Ultra-Precision Surface Machining with Reactive Plasma Jets

Atmospheric Plasma Jet Machining (PJM) is a technology for non-mechanical ultra-precision surface shape generation, shape error correction and smoothing based on atmospheric plasma jets. PJM is favorably applied to generate optical surfaces like aspheres, acylinders, or free-forms but also to improve the surface shape accuracy in a very fast and cost-efficient way. For that purpose a mainly fluorine containing plasma jet is brought into contact with a surface to locally remove material by a chemical reaction forming volatile products. Hence, the technology is limited to materials like silicon, fused silica and similar, or silicon carbide. Furthermore, the etch profile results from a convolution of the radical and the temperature distribution at the surface. Since the temperature distribution is also influenced by the plasma jet this leads to a non-linear dependence of the removal function of the plasma tool on its velocity. Using the dwell-time algorithm for deterministic surface machining by superposition of the local removal function of the plasma tool an advanced process simulation is necessary. In a first local approximation the velocity dependence of the removal function, which has to be determined previously, must be incorporated. Second order thermal effects due to inhomogeneous heating caused by the part geometry and the tool path can be managed by a sophisticated calculation of the surface temperature evolution during machining based on the Finite Element Method (FEM). With the help of this procedure the accuracy and convergence of the machining process can be significantly improved. In the article several examples of surface processing using plasma jet machining are presented. ((c) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)