Tungsten Carbide Punches Increase the Performance of Fineblanking of High-Strength Steel Sheet

12/11/2020

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Ideal Surface Integrity of Tungsten Carbide Punches through Advanced Punch Manufacturing

  Fine blanking tool Copyright: WZL Fine blanking tool, optimized for tungsten carbide applications

In cooperation with the Working Group Fineblanking, the Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen University has succeeded in improving the performance of fine blanking of high-strength sheet steel by using tungsten carbide punches.

Due to its high hardness and compressive strength, tungsten carbide offers the potential to fine-cut high-strength sheet metal materials with high process reliability and high durability. This significantly reduces tool wear compared with conventional punches made of high-speed steel. Since, in contrast to conventional high-speed steels, surface integrity is particularly important when machining tungsten carbide, the production process for manufacturing tungsten carbide punches was adapted to these special operating conditions.

A cooperation of the Chair of Manufacturing Technology of the WZL with Your-Tool (specialist for high-precision tooling applications) and Platit (manufacturer of coating systems) has now led to a promising production of tungsten carbide punches for a process-safe usage. The tungsten carbide for the successful test series was provided by Ceratizit Empfingen. The tungsten carbide test punches received their application properties as a result of the special production route.

  Tungsten carbide punch Copyright: WZL View of a tungsten carbide punch before coating with eroded surface and polished cutting edge

It all depends on the technology

The test punches were manufactured by the company Your-Tool using wire spark erosion in a water bath made of a corrosion-resistant carbide grade. For the wire spark erosion, a high-precision cutting technology for tungsten carbide machining, specially developed by Your-Tool, was used. Compared to standard technologies already available, this advanced technology now allows surfaces with a particularly small edge zone - a prerequisite for fine cutting with tungsten carbide punches. After fine-finishing in the last processing step, almost no white layer is produced and the edge zone is smaller than one micrometer. The potential for the formation of micro-cracks during the fineblanking process has thus been demonstrably minimized.

The cutting edge of a cutting punch is created at the transition between the eroded lateral surface and the ground front surface. To avoid chipping of the cutting edge and thus maximize wear resistance, high edge stability is essential. The sharp cutting edge is therefore usually finished by chamfering or rounding.

At the WZL, the cutting edges were rounded using a BP Smart brush polishing machine from René Gerber. A Gerber disc brush, with an abrasive medium integrated into the bristles, was used. After the cutting edge preparation, the cutting edge has a defined rounding with a uniform radius of 20 micrometers, which ensures good coatability and edge stability.

In the last production step, the piercing punches were coated by the Platit Company. The minimal remaining edge zone and the white layer were removed by careful wet blasting in preparation for coating. A PVD hard coating FeinAl further increases the wear resistance and minimizes friction with the sheet metal material.

When fineblanking high-strength sheet steel, the hardness and high rigidity resulting from the high modulus of elasticity of the tungsten carbide are beneficial. In contrast to steel punches, tungsten carbide does not compress as much in the fineblanking process, so that the coating does not flake off (eggshell effect) and can develop its full effectiveness.

  Section of the experimental process Copyright: WZL Section of the experimental process with various geometric elements

Successful experiment: hardly any abrasion on the tungsten carbide punch

In a test series of 10,000 strokes on the Feintool XFT 2500 speed fineblanking press, high-strength sheet metal with a thickness of six millimeters was fine blanked with process reliability. The tungsten carbide punch showed hardly any signs of abrasion, while a conventional HSS punch as a reference was almost at the end of its service life. The cost-effectiveness of fineblanking was therefore significantly increased thanks to a significant reduction in abrasion.

The project fineblanking of high-strength steel with tungsten carbide has proven the effectiveness of the cooperation of companies from different disciplines involved in the Working Group Fineblanking. In a joint effort, the WZL generated comprehensive knowledge about the machining of tungsten carbide and fine blanking with carbide punches, which enables users to apply the technology in industrial practice. In further work of the Working Group Fineblanking, the scientific findings are used to enable users to use carbide in their production and to re-duce production costs in the long term.