Summary

Hi,

This is a simple mechanical RPM meter I build and improved.
It uses a worm gear set with a high transmission ratio (1:50) in order to count revolutions.
All parts are designed with a rather high clearance in order to ensure that all moving parts run freely and don't influence the measurement.
The (friction) couplers for inner and outer diameters work, but are not ideal, because I wanted a 100% 3D printed object.

HOW TO MEASURE:
-Hold the device with respective coupler against a rotating part
-Start a timer (for example 10s or 30s) and count the number of revolutions the worm wheel made
-RPM=(revolutions/time[in s])x50x60s

I wouldn't recommend using this tool for more than 3000RPM, since you can barely keep track of the worm wheel at that point.

All parts can be printed with low infill, except the pin. With 20% or so it's not that strong, so you should rather print it at 50% infill. Platform adhesion (raft) is recommended here.

For better visibility you can fill in the gaps of the digits with paint/...