I designed a 3D printable gauge/ruler that can be used to measure the string action on a guitar or ukulele. It features measurements in both millimeters and fractions of an inch. It can be optionally printed in two colours (does not require a multi-colour capable printer). Or print in single colour and use a sharpie to fill the depressions.
There is a generous 1mm fillet along the bottom edges. This helps with elephant foot effect not interfering with the measurements. The reference edge is at the top of the print, and will be unaffected by the elephant foot. The digits are fairly large, so that they would print quite legibly with the common 0.4 mm nozzle.
The gauge itself is 107 mm long, and 50 mm wide.
The depressions are 0.6mm deep, and the whole gauge is 2mm thick, so if you want to print with two colours, insert a colour change for the layer after 1.4 mm. For example, if printing at 0.2 mm, you would need to change colour at layer starting at 1.6 mm.
I printed using PLA, with a 0.4 mm nozzle, at 0.2 mm layer height, and it printed great.
Using the Gauge
Rest the edge on the frets, and see which rectangle best fits under the string. On my ukulele, in the above picture, the action is 2.25mm above the 12th fret.
The design is released under the Creative Commons Attribution NonCommercial license. Download the zip file with the STL, STEP, and Fusion 360 files below. If you find my design useful, you can buy me a coffee.