Abstract: A channel is formed in the body of a machine tool holder, this channel extending to the area where the tool is working on the workpiece. A pneumatic flow away from the tool is established by means of a vacuum source connected to the channel. A pressure transducer is connected to the channel so that it is capable of sensing the pneumatic pressure therein. The transducer produces an electrical output in accordance with the sensed pressure, this electrical output being suitably processed to provide a control signal capable of shutting down the operation of the machine tool and simultaneously providing a warning signal.