Effect of Machining Cusps on the Stress Concentration Factor on Curved Surface

Recently multi-axis machining technology has improved significantly. It has become a widely accepted method of manufacturing components with complex, free form surfaces. Solid billet materials with negligible internal defects are used in this process. This provides increased durability and fatigue life over equivalent cast components. However, multi-axis machining using ball nose cutting tools leaves cusps as machining marks. The combination of tool size and step-over generates cusps with different depths and widths. Even though the cusps add extra material on top of the nominal surface, Finite Element Analysis (FEA) shows the maximum stress generated within the cusps is greater than that predicted from the cusp-free geometry. These stress concentrations generated by cusps can reduce the fatigue life and durability of a machined component. This paper reports FEA results; simulating the effect of cusps on surfaces with a range of curvatures for a fixed tool size and six different cusps depth. The FEA results show that the stress concentrations due to surface curvature and cusps can be determined independently and then multiplied together to give the combined stress concentration factor. The results of this analysis will allow designers to specify a maximum machining cusp depth on a curved surface.