A Study on Abrasive Waterjet Milling

Doktorsavhandling, 1997

Abrasive waterjets (AWJ) have in a short time become an indispensable industrial tool for many machining operations involving hard-to-machine materials. In general, the process is used for through-cutting but there is an awakening interest in utilizing the process for three-dimensional machining, or milling. This thesis presents a study on AWJ milling aiming at identifying the problems related to the abrasive waterjet,s use as a milling operation, and developing it towards a functioning milling process. The results are presented in five papers concerning the behavior and mechanisms of the AWJ milling process. An experimental investigation was performed in order to identify important parameters for controlling the jet penetration depth as well as the quality of the operation. Statistical methods were employed to find a quantitative comparable estimate of the effects resulting from a selected set of parameters. The process was found to be highly sensitive to disturbances. Parameter variations in abrasive mass flow, water pressure and traverse rate were shown most important. As a means of controlling the jet penetration depth and minimizing the influence of parameter variations, a discrete approach to abrasive waterjet milling was introduced. For this a conventional AWJ cutting head was used, producing a discrete machining pulse when supplied with a controlled volume of abrasives. Each pulse creates a unit cavity, and by superposing a number of such cavities, a larger scale milling operation may be produced. A part of the study deals with the typical surface characteristics produced by AWJ milling operations. The milled bottom surface typically exhibits a texture having three characteristic main features: a surface roughness, typically depending on individual particle impacts; a cross-feed waviness, typically of a wavelength equal to the cross feed increment; and a process waviness having wavelengths of several millimeters. AWJ machining of engineering ceramics may be a future application of interest. In a study on the use of the AWJ for ceramic wear testing, mechanisms active in the material removal of ceramics were identified. The presence of temperatures above the melting point of garnet 1230 °C was indicated, playing a part in the material removal process. Finally, a brief study was made investigating the viability of using an AWJ slot milling process for replacing a problematic conventional milling process in an industrial application, involving large-series production. The study indicated that AWJ slot milling was suitable for performing the operation. Since results were even improved using AWJ milling, it may be a preferred choice for the operation. Hence, this study indicates that AWJ milling may now be applicable for large-series production. However, more demanding milling operations still require further development of the AWJ milling technique.