has a resonant frequency of about 60 khz and useful response up to about 70 khz other accelerometers can be used the output signal from sensor 10 is applied to a band pass filter bpf 12 which has a passband generally selected to pass frequencies indicative of abnormal tool conditions e g a broken or worn tool and reject frequencies indicative of normal machining and background noise typically this passband is between about 30 to 70 khz although other passbands can be used depending upon the material of the tool and workpiece the cutting speed etc the signal from bpf 12 is applied to an energy detector comprising a full wave fw rectifier 14 and a low pass filter lpf 16 an fw rectifier is preferred since it has a high sensitivity however other types of rectifiers e g a half wave rectifier can be used in turn the unipolar output from rectifier 14 is applied to low pass filter lpf 16 which typically has a cut off frequency of about 500 hz the output signal from lpf 16 which has a time varying amplitude representative of the varying energy in the signal from bpf 12 is applied to an analog to digital converter adc 18 which has a typical sampling frequency of about 2 khz it will thus be appreciated that lpf 16 in addition to its function as part of the energy detector is an anti aliasing filter for adc 18 if desired these functions can be.