A chain link mandrel for rotary draw bending: experimental and finite element study of operation

Rotary draw bending is one of the methods used for bending of thin wall tubes. In this process, the mandrel is positioned inside the tube, and the bend die, clamp die, pressure die, and wiper die surround the tube. The bending is conducted around the bend die. Conventional mandrels are mostly ball link type whose manufacture needs high precision and cost. Links of ball link mandrels are exposed to failure in necking areas due to particular form of the links. In this paper, a new mandrel type called chain link mandrel is presented, manufactured, and tested. The main difference of this mandrel with conventional mandrels is in the degrees of freedom of the mandrel's ball segments. Finite element simulation of the bending process, using proposed mandrel and ball link mandrel, has been conducted in ABAQUS software, and some parameters, including required bending moment for tube bending, tube's ovality, the variations in tube wall's thickness, and spring back have been measured and compared between two types of mandrels. The clearance between external surface of the mandrel and internal wall of the tube and other influential process parameters has been set equal in both simulations. The simulation results showed that the amounts of thinning in outer wall of the bend, thickening in inner wall of the bend, tube's ovality, and spring back are equal for both of mandrels. This fact proves the proposed mandrel's effectiveness, despite its lower cost and simplicity compared to ball link mandrels. Experimental tests are also carried out, and the findings are compared with those obtained in the finite element simulations. There is good agreement between the simulation and experimental results. Practical operation of the proposed chain link mandrel shows that it has good performance for bending thin wall tubes. This mandrel is less expensive and stronger than the conventional ball link types, and the quality of the tubes bent by using this mandrel is acceptable.