Mask-Less Pocket Milling of Composites by Abrasive Waterjets: An Experimental Investigation

Abstract

Milling pockets in advanced engineering composite materials (AECMs) by conventional methods is difficult due to machinability issues, such as heterogeneous constituents, softening of heat sensitive resin matrix, delamination, fiber pull-out, carcinogenic gases, and dust. On the other hand, waterjet (WJ) machining, among the unconventional approaches, is well known for machining a wide range of AECMs due to its unique features, exertion of low cutting force, low heat generation, no dangerous fume development and low airborne dust. However, from the preliminary studies, it was found that the conventional pocket milling tool path strategies existing in standard computer aided design (CAD) packages cannot be employed directly in milling AECMs with the WJs due to various reasons, such as less strength of composite materials in the transverse direction to the fiber orientation, and aggressive nature of high energy WJs. To address these issues, a novel jet path strategy for mask-less milling of pockets was proposed. This approach takes into account the nature of the highly aggressive fluidjets, the physical structure of the AECMs, and the limitations of the existing hardware controllers, which are not specifically designed for jet machining, while manoeuvring the jet over the surface to avoid undesired excessive erosion and contributes to the elimination of sacrificial masks. The proposed strategy is demonstrated by pocket milling of difficult to machine AECMs (glass and carbon fiber) by the WJ and abrasive waterjets (AWJ) along with geometric analysis on the pockets. The influence of various process parameters, such as water pressure, jet traverse rate, standoff distance (SOD), number of passes, on the milled surfaces was studied. Furthermore, the damage at various regions of the pocket was analyzed by scanning electron microscopy, to find out the causes of surface damages and re-cast of resin layer to address the damage was suggested. The effect of consideration of composite material's physical structure while milling was successfully demonstrated by generating pockets with minimum damage to the reinforcing fibers and delamination. A selection procedure between AWJs and WJs was proposed depending on the scaling of the targeted milling depth and precision required. Finally, modifications to the machine tool hardware controllers are suggested for efficient milling of AECMs.