Modelling and analysis of contact stress for automated polishing

Abstract

Though research on force control of polishing processes is archived in the literature, the objective has been on maintaining a constant force on the polishing tool. However, this constant polishing force does not guarantee a constant contact stress between the polishing tool and the part being polished. It is the contact stress that determines the quality of the polished part, not the force exerted on the polishing tool. Under the constant force on the polishing tool, the contact stress will change from point to point if the part geometry varies, which is the case for most industrial parts. If the contact stress is too high, the part will be over polished. Therefore, the goal of this paper is to model and analyze the contact stress for an automated polishing process. To do so, first, the Hertzian contact model is applied, which calculates the contact stress under a constant force on the tool. This model is established in terms of the part geometry, based on which a pressure distribution map is simulated. Second, with this pressure model at hand, the tool speed and torque model is established, and their distribution maps are also simulated in terms of the part geometry. These maps are then used to generate the set points for tool force and tool speed under the constant contact stress. These set points can be augmented into the conventional NC code to form a complete CNC code for polishing. The effectiveness of the parameter planning is assessed through open loop control. The parameter planning method allows polishing without significantly changing the part profile, whereas without the parameter planning, the part profile is changed considerably.