High speed blanking: An experimental method to calculate the induced cutting forces

To increase the productivity, blanking professionals are interested to control a new machining process that involves the high speed cutting at around 10 m/s. Nevertheless, the inherent hard dynamic of such a process makes it difficult to develop a high speed punch press. The cracking or fracture phenomenon governing the cutting process has to be well understood in order to cor- rectly design the machine support and the tools. In order to observe this phenomenon and to con- trol the cutting speed a specific experimental device has been developed. In this study, the goal is to calculate the shear cutting forces imposed on the specimen. A new method allowing the calculation of shear forces and taking the proposed test configuration into account is explained and is validated through numerical simulation. This technique has been successfully implemented for the determina- tion of shear forces of a micro alloy steel S600M. The high speed blanking is a new process that mechanic industry would like to better control. Indeed, without lubricant, this process allows obtaining work pieces which have geometrical and dimensional characteristics really satisfying and blank surfaces are in good condition. Moreover the minimisation of the distance between each work pieces because of less deformation could make saving material. Currently two techniques are used: classical and fine blanking. With those process the punch has a very little stroke and speed (1). Fine blanking maintains sheet metal during the cutting and gives the best geometrical and dimensional quality. However it is very slow compare with classical blanking. The high speed blanking tries to find a compromise between those two techniques: high productivity with good geometrical and dimensional quality. Moreover, avoiding lubricant, costs could be cheaper. Indeed, using lubricant during fabrication requires to clean work piece and to collect oil and waste because of environmental laws. So industry would be more efficient. However, the rise of the contact speed between the punch and the sheet metal induces rupture or mechanism of cracking more different than what we find in classical blanking. Consequently, it means physical phenomenons during the cut are not really good determined. The lack of expertise generates earlier tool's break and a bad prediction of geometrical and dimensional quality of work piece. In this way, a specific experimental device allowing the cutting conditions control and the phenomenon observations has been developed (2). The punch speed range is 5 to 20 m/s. Different sensor equips the set-up: gauges associated with a Hopkinson tube which allows measuring strain, laser-vibrometer indicates the punch's speed and a camera which realizes picture during the cut. In order to observe the cutting zone, we prefer, in a first time, to realize only open-cut called shearing. Later, we would try to understand closed-cut called stamping, for industrials. When the punch touches the sheet metal, a compression wave was created. The measure of strains during its propagation in the tube and the elastic wave theory let us calculate forces at the interface die/Hopkinson tube. Like industrial technique, a die is needed to maintain the sheet