Fixtureless tube bending with on-line feedback and statistical dimension control

The traditional method of bending aerospace tubes is a trial-and-error method. Several trial bends are made to build the springback prediction model prior to bending the actual part and a fixture is needed to inspect each bent part. This trial-and-error method has the problems of high scrap rate of expensive materials, low efficiency, large number of costly fixtures and operator experience dependency. This thesis research develops a new method in which tubes are made accurate enough and guaranteed within tolerance without an inspection fixture. To realize this goal, an on-line measurement system is added to the bending machine to measure the springback of every bend and a springback prediction model is built based on in-process measurement. Bend-rebend control is implemented to eliminate trial bends. An adaptive bend correction algorithm is developed to statistically improve the overall tube accuracy. A process control software is developed to perform tasks regarding springback data collection and retention, empirical springback and relaxation models, statistical tolerance analysis with Monte Carlo simulation and bending process capability evaluation. The process control software makes decisions on whether to apply adaptive bend correction and whether to stop a bending process since the tube is out of the tolerance. At the end of the bending process, the bent tube accuracy is calculated and it can be known whether the tube is within tolerance without inspection with fixtures.