Robot manipulators play an important role in industrial automation. Various aspects of robotic systems were subject of intensive investigations in the past, but the vibration problems in robotic machining processes have been rarely treated in the available literature. In this paper we present dynamic modeling of an ongoing research to study chatter vibration in robotic grinding process using a portable manipulator for rectifying the surfaces of hydro-electric equipments. This special-purpose robot manipulator was developed to automate on-site repairs such as grinding of eroded surfaces, depositing overlay welding and hammer peening. In this study, the structure of robot as the tool holder mechanism of the machining operation is modeled by articulated rigid bodies with flexible joints. The dynamic equations of the 6DOF flexible-joint manipulator are established using Lagrangian formulation. Impulsive grinding forces and periodically perturbed excitations existing in the process are exerted on the model to simulate its response. As an intuitive estimation for joints' stiffness parameters of the model, payload test experiments were performed on the robot. To validate predictions of the dynamic model regarding vibratory behavior, modal testing experiments were performed and measured natural frequencies and mode shapes were compared to their analytical equivalents. Some future trends of the research work are also addressed.