New monitoring methodologies in the shearing process
Blanking is a process which consists in the separation by cutting of a sheet metal to obtain individual components with particular shapes.
The process takes place by subjecting the material to high stresses, beyond the breaking limit, which deform the material in shear until a fracture is created.
It allows you to shape the sheets through the use of punches, which deform and cut, and dies that give the shape.
The shearing has the purpose of cutting out a flat geometric figure, thanks to molds mounted on presses.
A punch, connected to the slide of the press, penetrates the die cutting the sheet according to the figure corresponding to the shape given by both the punch and the die. The contours of the punch and die hole act as cutting edges.
When the shear breaking voltage of the material is reached, the actual fracture is generated, which separates the sheared object from the rest of the sheet. In this phase the punch applies the maximum force.
From this moment on, the force decreases and the punch pushes the sheared piece into the die, until it falls into a collector.
Quality controls are carried out through destructive tests on randomly selected components. There is also a new alternative is that of real-time monitoring during production. Measurements carried out during processing make it possible to obtain quantitative measurements, so as to be able to anticipate the presence of potential damage or critical issues.
It is possible to identify some distinct phases during the shearing process:
- approach of the punch to the sheet metal,
- phase of elastic deformation,
- plastic deformation,
- onset of fractures,
- propagation of the fracture, up to the separation of the sheared sheet.
- the first factor is given by the material to be processed and its characteristics,
- secondly, the process parameters must be considered.
To obtain a valuable product, the quality of the cut surface and the accuracy of the workmanship must be considered.
New monitoring processes
It is particularly interesting to use acoustic emissions (abbreviated as “AE”) to monitor the various stages of the process.
This technology is able to quantify the microstructure changes in the material during processing.
Another interesting monitoring technique is that of correlating digital images (Digital Image Correlation or “DIC”).
This methodology relies on high resolution images to measure displacements.
The machined material is 6082-T6 aluminum, used for structures and in the transport sector due to its high corrosion resistance, high strength and good weldability. The shearing was performed using a universal machine with a maximum load of 50 kN sensorized by the authors. Images were recorded by a CCD camera to track the entire process.
Acoustic emissions were measured by two sensors between 50 and 400 kHz, were filtered and amplified. The tests relating to the acoustic measurements have highlighted the importance of processing this signal by eliminating the background noise and extracting indicators from the recording to be used in the analysis phase.
Metallographic inspections were also carried out using a scanning electron microscope (SEM) to investigate what was suggested by the monitoring sensors during the various stages of the process.
By analyzing the signals recorded during blanking, it is possible to obtain a complete description of what happens during machining.
Image measurement (Image b) makes it possible to identify, quantify and above all locate the initiation and propagation area of the crack, identifying the most stressed areas. The acoustic emissions instead (Image a) can be useful to identify the transition between the phases of the process. In this way it is possible to measure the duration of each single phase to be used as an index of quality and stability of the process.
From the point of view of acoustic emissions, the fracture phase is particularly dense with high-frequency and high-energy events.
This is because a great deal of energy is instantly released when the crack generated by the punch is formed. By coupling the two measurements in real time it is in fact possible to accurately describe the shearing process. These measurements have proved to be interesting indicators of what happens inside the material during transformation, so as to be able to measure and trace its evolution.
The study proposed by Chinese researchers shows an interesting process monitoring solution. Through non-destructive and relatively simple measurements it is possible to investigate what is happening during a traditional shearing. This reduces the need to carry out numerous measurements on finished components, wasting time and money.
The sensorization of machinery opens up new scenarios for manufacturing.
These measurements carried out directly during processing make it possible to identify anomalies during the transformation process due, for example, to non-compliant materials or machinery breakdowns. So as to avoid delivering defective components that could be rejected or cause failure.