Precession polishes to new precision

Precession polishes to new precision e measured in single atoms. Tom Shelley reports Glass may be automatically polished to a roughness of 0.5nm – not much more than the typical spacing between atoms in a solid – and stainless steel to a roughness of 1.5nm or about five typical interatomic spacings. As well as allowing the manufacture of optical components to an extraordinary degree of precision, it will engender new levels of smoothness and reliability in mechanical components, especially joints in human prosthetics. The techniques are being jointly developed by Zeeko, a company spun out of University College, London, and that institution's Optical Science Laboratory. Zeeko technical director and UCL reader in optical science, Dr David Walker, explains that the secret is to use a combination of precise numerical control and a patented precession technique.The polishing is undertaken by an inflated rubber tool pressed against the work piece and spun at up to 2,500rpm. The tool is covered with a proprietary surface material such as alumina, diamond slurry and cerium oxide. If the tool is spun co-axial with the workpiece, it presents a much lower contact speed toward its centre than towards its periphery. Its rotation axis is, therefore, at an angle to the workpiece, and rotates round the central axis in the same way that the axis of a spinning top angles itself to the vertical and slowly moves around it. One of the original intentions is to improve the lifetime of components in adaptive optical systems – essentially brittle materials that are required to flex within very short time scales. In bearings, improving surface finish is always a great help countering wear, by reducing the incidence of debris and eliminating surface imperfections, that might initiate fatigue cracks and lead to lives shorter than might otherwise be expected. Dr Walker expects the technique to similarly "revolutionise the finishing of prosthetic knee and hip joints" and also turbine blades, which, like the prosthetic joints, are currently finish polished by hand. In the case of prosthetic joints, smoother surfaces should offer improve functioning and further extend useful life by reducing mutual damage resulting from imperfect surfaces rubbing against each other. Other possible applications include custom engineered bearings and plastic injection mould tools. The first prototype machine has been sold and a second machine, with a 600mm workpiece capability, is now under construction, funded by PPARC and the Ministry of Defence. The machines are capable of polishing a knee joint in a few hours, although a more typical time for optical components is half an hour or less. A vital part of the development is the software used to numerically optimise the tool path. The research activity could form one element of a proposed Institute for Form and Texture proposed by UCL, also taking on board synergistic developments in surface physics and coating technologies. animation Design Pointers Numerically controlled polishing can be used to reduce surface roughness on glass to 0.5nm and on stainless steel to 1.5nm Consequent results are improved fatigue life and extended service life in bearings Target applications include components for adaptive optical systems, prosthetic knee and hip joints, turbine blades and injection mould tools