Machining strategies for hole making in composites with minimal workpiece damage by directing the process forces inwards

Mechanical machining of fiber reinforced plastics has been subject to research for many years. Especially drilling with its resulting workpiece damage such as spalling or delamination still is critical. Approaches to reduce damage usually aim at reducing axial thrust forces and thus reduce the damage-causing effects. In this article two milling strategies for hole making with standard tools are presented, which actively direct process forces toward the center of the workpiece when machining the outer layers: a three-axial combined process of circular and spiral milling as well as a five-axial process called wobble milling. Better machining results are expected, as the material may act as its own back-up, thus reducing especially surface damages. Basic considerations and calculations regarding the direction of the theoretical process forces in dependence on the tool movement are given. Machining experiments have been performed on short glass fiber reinforced polyester and the resulting workpiece damage has been evaluated to assess the potential of the new strategies. Additionally some computer tomography scans have been obtained to qualitatively assess the machined surfaces. The experimental results support the presented idea: machining damage can be significantly reduced by machining strategies which direct the process forces inwards as compared to the reference process of circular milling. The results also indicate that the damage decreases with an increasing ratio of process forces which are directed toward the center of the workpiece.