The increased use of high- and ultra-high-strength steel sheet materials in passenger car body construction necessitates the development of novel cutting processes or the adaptation of existing processes to these materials. At the institute, cutting methods are tested and further developed through both simulated and experimental investigations under laboratory and series production conditions, in both single and long-term tests. Central to the research are questions regarding the appropriate determination of process parameters, suitable tool materials, and more.
a) mould structure and b) max. punch deflection applied over the sheet position angle
In the production process, the trimming and punching of already formed structural components frequently deviate from the optimal 90° angle between the sheet metal and the punch. When this angle deviates, it is referred to as the die clearance angle. A significant issue arises with lateral forces during punching at non-90° angles. These lateral forces cause horizontal deflection of the punch, which can result in the deflected cutting punch colliding with the cutting die in extreme cases. Consequently, industries frequently attempt to circumvent cutting with die clearance angles by employing costly blankholder tools. Slide tools are already utilized in the industry for relatively minor die clearance angles due to the paucity of empirically validated data on process limits.
As part of a research project funded by EFB/AiF, a test tool was developed to measure horizontal punch deflection during the process. The process limits (maximum permissible die clearance angles) were accurately determined for test materials HC340LA, DP600, and DP1000, depending on the investigated parameters such as "cutting gap," "die clearance angle," "punch diameter," and "punch length." One method to reduce the measured punch deflection by approximately 50% was found to be the use of suitable punch edge preparations. A subsequent project is planned to further investigate the findings and optimize punch edge preparations through simulation and experimentation.
Duration: 11.2020 - 21.2022
In industrial practice, the quality requirements for sheared component edges (external trims, cutouts) are highly specific. In many cases, these cutting surfaces are permitted to serve as functional surfaces. These requirements include tight tolerances for the allowable edge radius, smooth cut portion, burr height, and fracture surface heights. Precision cutting methods such as fine cutting, accurate cutting, counter cutting, or two-stage trimming are typically employed to enhance cutting surface quality.
However, compared to standard cutting methods, these precision techniques involve higher tool and process complexity. They also necessitate additional axes in the cutting press, which results in more intricate tool designs and lower production throughput, ultimately leading to higher component costs.
The re-cutting process represents an economically viable alternative in follow-on compound tools and smaller stage tool sets, as it enables the achievement of the desired cut surface quality. This process results in relatively minimal work hardening while still exhibiting comparatively high smooth-cut portions. Nevertheless, the implementation of the two-stage re-cutting process in large-scale tooling presents certain challenges. In order to achieve the desired results, at least two tooling stages (pre-cutting and re-cutting) are required, and the component or sheet must be positioned with great precision for the second cut. The newly developed solution approach within the framework of the research project represents single-stroke re-cutting. The fundamental concept of pre-cutting and re-cutting in a single stroke involves dividing the cutting punch into two active elements: an internal pre-punch and an external, spring-loaded cutting sleeve. This results in the cutting punch separating the slug with a relatively large cutting gap (pre-punching), while the process-specifically dimensioned cutting sleeve cuts out the hardened edge area of the hole as a ring. Following the re-cutting process, the spring preload force of the cutting sleeve ensures the reliable removal of the waste ring from the pre-punch. Consequently, the objective of this project is to develop a straightforwardly standardized tool unit for pre-cutting and re-cutting in a single press stroke. The objective of this tool unit is to optimize both the cut surface quality (especially smooth cut portions) and the residual forming capacity of sheared sheet metal component edges. The subsequent figure schematically illustrates the design principle of the tool unit to be developed.
The design of the cutting sleeve was subjected to a finite element analysis. In this context, an initial strength analysis was conducted. Subsequently, the punching unit for the single-stroke re-cutting was constructed in the institute's own workshop. The initial punching tests demonstrated the functionality of the tool unit for pre-cutting and re-cutting. Following the successful commissioning of the tool, the wear behavior of the tool unit will be tested under conditions simulating series production until the conclusion of the project. Furthermore, an assessment will be conducted on the cut samples to evaluate the achieved cut surface quality and the residual forming capacity of the sheared sheet metal edges.
Duration: 04.2021 - 09.2023
In light of the ongoing increase in the demand for the quality of sheet metal components, the quality standards for component functional surfaces affected by shearing must now meet higher requirements. In industrial practice, such high-quality standards for cutting edges or surfaces are characterised by a small edge radius, a high proportion of smooth cuts, the absence of burrs, low fracture surface heights, and tight manufacturing tolerances. Moreover, in addition to the objective of achieving high cutting surface quality in the production of sheared components, the productivity of the employed process is of paramount importance. In addition to output quantities, productivity is also defined by low tool costs and minimal tool maintenance costs.
In this context, conventional shearing or punching with single-acting presses is considered one of the most productive cutting methods. However, a disadvantage of conventional shearing is that it only achieves cutting edges with relatively coarse tolerances (IT11) and maximum smooth cut proportions of up to 50% of the sheet thickness. When production and method planners aim for higher component qualities, precision cutting methods such as fine cutting, exact cutting, or post-cutting are typically required. Fine cutting results in an increase in compressive stress levels within the shearing zone. This enables the attainment of smooth cut proportions of up to 100% and component qualities in tolerance class IT7, for instance. In contrast to conventional cutting, the elevated tool and process intricacy inherent to these precision cutting methods translates to markedly diminished output quantities and elevated component costs.
In light of the aforementioned circumstances, a novel cutting process, designated as "hollow cutting," is developed within the context of the ongoing research project. In this process, conventional cutting punches are designed with a concave face to increase the compressive stress level in the shearing zone, utilizing a relatively delicate punch cutting edge. This application represents a cost-effective alternative to existing special cutting methods, as it simply requires the replacement of conventional cutting punches in standard cutting tools with hollow cutting punches. The following illustration provides a summary of the project's objectives.
Extensive numerical and experimental investigations have demonstrated the effectiveness of the new cutting process. The figure below illustrates that, in comparison to conventional cutting, hollow cutting enables significantly higher proportions of smooth cuts and greatly reduced edge deformation heights. Endurance tests have demonstrated the potential of hollow cutting for industrial applications involving high-strength sheet metal (DP600). The initial implementations are pending at the project partners.
The project findings will be published in a comprehensive AiF final report in 2023.
Duration: 11.2020 - 21.2022
Duration: 07.2023 - 06.2025