Researches On The Chipping Mechanism Of Locally Heavily-loaded Fine Blanking Tools

Fine blanking process is a near-net forming process, which has been widely applied in various industrial fields. While engineers are often troubled by chipping of the fine blanking tools when forming components with complex features, such as multi-tooth. The frequent and unpredictable chipping of locally heavily-loaded tools has prevented the fine blanking technology from further development on the forming of more complex parts and high strength steel parts. Hence the researches on the chipping mechanism of locally heavily-loaded fine blanking tools and how to prevent and handle chipping are important both on theoretical and industrial level.Through indirect online detection of the morphologic development of the bottom surface of fine blanking tools, the common sense that the fatigue crack of chipping was initiated from the bottom surface at which the main load is applied was disproved. Then, through systematic microscopic observation of the crack, void and valley features on the side surface, cross section and bottom surface near the cutting edge, it was conformed that the fatigue crack of chipping was initiated from a position on the side surface under certain distance from the cutting edge.A hypothesis of the chipping mechanism of locally heavily-loaded fine blanking tools was proposed, and then validated by stress-strain CAE analysis of a fine blanking tool model and residual stress tracking of an in-service fine blanking tool. The core idea of the hypothesis includes: The manual grinding of the side surface of the tool that is widely applied in Chinese fine-blanking industry is insufficient to clean the defects induced by WEDM, and it is highly possible that those defects become the crack initiators of chipping cracks; Micro plastic deformation zone could be formed on the cutting edge where stress concentration was induced by the complex features, such as sharp corners and multi tooth. On the side surface of the tool, the boundary region of the plastic deformation zone will stand not only the maximum stress in the loading stage, but also the most serious residual compressive stress degradation in the unloading stage. Both factors selectively promote the crack growth in this region and are the main driving forces of chipping.At last, three fine blanking process improving solutions were proposed,namely improving the machining quality of tool side surface, optimizing the chamfering design of the cutting edge and load reduction of heavily loaded tools.