Strategy development for chatter-free milling of Ti-6Al-4V thin-walled surfaces using stability region diagram (SRD)

The uncertainty in the prediction of machining stability increases with the increasing flexibility of the workpiece. The past research showed that the process cannot be stabilized even using the largest pockets in the stability diagrams due to high flexibility of the parts. Therefore, the present research emphasizes extensive experimental analysis where the uncertainty of the stability lobe diagrams was brought into the light, especially in the case of machining of flexible parts. This led to the introduction of a new term “Stability Region Diagram (SRD)” in the present research. The three generalized machining scenarios for chatter-free machining of thin-walled features on low rigidity Ti-6Al-V4 were experimentally analyzed. These include Scenario-I, where workpieces have identical pre-machining stiffness and natural frequency that varies during and after machining. In Scenario-II, workpiece stiffness and natural frequencies vary initially but are identical after machining. In Scenario-III, workpieces have identical pre-machining stiffness and natural frequency but vary during and after machining over central 1/3 part, straddled between bosses. Various machining strategies with an optimum combination of cutting speed-feed-radial depth of cut were developed using stability region diagrams (SRD) to achieve stable machining throughout the length of the cut along the flexible workpiece surface. For Scenario-I and II, stable machining is possible at widths of cut lesser than or equal to the final thickness of workpiece and at a constant spindle speed of 4,000 rpm throughout the length of the workpiece. However, while machining at widths of cut more than the post-machining thickness of the workpiece, the stable machining is possible using a spindle speed ramp-up technique. In the Scenario-III, the surrounding uncut material is found to make machining unstable which can be improved by spindle speed ramp-down technique.