Ultrasound helps diagnose machining health

Ultrasound helps diagnose machining health

Researchers at Lasson Technologies Inc., Culver City, Calif., have developed an ultrasonic method to monitor and diagnose machine tool vibrations in real time. They have already demonstrated success at low spindle speeds, and they are working to refine th

Lasson Technologies simulated a milling operation at low and high spindle speeds to test its ultrasonic fluid probe.


Researchers at Lasson Technologies Inc., Culver City, Calif., have developed an ultrasonic method to monitor and diagnose machine tool vibrations in real time. They have already demonstrated success at low spindle speeds, and they are working to refine the technology for high-speed applications.

Today, many shops control vibrations by placing sensors on the machine tool and/or the workpiece. These sensors may include contact-strain gages/accelerometers, capacitive probes, inductive probes, fiberoptic-displacement sensors, laser-Doppler vibrometers, and acoustic-pickup devices such as microphones.

But these devices have limitations. For instance, contact accelerometers or other static-testing sensors won't fit on tiny cutting tools. And while optical probes work on rotating tools, they are best suited for fixed surfaces. That's because rapidly moving surfaces create speckle noise that overwhelms an optical probe's signals and significantly degrades sensor performance.

Realizing that working at optical wavelengths creates a speckle problem, Lasson considered ultrasound. "The wavelength of sound waves is 3 to 4 orders of magnitude larger than optical wavelengths," says Dr. Marvin Klein, Lasson president. "That means a surface that is rough to an optical beam could easily look smooth to an ultrasonic beam." Simply put, the speckle problem disappears. The other major issue is attenuation — basically, an ultrasonic wave launched in air tends to weaken. To compensate for this, the company directs a cooling-fluid stream at the cutting tool. This stream serves as a coupling medium to guide the ultrasonic wave.

In 2000, the National Science Foundation, Arlington, Va., gave Lasson the funds to test the feasibility of this idea. The research was successful, but the company only proved the concept worked at low spindle speeds — up to 1,600 rpm. In 2002, the Missile Defense Agency, Alexandria, Va., funded Lasson's development of an active ultrasonic fluid probe for vibration monitoring at higher speeds. The company uses a megahertz-frequency ultrasonic transducer to propagate an ultrasonic signal through the cooling stream. This signal reflects off the cutting tool back through the stream, where it can be detected and analyzed.

If the cutting tool does not vibrate, the reflected ultrasonic wave has the same frequency spectrum as the incoming ultrasonic wave. If the tool vibrates, the frequency spectrum of the reflected ultrasonic wave is shifted. Lasson researchers measure tool vibration by comparing the transmitted ultrasonic carrier with the reflected ultrasonic signals.

They further investigated this technique at various speeds using a rotary tool. Researchers gathered data with the tool remaining stationary, rotating at low speeds, and rotating at 20,000 rpm. Initial findings show that the increased noise levels at higher speeds don't interfere with the signals.

Lasson is now working to refine its ultrasonic fluid-probe technology and is working with Cincinnati Machine, Cincinnati, and Manufacturing Laboratories Inc., Las Vegas, a company that specializes in machine tool characterization and machine tool vibration monitoring.

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