The Characterisation of the Free-Bending Technique

• Main Author: Gantner, Peter
• Published: Glasgow Caledonian University 2008
• Subjects: 739
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486486

The increasing requirements in the automotive industry regarding safety, weight and economic efficiency involve the application of stiff structures made from hollow sections, manufactured by bending tubes and profiles into complex shapes. These geometries comprise different radii in several planes, including compound bends, and demand new techniques for manufacture. To this end, the free-bending technique has evolved which has the potential to realise almost arbitrary bending geometries combined with fast bending speed. This research provides a general insight' into free-bending and investigates the possibilities of this novel bending technique in respect to applications in the automotive industry. The principles of the free-bending technique are explained and a comparison is drawn with other bending techniques. The Nissin CNC Bender was used to carry out the bending tests in order to investigate the free-bending technique with regard to its geometrical capabilities, bending speed, surface quality and the resulting ovality of the tube after bending. For this a try-out geometry similar to an engine cradle was bent using steel, stainless steel and aluminium tubes and the repeat accuracy was examined. For feasibility studies, a FEA model was developed and validated with the results from the tubes bent from a second bending test series where the geometrical results including wall thickness and ovality were investigated. From this FEA model the elastic and plastic forming ofthe tube during the process was examined. In order to determine the necessary bending control data for arbitrary bending geometries, an analytical model based on the geometrical data (bending line and tool set-up) was derived. Finally a third test series with rectangular aluminium profiles was carried out and the profiles' geometries, the wall thicknesses and the strains were evaluated and compared with the FEA results. The free-bending technique has proven its potential to realise almost any arbitrary bending geometry with a bending ratio of the centre line radius with respect to the initial tube diameter of2.1 using a ball mandrel. Free-bending has the added advantage in achieving compound bends without the recourse to special tools or tool changes for different bending radii. Furthermore, the improvement of up to 100 %in bending speed compared to rotary draw bending can be achieved dependent on the composition of bends in the geometry. However, due to its sensitivity to changes in material behaviour, lubrication or geometrical tolerances, it is necessary to have stringent control for parts with small bending tolerances. This research has contributed to new knowledge with regard to the possibilities and limitations of the free-bending technique and serves as a decision aid for the application ofthis novel technique in series automotive production. Furthermore, the FEA model in conjunction with the analytical model to determine the process control data enables the evaluation and the realisation offeasibility for any required bending geometry before the production ofexpensive bending tools.