Chatter vibrations during machining operations are large-amplitude self-excited vibrations, which result in the loss of physical contact between the workpiece and the tool. This paper presents an electric contact sensor, which is used to detect the cutting and noncutting periods during the operation. The spectrum of the signal of the electric contact sensor is free from environmental noise and can be used to select the chatter frequencies as the boundary of stability is approached. The detection is automatic with the use of an analog phototransistor that helps the time-periodic sectioning of the measured signals. The presented approach makes chatter detection possible before harmful vibrations arise.