Acoustic detection of tool break events in machine tool operations

A system for detecting cutting tool break events while machining a workpiece comprising:

• an accelerometer which is most sensitive to frequencies around a resonant frequency and is positioned on a machine tool to sense vibrations at the tool-workpiece interface and convert these and other vibrations to an electrical signal;
• an analog preprocessor including means for high-pass filtering said vibration signal to discriminate against lower frequency machinery noise, and means for rectifying and low-pass filtering said signal to detect the energy in a band around said resonant frequency;
• means for sampling the unipolar output signal of said analog preprocessor and converting each sample to digital form;
• digital circuitry to detect major tool break events capable of marring the workpiece and prevent false alarms on minor tool break events and spurious noise;
• said digital circuitry comprising means for calculating the running mean signal level of a selected number of samples, a transient detector to compare each new sample with the running mean and detect a transient increase in signal level that may have its source in a major tool break event, a mean shift detector to compare the mean signal level after and before said transient to detect a shift in background cutting noise, and means for checking that said mean shift persists for a given period and hence generating a tool break alarm.

The method of detecting tool breakage capable of marring the workpiece comprising:

• sensing the vibrations of a cutting tool insert during a machining operation and converting these and other machine tool vibrations to an electrical signal;
• preprocessing said vibration signal by high-pass filtering to discriminate against lower frequency machinery noise, and rectifying and low-pass filtering to detect the signal energy, the output signal containing a positive-going transient that may indicate a major tool break event;
• sampling said output signal and converting each sample to digital form;
• detecting said transient by comparing every sample with the running mean signal level of N previous samples, comparing the mean signal level after and before said transient to detect a mean shift and dismissing any transient not followed by a substantial change in background cutting noise, checking that said mean shift persists for a given period and dismissing any transient not meeting this second criterion, and generating a tool break alarm when both criteria are satisfied.