IMPAX-SK and Vibrasonic Acoustic Sensors

By: Ed Evensen and Jerry McCabe, of Process Technologies Group, Inc., and Thomas Kopka and Anton Schwer, of Schwer + Kopka GmbH
Topics: Acoustics, Wire Cracks, Die Wear, Sensors, IMPAX-SK

Using Ultra-High Frequency Acoustic Emissions to Monitor Fastener Making Machines

Acoustic sensors For monitoring acoustic emissions

Introduction

The current era of "zero defect" quality requires that production faults must be recognized and eliminated as early as possible during the manufacturing process. This applies particularly to the fastener industry where most end users have introduced automatic assembling processes. Every single defective fastener immediately causes costly stoppages of automatic manufacturing lines. In addition, fastener-making machines operate at very high speeds, making it even more important to incorporate the quality control into the process itself.

For a good number of years, electronic process monitoring systems have been used on fastener producing machines such as headers, thread rollers, and multi-die presses. These systems typically measure the forming forces, and compare them to learned target values. The introduction of the "envelope curve" technique to process monitoring was a major breakthrough to improve the monitoring accuracy as compared to conventional peak force monitors. Still the assessment must be made, that certain type of faults remain undetected even with the sophisticated "enveloping" techniques.

Small defects such as cracks, tool chipping, minute die failures and even tool wear are demanding new approaches from process monitoring systems.

The New Approach:
Acoustic Emission Monitoring

Before the implementation of electronic process monitoring system, operators used to listen to the sound of their running machines. A change in the sound of each machine told the operator whether there was a problem or not. Today, this source of information is often no longer available due to the fact that many of today's modern machines are fully enclosed for noise protection purposes. On the other hand, this information is no longer necessary because conventional monitoring systems are quite capable of detecting, all common defects. Still this "listening" technique reminds us of the acoustical characteristics of a process. New developments suggest the use of "Acoustic Emission Monitoring" to be the decisive step toward zero-defect production in fastener making. Scientific research has shown that tooling failures such as breaking, chipping, cracking, or wearing will emit strong acoustical signals, which in turn can be monitored for process deviations.

Acoustic emission can be described as a type of energy waveform that will travel and expand through any elastically or solid structure. Metal parts offer ideal conditions to the transmission of acoustic emission. Acoustical signals are divided into three (3) primary groups depending upon their frequency.

1. Infra-Sound	up to 16 HZ (16 cycles per second)
2. Hearing Sound	16 to 16,000 Hz
3. Ultra-Sound	above 16,000 Hz

Infrasound is produced by very slow processes only, and cannot be recognized by the human ear. Since the manufacturing processes concerned (such as heading machines) are operating at much higher rates, infra-sound can be neglected here. Hearing Sound includes all those frequencies, which the human is capable of recognizing. All cold forming people are aware of the fact that there is plenty of this "noise" present in a fastener plant, and the noise is often used by the machine operator to determine problems. Ultra-Sound, on the other hand, is well above the detectable range of man, and requires special sensors if it is to be measured.

If punches break or dies crack, they will produce sound waves, which are referred to as "Acoustic Emissions" (AE). These sound waves are mainly of the ultra-sonic range with fairly high frequencies therefore, any process monitor using acoustical signals must concentrate on the Ultra Sonic range in order to be effective. Furthermore, any machine noise that typically is found in the Hearing Sound Range is filtered away, and thus, it cannot interfere with the monitoring signals.

Cracks are the Source of Acoustic Emissions

The most important source of acoustical signals in connection with process monitoring is the cracking of metal. A crack releases a high value of binding energy that typically appears as different waveforms. The two most significant types are called "surface waves" (expanding like wave rings on a water surface) and "longitudinal waves" which travel in a ball shape pattern throughout the material.

Acoustic "waveforms" caused by cracking metal

A better understanding of this principal may be obtained by imagining the ties between the metal molecules acting like rubber bands which expand and contract under a load. If those bands are over-extended, they will snap off and release their "binding energy" It is this rather powerful energy, which travels as an acoustical wave through the material and can be picked up somewhere else in the structure by means of suitable acoustic sensors.

The transmission of sound waves is affected by criteria such as the reflection of the waves on corners and boundaries, and the dampening or absorbing effects of the surrounding materials. These effects must be considered when selecting the proper location for the acoustic sensor. It is desirable to have as few material mating surfaces as possible between the "crack" and the sensor in order to keep information loss to a minimum. This absorbing effect requires the sensor to be placed as close as possible to the tool.

Monitoring Acoustic Emissions

The IMPAX-SK Vibrasonic system has been developed to monitor fastener-making machines through measuring the acoustic emission of the process. The system can be used as a stand-alone module, or as an addition to an existing process monitor. The system utilizes Acoutronic ultrasonic sensors and a special electronic signal-processing unit.

A typical system screen for a multi-die cold forming machine is shown below where the conventional multi-channel process monitor has been enhanced with an acoustic module. The acoustic emission monitoring function is activated automatically by the system, and it requires no additional set-up work for the machine operator. Optical warning limits and the visual presentation of the acoustical signals facilitate the use and the acceptance of this new technology.

Acoustic monitoring on a multi-die cold forming machine
Shows both the acoustic signal and the normal force curve.

The Acoutronic sensors are available in different styles in order to accommodate a variety of sensing locations for each individual application.

Higher Quality and Better Efficiency

The examples described below clearly explain how the use of acoustic emission monitoring contributes to improved fault detection, reduced tooling expenses, reduced machine down time, and lessened overall manufacturing costs. In many cases, acoustic emission monitoring has even been able to give early warnings of an upcoming tool failure, such that the machine could be stopped before the breakage had occurred.

Punch Breakage in Extrusion Processes

Everyone involved with the extrusion work is familiar with the fact the extrusion punch tends to break during the backward extrusion operation. The process monitors used today typically do not detect the breakage at this moment in time. They allow the machine to make another stroke because the forging load had not changed during the initial bad stroke. The broken punch tip remains stuck inside the part and transfers into the next forming station. A major smash-up now is inevitable. Once this occurs, the process monitor will react, but quite a few tools, including the transfer fingers, may have been destroyed by then.

Detecting Die Wear on a Multi-Station Machine

Another application of the VIBRASONIC system is on a five (5) die cold forming machine. The Acoutronic sensor was placed in a central location underneath the die-block which monitored all five (5) dies with just one single acoustic sensor. It is interesting to view the "typical" sound signals (such as transfer cam noise, wire cut-off, and the friction and scratching during the ejection of the part) to see how they relate to the running operation of the machine and the process. On the other hand, there is very little high frequency, acoustic activity during the actual forging process itself as long as good parts are being made.

The acoustic monitoring technique was able to easily detect small process inconsistencies resulting in poor quality. The bad parts showed slight scratch marking along the side of the shaft, which were caused by a worn-out extrusion die. The added friction during insertion and ejection of the blank produced significant acoustic emissions resulting in a number of distinct spiked signals. The force monitoring systems used in parallel to the acoustics were not able to detect the scratch marks, as no relevant changes in the force signal were seen.

Die Chipping on a Two-Blow Cold Header

Acoustic sensing was also applied to a series of 2-blow cold heading machines in order to evaluate to what degree smaller punch and die breakages, chipping, or even flaking could be detected. The Acoutronic sensor was placed each time on the stationary die block.

In this type of cold forming work, it is typical for a small portion of the die face section to chip, and conventional process monitors do normally not detect this. The VIBRASONIC system clearly senses the die chipping. The acoustic signal increased significantly during the very stroke when the chipping occurred, and the machine could be stopped immediately. Again, the force monitoring did not react to this, because no appreciable difference could be seen in the force curve. The same sensor location proved also capable of detecting smaller punch problems. The close contact between punch, wire blank, die and die block guarantees a clean transmission of the acoustical signals from the punch to the die-side ACOUNTRONIC sensor.

Conclusion

The results described above prove that acoustic emission sensing is a valuable addition to conventional process monitoring. Neither technology will be able to fully replace the other, but the combination of both is an important step towards zero-defect production. The improved capabilities in detecting faults and the fact that these faults are often detected before they cause any damage, makes the VIBRASONIC system a very appealing means of reducing manufacturing costs. Critical defects such as small breakages, chipping or even die wear, are now within the reach of process monitors. The investment in a new monitoring system combining force and acoustic sensing, will often pay for itself by preventing one major smash-up.

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