Acoustic emission monitoring for stamping presses
Optimizing production for better quality, longer tool life
Sheet metal forming companies face constantly increasing demands for higher production speeds from their machinery and more complex parts. At the same time customers demand higher standards of quality and stringent price controls, even as production volumes swell.
If a deviation from standard impulse sequences is detected, the process-monitoring system immediately reacts and automatically switches the machine off before any damage occurs to the tool or the machine.
Maintaining quality and eliminating costly waste require efficient process-monitoring systems. Acoustic emission (AE) monitoring is one process-monitoring development that can be used in conjunction with conventional monitoring processes such as force or pressure measurements and tool protection. AE signals detect process faults and identify tool failures early, while helping to increase monitoring accuracy and optimize operator adjustments of the machine and tooling.
Each stamping press in a facility makes specific, or signature, noises. These noises occur in different places—such as near the workpiece, the tool, the bearings, or the drive—at different times in the process. This typically results in a recurring impulse sequence for each tool; with each stroke of the press, that sequence defines the normal, fault-free signature of the process.
The operating noise signature is detected by acoustic sensors. If the signature "sounds" normal, then everything is fine. For example, typical AE impulses result from:
- The impact of the stripper plate and punches on the material.
- The cutting shock of the blanking and piercing punches.
- The stripper plate lifting off the material.
Two piezoelectric sensors, fitted on the upper tool and the stripper plate, usually are sufficient to measure the AE signals.
Operators record and analyze AE signals from their machines during production. If a deviation from standard impulse sequences is detected, the process-monitoring system immediately reacts and automatically switches the machine off before any damage occurs to the tool or the machine (see Figure 1), such as:
- Broken punches.
- Chipped or broken dies.
- Broken springs and tooling elements.
- Slug or scrap returns and buildup.
- Tool impacts.
- Friction between tools and the material.
- Broken material.
Typically, two piezoelectric sensors, fitted on the upper tool and the stripper plate (see Figure 2), are sufficient to measure the AE signals. Only very large tools (wider than 800 inches), tools with several strippers, and modular tools require additional sensors. Thus, detecting slug returns and buildup, for example, requires one ultraemission (UE) sensor per tool module. They are screwed onto the tool halves in the direction of the ram movement.
Monitoring and Optimizing Processes
AE measurements give the press operator information on what is happening in the dynamic stamping process. This allows the operator to optimize production processes quickly and directly.
For example, incorrect stroke rates on an automatic stamping press create vibrations that can cause the punch to wear and reduce its serviceable life. AE recognizes these resonances and simplifies the machine and tool adjustment for the operator, so that the goals of extended tool life, improved quality, and reduced scrap and tooling costs can be achieved more easily.
Modern process-monitoring devices use AE signals to calculate a process quality (PQ) factor. This value helps the operator evaluate and optimize the consistency and stability of the reshaping process. A stable process is the prerequisite for uninterrupted, fault-free production.
These devices have several features to make the monitoring process efficient:
- Crash limits prevent damage to the stamping tools. With each stroke, the tools are monitored for forced ruptures.
- Process trends visualize the stability of the process and identify process variations or abnormalities.
- Dynamic envelope curves help ensure part quality during continuous operation. Process errors such as broken punches or slug returns and buildup are detected.
Multisensor Monitoring Using Envelope Curves
In addition to AE sensors, effective process-monitoring devices also should support conventional force sensors for the evaluation of force or pressure measurements. To accomplish this, process monitors feature the double dynamic envelope curve, or UE monitoring, evaluation method.
Ultraemission detection requires double envelope curves for detection and process limits. These double envelope curves form dynamic envelopes, while inner envelopes serve as intervention thresholds.
This approach calls for a doubling of typical stamping process force curves. In typical process monitoring, software displays manufacturing information in a computer display plot, or force curve. Usually there is one force curve for the user-selected quality parameters for a process and another that depicts the actual manufacturing process. If the actual process values drift outside the quality parameters, the machine operator is notified, or the machine is shut down.
Ultraemission detection requires double envelope curves for detection and process limits. These double envelope curves form dynamic envelopes (see Figure 3).
The inner envelopes serve as intervention thresholds. Exceeding these limits creates a warning message and causes defective parts to be diverted via a sorting gate and dropped from the production run. This is an indicator of a worsening process.
The outer envelopes serve as process limits. If the stamping process exceeds these limits, the machine is stopped immediately. Binary sensors such as light barriers and proximity switches can be connected for feed and ejection control. The switching behavior of these transmitting devices is shown on the device's display and monitored.
Protection Through Retrofitting
Many automatic stamping presses already are equipped with appropriate machine protection devices. Retrofitting an automatic stamping machine with additional AE monitoring technology is a fairly simple procedure.
AE monitoring not only protects the production machine itself, but it also protects the tools against breakage, helping to ensure the production of high-quality parts. The dynamic process behavior becomes transparent, and the machine and tool can be optimally adjusted.
Ferdinand Oppel is managing director and Dr. Thomas Terzyk is managing partner of Prokos GmbH, Vahrenwalder Str. 7, 30165 Hannover, Germany, 0511-9357-400, fax 0511 9357 100, email@example.com. Chris Kane is vice president of Brankamp Process Automation Inc., 222 Third St., Suite 3200, Cambridge, MA 02142, 617-492-1692, fax 617-497-5675, .
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.