One major aim in the field of production technology is the reduction of scrap. This requires comprehensive monitoring of both the processes and the resulting product quality. For this purpose, process data, which is sensitive for the result of the manufacturing process, must be evaluated and monitored. In order to do so, Fraunhofer IWU assessed different in-process data acquisition systems in the context of sheet metal forming. Within FormPlanet project, one of the approaches investigated in the context of shear cutting processes was the tool integrated measurement of the process forces. This method seemed to be valid for the in-process detection of the wear of the punch. This blog post describes the methodology and the results of this study.
Innovative in-process measurement for detecting wear on cutting punches
The quality of a part processed by a shear-cutting process mainly depends on the dimensional accuracy of the cutting line and the quality of the cut surface. These properties are influenced by the material properties as well as on process parameters like cutting clearance and cutting-edge radius.
However, these process parameters change due to the increasing wear of the active tool parts (especially cutting punches). If the wear exceeds a critical value, the resulting part quality is reduced. In the further progress, this can lead to complete failures of the active part of the tool. This leads to increased scrap rats and consequently to unpredictable tool downtimes. To avoid this, the active parts of the tool are preventive periodically replaced. However, this does not exhaust the maximum lifetime of the tools.
In the presented approach, the wear condition of the cutting punches will be detected by an in-process measurement of the process forces. For this purpose, fundamental investigations were carried out within the framework of the FormPlanet project in cooperation with the company TRSystems GmbH.
In the first step, piezo electric force sensors (PSA sensors) were integrated into a cutting tool available at Fraunhofer IWU. For this purpose, the used tool was modified and appropriate sensors were integrated into the pressure plate. The used setup is shown in Figure 1. The used PSA sensors are the ‘PSA-20 mm’ sensors from TR-Systems GmbH.
Figure 1. Experimental Setup with PSA sensors.
For the evaluation of the system, cutting tests were performed on a Bruderer BSTA 25USL press using the described tool set-up. The tests were run with carbide punches in a ‘sharp’ and a ‘blunt’ condition. The sharp punches were used for a maximal number of 500 strokes and the blunt punches for more than 100000 strokes. The difference of the two punches can be clearly seen in Figure 2. The blunt punch shows cracks on the cutting edge and adhesive wear. The tests were carried out with a stroke rate of 250 strokes per minute and the force-crank angel curves ware recorded for each stroke using the ‘Compact Press’ software from TRSystems GmbH. The stainless steel 1.4301 in sheet thickness t = 0.3 mm was used as sheet metal material for the experiments.
Figure 2.Sharp punch (left) and blunt punch (right)
Successful preliminary results
Figure 3 shows the force-crank angle curves for the two different punch conditions. It is clearly visible that the process force increases with increasing wear. For the specific case, an increase of more than 25% of the maximal force was measured for the blunt punch. The differences are even clearer when the area under the measuring curve is evaluated. Here, the area increases by almost 60% from the sharp to the blunt punch.
Figure 3. Sharp punch (left) and blunt punch (right)
The evaluation methodology was implemented in the software ‘compact press’ of the company TRSystems GmbH and allows an analysis for each stroke as well as a statistical analysis over several strokes.
The investigations carried out show that process force measurement directly integrated in cutting tools is a promising method for detecting wear on cutting punches. In the next step, in-process measurements must be used to determine corresponding threshold values at which the punches must be replaced. By using this method, scrap can be reduced, and tool downtimes can be avoided.
Matthias Riemer:
Dipl.-Ing. in mechanical engineering at TU Dresden. Part of the sheet metal forming department at Fraunhofer Institute for Machine Tools and Forming. Main research topics are monitoring of forming processes as well as the in-process material characterization.
Fraunhofer IWU
Fraunhofer Institute for Machine Tools and Forming Technology (IWU) is a leading institute within Fraunhofer applied research organisation working on the development of efficient value chain processes in the machine tool, vehicle and component production sector. The institute is in charge of developing a testing machine to perform reliable FLC-characterization and using an advanced 3MA system for non-destructive quality inspection of components.
