Development of Precision Polishing Machine Based on Parallel-Kinematic System

Increasing demand on mass production of high precision functional surfaces (curved or freeform) has pushed the development of relatively low cost and high precision surface finishing processes for mould manufacturing. For some applications such as small optical lenses, the existing bonnet polishing (BP) process has limitation in polishing the inner surfaces of moulds due to tool head size and generation of mid-spatial frequency errors. In this paper, a novel precision polishing machine suitable for polishing inner surface of mould was developed based on a parallel-kinematic precision positioning system (PI Hexapod H840.D2). The hexapod was held in a machine frame, which allows precision movement in 6 degrees of freedom. An air bearing, electrically driven spindle (speed range from 500 rpm to 8000 rpm) was mounted to the Hexapod to drive the polishing tool. A rubber bonnet tool with a polyurethane pad was used to engage the part surface. In order to test the performance of this prototype machine, the machine was used to polish flat surfaces of P20 steel alloy which is widely used to manufacture plastic injection moulds. All polishing processes were carried out using diamond paste with particle size of 3 μm. Pre-polishing results showed that the feed rate of the hexapod, spindle rotating speed, tool offset and number of polishing passes are the main factors affecting the polished surface quality. A Taguchi approach was used to study the influence of these four main polishing parameters on the machined surface. After optimisation of the processing parameters, the polishing machine can consistently achieve ≤ 10 nm surface roughness Ra.