A system for monitoring the vibration of a power driven cutting tool during machining first produces a signal proportional to the vibration spectrum of the tool over a preselected band width. This is passed through a digitally controlled attenuator which is adjusted to provide an output signal at a preselected level. The degree of attenuation is stored, as is a reference value which is generated as a function of the attenuated signal. Then, during subsequent machining operations under the same machining conditions the attenuator is controlled to provide the same degree of attenuation and the reference value, which is in the form of a digital signal, is compared with the rectified and digitized output of the attenuator. When the latter exceeds the reference value a control signal is generated which either provides an audible or visual warning that the cutting tool is worn and therefore due for replacement or initiates an automatic tool changing operation. The system effectively monitors the wear of the tool and, in providing its own reference, i.e. the reference value which is generated, caters automatically for changes in the machining conditions such as use of a different tool or change in workpiece material.