Measurement of forces on multi-point forming tools using Fibre Bragg Grating sensors

Multi-point forming uses forces applied to a tool, comprising of multiple pins set at different heights, to form sheet metal for panelling in white goods, automotive bodywork, aircraft frames and so on. The use of multiple pins allows for rapid change over and flexibility in the tool making it suitable for small-batch and prototype component manufacture. To explore the relationship between ‘springback’ of the sheet metal on release from the tool, and the applied pin force, it is first necessary to understand and measure the forming forces. This article presents a novel method of measuring forming forces on individual pins in a multi-point forming tool using fibre Bragg grating sensors, monitoring the elastic strain on the selected pins during the forming process. The operating principles behind forming force measurements using fibre Bragg gratings are introduced and a relationship is developed between springback in the formed part after the final unloading and the forming force as measured on selected individual pins under different compression ratios (30%, 40%, 50% and 60%) of the elastic cushion between the tips of the pins and the workpiece. Experiments were performed to validate the proposed measuring method, and results indicate that forming forces detected by the proposed method correlated well with the results obtained by numerical simulation. This suggests the proposed method has good potential for real-time measurement and monitoring of forming force distribution in multi-point forming tools during the forming process.