Enhancing the accuracy of in-process springback measurements of complex tube bending processes using cost-effective embedded sensors

Geometrical errors due to springback are the main causes of variability in bending of thin-walled tubular profiles. This limits the process capability and determines an increase of the overall production costs due to both scrap or rework, and to the quality control operations. Despite the evolution of bending processes in the last decade, with the adoption of full-electric, highly automated CNC machines, the workpieces' geometry is usually still verified off-line downstream from the forming stage. This paper presents an innovative on-machine, integrated measuring approach for fast in-process inspection of bent tube geometrical features through cost-effective micro-electromechanical system inertial measurement sensors. These embedded sensors are affected by systematic errors due to both measuring principle and process-related effects; therefore a methodology to correct such errors is proposed. Error compensation is applied to drift and cross-axis sensor errors, as well as to mandrel kinematics and wear effects. An additional sensing system, placed on the tube end for monitoring of the metrological performances and to enabe quick adjustments, is also presented. The results of the validation tests highlight how the proposed system can be used to measure on-machine the final workpiece angle. The final mean absolute errors, for bending angles between 15 and 90 , were, respectively, 0.03 and 0.1 for the sensors embedded in the mandrel and on the tube end depending on the configuration, which are lower than the maximum deviations actually accepted in such processes. However, the performance of the former can be further improved by the proposed periodic quick adjustment procedure. The newly developed on-machine metrology solution enables not only a shortening of the quality control chain, but also significant economic benefits, and a case study is documented.