Off-line compensation of the tool path deviations on robotic machining- Application to incremental sheet forming

Author(s): 
J. Belchior, M. Guillo, E. Courteille, P. Maurine, L. Leotoing, D. Guines (Universite Europeenne de Bretagne, Institut Maupertuis)

In order to reduce manufacturing costs and to improve production flexibility, the industrial robot manipulators are nowadays involved for processes such as machining, assembly or forming. Robots can be used for incremental sheet forming (ISF) which is an interesting process for small series production and prototyping. In this paper, a coupling methodology is involved and improved to correct the tool path deviations induced by the compliance of industrial robots during an incremental sheet forming task. For that purpose, a robust and systematic method is first proposed to derive the elastic model of their structure and an efficient FE simulation of the process is then used to predict accurately the forming forces. Their values are then defined as the inputs of the proposed elastic model to calculate the robot TCP pose errors induced by the elastic deformations. This avoids thus a first step of measurement of the forces required to form a test part with a stiff machine.

An intensive experimental investigation is performed by forming a classical frustum cone and a non-symmetrical twisted pyramid. It validates the robustness of both the FE analysis and the proposed elastic modeling allowing the final geometry of the formed parts to converge towards their nominal specifications in a context of prototyping applications. modeFRONTIER has been used to identify the FANUC robot stiffness parameters. The identified stiffness values obtained with the optimum design are used to evaluate the elastic displacements of the robot for the verification load configuration. The process FE simulation performed with the ABAQUS software computes the estimated forces required to form the part assuming an ideal stiff robot structure. The advantage of this approach is to avoid the measuring of the forming forces during a first run without any compensation. The obtained experimental results show the relevance of this approach since the TCP pose accuracy could be improved up to 80% for the forming of a frustum cone and a non-symmetrical part (a twisted pyramid) made in an aluminum sheets. This allows the final geometry of the formed parts to converge towards the nominal specifications required by a proto- typing application.