Acoustic tool break detection system and method

An acoustic monitoring system to detect cutting tool break events while machining a workpiece comprising:

• a broadband 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;
• analog signal processing means including means for high-pass filtering said vibration signal to discriminate against lower frequency machinery noise, and means to rectify and low-pass filter said signal to detect the signal energy in an acoustic frequency band between 30 KHz and 100 KHz;
• means for sampling the unipolar output signal of said analog processing means and converting each sample to digital form;
• a digital processor to detect major cutting condition changes occurring gradually over a period of time that can damage the workpiece or machine tool;
• said digital processor comprising means for calculating the running mean signal level of a selected number of samples, means for activating the lower threshold test of a threshold crossing detector after the mean signal level rises above a much higher flag-set threshold and remains there for a preset number of consecutive signal samples, said detector determining when the mean signal level crosses lower or upper break detection thresholds where the former is just above the continuous traverse noise level of said machine tool and the latter is just below the system saturation level, and means for testing that the mean signal level persistently remains outside of said thresholds, and generating a tool break alarm.

The method of acoustically detecting tool breakage capable of damaging the workpiece or machine tool comprising:

• sensing vibrations at the tool-workpiece interface and converting these and other machine tool vibrations to an electrical signal;
• preprocessing said vibration signal to discriminate against lower frequency machinery noise and rectify and low-pass filter said signal to detect the signal energy in an acoustic frequency band below 100 KHz;
• sampling the processed vibration signal and converting each sample to digital form;
• and
• analyzing said signal samples to detect gradually occurring, but substantial, cutting noise changes due to tool break events capable of damage, comprising setting upper and lower cutting noise thresholds, determining when the running mean signal level of N samples drops below said lower threshold, which is just above the continuous traverse noise level of said machine tool, and generating a tool break alarm after a preset number of consecutive signal samples and the cutting noise are outside of said thresholds.