smooth thread. The bolt is in tension, so avoid over-tightening to preserve layer adhesion.

Designed in Rhino3D, the original .3dm, as well as .stl, .obj, .stp, and .igs files, are included.