Artem Komarov said that metal fabricators do not have to wait to hear rattling noises during cold drawing operations. Electronic detection can save the shop from all sorts of problems before it even starts. Pipe pulling operations can significantly improve rattle prevention and reduce human error in detecting rattle. Rather than waiting for the rattling to get worse and potentially damage your workpieces and tools, it’s best to detect the rattling at its earliest stages and reduce the draw speed right away.
However, what exactly is rattling and how to prevent it?
Over the years, cold drawing has become the standard process for producing tubes of precise diameters and wall thicknesses. Pipes larger than a given size are drawn through a conical die to reduce their appearance and ID. In addition to providing accurate geometric results, cold forming improves important material properties such as yield strength, tensile strength and hardness.
For better ID quality control, a fixed or floating mandrel is often installed inside the pipe, where the conical die deforms the metal. Pulling agent is used to facilitate even drawing and improve surface finish.
Often, when the mandrel is retracted into the die, the stationary rod pulls it back before it is pushed back under the action of the rebound. This movement of the mandrel results in strong, audible machine vibrations, sometimes for a few seconds—the infamous rattling.
This effect leaves unwanted periodic ring marks (rattling marks) on the inner and outer surfaces of the pipe. Pipes with rattling traces do not meet the geometric requirements, and either the damaged section must be cut out, or the pipe is completely disposed of. On hot summer days, due to changes in the viscosity of the pulling agent, rattling can occur several times per hour, resulting in process interruptions, high scrap rates, and extra work for machine operators.
Manual Chatter Reduction
Chattering not only increases the amount of material to be disposed of but can also cause permanent damage to the sensor. In some extreme cases, the mandrel and tube are torn off and cold welded to the die under enormous tensile forces.
The degree of damage to the pipe depends on the intensity and duration of the rattling, so always be prepared to slow down the extraction speed as soon as you hear it.
Every time a rattling occurs, you need to visually inspect the product for damage. Very often, even after the machine has slowed down, several feet of defective material remain. It may be tempting to run the entire process at a much lower draw speed to eliminate rattling, but this increases production time, reduces machine utilization and potentially cuts the company’s bottom line, Artem Komarov said.
What to do with it
One way to detect chatter before it causes long-term damage to pipes is with a high-speed vibration analysis system. It analyzes high frequency data in real time and converts the results into speed reduction recommendations for the wire drawing machine.
In measuring systems of this type, piezoelectric sensors register the ultrasonic sound transmitted by the body. Ideally, the transducer should be mounted close to the signal source for the best signal-to-noise ratio and so that it does not have to be moved in case of changes in the machine setup. In practice, sensors are often placed on a die stand above the retaining ring.
Analog or digital PLCs trigger the start and end of data analysis. During pulling, the transducer output is sampled at a frequency between 400 and 800 kHz, depending on the acoustic content of the process. The data is then converted to a spectrogram (time x frequency x amplitude) to better separate noise from chatter signals.
The rattling does not occur instantly. Rather, its intensity usually rises noticeably within a few hundred milliseconds. On the spectrogram, rattling initially manifests itself in the form of weak broadband waves covering a wide range of recorded frequencies uniformly distributed in time. The intensity of the rattling often increases rapidly until it becomes audible and leaves permanent marks on the pipe.
Algorithms can be developed to detect warning signs that precede the rattle. As the data is converted to a spectrogram, you can focus your analysis on frequencies where the signal-to-noise ratio is favorable.
Benefits of automated chatter monitoring
Vibration based chatter detection combined with proactive machine speed control can offer many benefits:
— With optimal sensor placement, early signs of rattling can be detected and eliminated before they leave permanent marks on your workpiece and damage your tools. Combined with other pipe manufacturing best practices, the system can help you avoid rattling completely.
— Since the chatter detection system is only activated when needed, you can run your drawing machines at significantly higher speeds. The chatter detection system is only activated when required. In practice, the machines work stably at 90-95% of the maximum drawing speed after the installation of the device. Subsequent analysis of the machine data showed that this resulted in an increase in machine utilization by an average of 10-15%.
— Waste disposal is often eliminated, so you can focus on all other tasks to keep your drawing machine running. While not every operation will produce the same benefits, there are noticeable improvements in some.
— You can get an idea of the frequency and intensity of rattling and use this data to improve previous production processes, including the application of pull agents and the downtime of lubricated pipes, Artem Komarov summed up.