Development of precision polishing machine based on a hexapod

Increasing demand on mass production of high precision parts, has pushed the precision manufacturing industry to develop relatively low cost, reliable precision finishing processes to meet market requirements. A gap within the provision of low cost polishing processes has been identified; namely parts with a radius of curvature less than 10mm will be increasingly present in new mass produced products, yet such parts cannot be efficiently polished with existing bonnet polishing (BP) process due to tool head size and the tool holder being bigger than the part radius. Moreover in some applications such as optical lenses the mid-spacial frequencies errors present in the surface severely affects the optical performance of the lenses. Therefore, bonnet polishing with physical contact is more appropriate process for these types of surfaces.
A novel low cost precision polishing machine is presented in the present paper. The machine is a form of bonnet polishing. The machine is composed of a Hexapod H840.D2 (PhysikInstruments) which allows precision movement in 6 degrees of freedom held in a machine frame. An air bearing, electrically driven spindle (speed range from 500rpm to 8000rpm) is mounted to the Hexapod to drive the polishing tool. A rubber bonnet tool with polyurethane pad is used to engage the part surface. In order to test the performance of this prototyped 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 sizes of 3µm. 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. Moreover, the Taguchi approach was used to investigate the four polishing parameters. The optimisation of the parameters allows the polishing machine to consistently achieve ≥10 nm surface roughness Sa.