Ultra-precision Abrasive Machining of Functional Structured Surfaces

This thesis details the research undertaken to investigate the feasibility of the adaption of macro-scale grinding techniques to the production of micro-scale surface structures. The ability to utilise these techniques in conjunction with equipment normally associated with ultra-precision grinding presents an opportunity to increase the manufacturing capability of existing ultra-precision machining centres at relatively low cost. Grinding is a process well suited to the manufacture of surface structures in substances such as hard or brittle materials, for example tool steels and ceramics, which can be difficult or costly to work. This is of particular value when considering the production of moulds for parts incorporating functional surface structures, which are becoming increasingly common.

A wide variety of parts can incorporate functional structured surfaces which utilise micro and nano scale structures to provide beneficial characteristics. Structured surfaces are currently being used to affect properties such as drag reduction, wettability and reflectivity. However, the manufacture of structured surfaces either directly onto the part or onto moulds and other forming tools presents several challenges. The manufacture of the individual micro and nano scale features requires specialist production techniques which can prove costly and difficult to apply over larger areas.

Recently macro-scale grinding techniques using grinding wheels incorporating specialised geometry have been demonstrated which can be used in the production of deterministic surface textures. Adapting these techniques for micro-scale production of surface structures presents several challenges. The tools and equipment used must be sufficiently accurate and capable of a high degree of control while also being cost effective and rugged enough to be utilised in a typical CNC ultra-precision machining environment. The scalability of the process also presents a unique challenge in the physical interaction of the tool and workpiece. Conventional techniques used to analyse the performance of ultra-precision grinding are unsuitable or insufficient to characterise the micro-scale structures generated by such a process.

The original contribution of this work is to address these challenges and demonstrate a system capable of dressing grinding wheels by utilising a novel offset technique with a continuously rotating single point dressing nib. Grinding wheels have been dressed with specialised geometry and used to produce micro patches with aspect ratios of 10:1 and feature heights of 40+ μm. The system presented is cost effective and could be retrofitted onto an existing CNC ultra-precision machining centre for approximately £10,000 to £15,000. While not capable of producing every distinct type of surface structure; this system has the potential to greatly increase the ability of small manufacturing companies to incorporate a specific range of functional surface structures into their products without the expense of large capital outlay on specialised equipment. This would greatly impact on their ability to manufacture high value added parts.