Machining with Flexible Manipulators: Critical Issues and Solutions

Abstract

The automotive industry represents the fastest-growing market segment of the aluminium industry, due to the increasing usage of aluminium in cars. The drive behind this is not only to reduce the vehicle weight in order to achieve lower fuel consumption and improved vehicle performance, but also the desire for more sustainable transport and the support from new legislation. Cars produced in 1998, for example, contained on average about 85 Kg of aluminium. By 2005, the automotive industry will be using more than 125 Kg of aluminium per vehicle. It is estimated that aluminium for automotive industry alone will be a 50B$/year market. Most of the automotive aluminium parts start from a casting in a foundry plant. The downstream processes usually include cleaning and pre-machining of the gating system and riser, etc., machining for high tolerance surfaces, painting and assembly. Today, most of the cleaning operations are done manually in an extremely noisy, dusty and unhealthy environment. Therefore, automation for these operations is highly desirable. However, due to the variations and highly irregular shape of the automotive casting parts, solutions based on CNC machining center usually presented a high cost, difficult-to-change capital investment. To this end, robotics based flexible automation is considered as an ideal solution for its programmability, adaptivity, flexibility and relatively low cost, especially for the fact that industrial robot is already applied to tend foundry machines and transport parts in the process. Nevertheless, the foundry industry has not seen many success stories for such applications and installations. Currently, more than 80% of the application of industrial robots is still limited to the fields of material handling and welding. (Figure 1) The major hurdle preventing the adoption of robots for material removal processes is the fact that the stiffness of today’s industrial robot is much lower than that of a standard CNC machine. The stiffness for a typical articulated robot is usually less than 1 N/μm, while a standard CNC machine center very often has stiffness greater than 50 N/μm. Most of the existing literature on machining process, such as process force modelling (Kim et al., 2003; Stein & Huh, 2002], accuracy improvement (Yang 1996) and vibration suppression (Budak & Altintas, 1998) are based on the CNC machine. Research in the field of robotic machining is still focused on accurate off-line programming and calibration (Chen & Hu, 1999; Sallinen & Heikkila, 2000; Wang et al., 2003a, 2003b). Akbari et al. (Akbari & Higuchhi, 2000) describe a tool angle adjustment method in a grinding application with a