Grover mini-Locking Rotomatics 406BC Guitar Machine The introduction of the new Grover Locking Rotomatics gives you the ease of use provided by a standard tuner and the positive locking of our internal cam post. Simply insert the string through the string hole, pull to tension by hand and begin tuning. As you start to tune, only the locking cam in the string post turns. Once the string is locked, the string post begings to turn allowing you to bring the string to pitch. In less than one revolution of the string post, you will be in tune, to stay in tune. Gear ratio 18:1.