This small mechanical revolutions counter was built to determine how fast a small steam engine, electric motor, or machine tool spindle is running. It uses a worm shaft and round gear with a 100:1 ratio. By timing ten rotations of the dial face with a stopwatch and dividing the number of minutes, it’s fairly easy to determine the rpm of any rotating object. The stand helps to align the device with the rotating shaft. Plans for this mechanical revolutions counter can be found in volume three of The Shop Wisdom of Rudy Kouhoupt.