Acoustic detection of tool break events in machine tool operations

Claim 1

A machine tool monitor for detecting cutting tool break events while machining a workpiece comprising: a broadband vibration sensor which generates an electrical signal representing vibrations at the tool-workpiece interface; analog signal processing means including a high-pass filter to attenuate lower frequency machinery noise, and a full wave energy detector to rectify and low-pass filter said signal; means for sampling the unipolar output signal of said analog processing means and converting each sample to digital form; and digital circuitry comprising means for comparing every sample with the running mean signal level of a given number of previous samples to detect the presence of a positive-going signal transient that may indicate a tool break event, means for comparing the mean signal level after and before said transient to detect a shift in mean level and a substantial change in background cutting noise, and means for generating a tool break alarm when the shift in mean level persists for a preselected period.

Claim 6

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.

Claim 7

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.

Claim 12

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.

Claim 13

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.