Investigation of the Mechanical Punch Loads during Fine Blanking of High-Strength Steels with Cemented Carbide

Abstract

Abstract The increasing ecological demands require emissions reduction of automotive vehicles. This challenges the automotive industry for the increased application of light-weight components made of high-strength steels. The processing of high-strength steels brings many manufacturing processes to its mechanical limitations. In fine blanking, conventional tool materials lack on compressive strength in order to separate high-strength sheet metal. Nowadays, high-speed steels used for tool applications must be substituted with materials having a higher compressive strength, such as cemented carbides. However, with the increased compressive strength the carbides show a significantly lower ductility. During a repeated stripping off process, the low ductility of cemented carbides leads to a premature punch fracture and with this reduced economic feasibility of the process. The stripping off force required for the final punch ejection is generated by the clamping force applied by elastic deformation of the scrap material. Currently, phenomenological models exist allowing only approximate prediction of the clamping force. The precise prediction is not possible, since the clamping force depends on plenty parameters, such as sheet material strength and thickness, as well as punch geometry, ambient temperature, nest positioning, etc. A comprehensive investigation of the influence of these parameters on the punch load is strictly required. In order to enhance the applicability of cemented carbides as an alternative punch material in fine blanking of high-strength steels, a tool for process forces quantification, such as clamping and stripping off forces during one cutting-ejection cycle, must be provided. To achieve this goal, a comprehensive numerical study on the influence of process parameters such as sheet metal strength, sheet thickness and cemented carbide grade on the cutting, clamping and stripping off forces is performed. The developed three-dimensional numerical model of a fine blanking process with an elastic punch is validated by means of the experimentally measured punch force during an industrial fine blanking process of spacer discs. Based on the obtained numerical results, a basic correlation between the clamping force during the cutting step and the stripping off force during the ejection step are analyzed. The identified tendencies are used to create an analytical model of the clamping force and a regression model of the stripping off forces. The developed models allow for improved prediction of the expected process forces during fine blanking of high-strength steels and with this for the selection of the appropriate grade of cemented carbide as a punch material.