DEVELOPMENT OF A LOW COST PRECISION POLISHING MACHINE BASED ON PARALLEL KINEMATIC SYSTEM

The increasing demand on mass production of high precision parts, has pushed the precision manufacturing industry to develop reliable precision finishing processes such as Bonnet polishing to address market requirements. Indeed, the nature of the surface to be polished plays an important role in the design of a possible polishing machine. A gap within the research in polishing for precision industry needs has been identified. Small parts with <50mm x 50mm and possible freeform curvature containing small slopes cannot be polished with available bonnet polishing (BP) processes on market. This is caused by the tool head size and the tool holder being bigger than part curvature or the part itself. Although, the BP process has a huge potential for surface roughness improvement and form accuracy, it is generally seen in industry as an expensive solution for a non-deterministic finishing process. Therefore, this project has sought to develop a BP machine to cover the gap with an innovative and inexpensive design. In order to develop a machine which responded to the market expectations all possible requirements were listed from a customer point of view. Based on the requirement, a machine concept was produced. Market analysis helped to identify sub-systems of the machine. FEA analysis of the design was performed to check for stress distribution and displacement due to its own mass. Additional assembly parts are designed and a prototype of the machine was produced. The designed machine is tested for its ability as precision polishing machine. Flat surfaces of P20 tool steel were targets for polishing to nanometric surface finishes. Empirical experiments helped to identify parameters which influenced the surface roughness. Taguchi method were then used to optimise the parameters for better surface roughness. Optimum parameters conditions helped to reach less than 10 nm Ra systematically and repeatedly. The samples were also polished using re-circulating slurry techniques, and the obtained results were discussed. Further, pre polishing, Grolishing processes capable of improving surface roughness from ground finish to mirror like finish were developed for cost effective manufacturing procedures. The material removal was analysed to identify parameters capable of improving surface roughness over a step grolishing process. Two grolishing procedures were developed. Both processes produced nanometric range surface finishes. Other variations in results were compared and discussed. Although, machine axis has the ability to produce freeform movement, tool holders need to be improved to facilitate the identification of the distance between tool origin and workpiece origin. Therefore, a new spindle holder assembly is produced to hold the tool and an optical measurement device DRI used to evaluate accurately the distance separating the tool-workpiece origin and further align the workpiece inclination with respect to the machine axis. A CAD-CAM package is also developed to generate programme capable of performing freeform curvature

Development of ultra-precision abrasive machining of functional structured surfaces

Current techniques for the manufacture of functional micro scale surface structures are both time consuming and expensive. Techniques for Ultra-Precision (UP) abrasive machining of functional surfaces, using specially manufactured tools, have previously been described for example in the grinding of riblets. However these methods are both costly and limited in their functionality. Using conventional UP grinding tools and a novel dressing technique, adapted from macro scale grinding, provides an alternative solution offering both low cost and ease of adaptability. Such a system would be capable of producing regular deterministic surface textures over a large area to a depth of 10-20μm grinding in single or multiple passes. To achieve this, the grinding wheel surface is shaped with specialised geometries using a single point diamond dressing tool. The required geometry for a given surface texture can be modelled and used to design a dressing solution consisting of a dressing depth and feedrate. Adapting a conventional grinding machine (Precitech Nanoform 250 Ultra-Grind) for such a process presents many technical challenges to overcome. The quality of the finished surface will be dependent on the accuracy of the dressing process and the tool/workpiece interaction. Methods for controlling and measuring the dressing depth, feedrate, spindle speed and runout have been designed and trialled to determine the feasibility of the process. The results of the trials have been compared against the model to determine the deviation from ideal and hence the quality of the finished surface

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