A MACHINING STABILITY MODEL WITH PROCESS DAMPING FOR HARD-TO-MACHINE MATERIALS
A substantial fraction of current machining research is directed towards increased productivity. A fundamental limitation is unstable vibration, or chatter, during cutting. Stability lobe diagrams, which relate the allowable chip width to spindle speed, may be used to select stable cutting conditions based on the system dynamics. However, at lower cutting speeds the stability limit asymptotically approaches a nearly constant chip width (using standard stability analyses) and variations in spindle speed do not have a significant effect. Thermal and hardness characteristics limit hard-to-machine metals, such as titanium and nickel alloys, to lower spindle speeds to avoid prohibitive tool wear. At these speeds, the allowable chip width can be larger due to an effect known as process damping.This work develops an analytical stability model that includes process damping effects in turning and milling for single and multiple degree of freedom (DOF) dynamic systems. This model includes contributions from the system frequency response functions, as well as a process damping force normal to the cut surface, which is a function of the depth of cut, cutting speed, and an empirical process damping coefficient, C.Stability testing was completed using a parallelogram, leaf-type flexure to identify the process damping behavior for low-speed single DOF milling. A representative database of process modeling coefficients was established for the workpiece materials: AISI 1018 steel, 6Al-4V titanium, 304 stainless steel, and Inconel 718. Two inserted cutting tools were used with relief angles of 11° and 15°; the rake and helix angles were zero for both single-insert cutters. It was demonstrated that a reduction in the relief angle and an increase in flank wear on the cutting edge results in an increased process damping effect. Multi-DOF (MDOF) systems for turning and milling were also modeled. Stability experiments were performed using a custom double-parallelogram notch-hinge flexure and a finned 6061-T6 aluminum workpiece for milling. Similarly, orthogonal stability testing was completed for turning using a custom parallelogram notch hinge flexible cutting tool. Tubular 6061-T6 aluminum workpieces were machined to validate the MDOF turning algorithm. The results indicate that the multiple degree of freedom model is able to predict a stability boundary that best represents the cutting test outcomes using a single process damping coefficient. Finally, the versatility of the experimentally identified process damping coefficients was examined. To accomplish this, the coefficient identification for 6061-T6 aluminum using the single DOF parallelogram milling setup was first performed. Secondly, the same 11° relief angle milling insert was mounted to the MDOF turning flexure and the orthogonal turning tests were repeated on the same composition aluminum alloy.The effects of changes in system dynamics on the process damping coefficient was observed to be minimal for a reduction of approximately 32% in the system's natural frequency. The process coefficients were similarly identified for much larger changes in dynamics, i.e., a 200-300% increase in the system's natural frequency. It was found that the identified process damping coefficients remained relatively consistent for the dynamic systems tested.