Adaptive control to actively damp bistabilities in highly interrupted turning processes using a hardware-in-the-loop simulator

Interruptions in turning make the process forces non-smooth and nonlinear. Smooth nonlinear cutting forces result in the process of being stable for small perturbations and unstable for larger ones. Re-entry after interruptions acts as perturbations making the process exhibit bistabilities. Stability for such processes is characterized by Hopf bifurcations resulting in lobes and period-doubling bifurcations resulting in narrow unstable lenses. Interrupted turning remains an important technological problem, and since experimentation to investigate and mitigate instabilities are difficult, this paper instead emulates these phenomena on a controlled hardware-in-the-loop simulator. Emulated cutting on the simulator confirms that bistabilities persist with lobes and lenses. Cutting in bistable regimes should be avoided due to conditional stability. Hence, we demonstrate the use of active damping to stabilize cutting with interruptions/perturbations. To stabilize cutting with small/large perturbations, we successfully implement an adaptive gain tuning scheme that adapts the gain to the level of interruption/perturbation. To facilitate real-time detection of instabilities and their control, we characterize the efficacy of the updating scheme for its dependence on the time required to update the gain and for its dependence on the levels of gain increments. We observe that higher gain increments with shorter updating times result in the process being stabilized quicker. Such results are instructive for active damping of real processes exhibiting conditional instabilities prone to perturbations.

An Investigation of Tool Performance in Interrupted Turning of Inconel 718

The paper presents the result of an investigation of cemented carbides cutting tools performance in interrupted turning of Inconel 718. The discussion is based on case study revealed from industry experience. The spacing created for operational reasons cause interruption in turning process what has crucial effect on tool life and wear. Furthermore cutting parameters and tool geometry have significant influence on creating burr at the edge of spacing which leads dimensional deviation. The research based on two stages, preliminary and modelling, was introduced. In the first an effect of cutting speed, feed, depth of cut, rake angle and tool major cutting edge angle on wear and burr size was investigated. The second step was a regression based modelling of major parameter influence on various process aspects such as values of forces, tool wear and burr size. It was found that the consequential effect on plastic deformation of edge and tool performance have depth of cut. It also has been proven that higher cutting speed, lower values of angles provides better quality of machined element. The paper gives recommendations for turning parameters selection to acceptable burr forms, tool life and process performance.

A Three-Dimensional Transient Thermal Model for Machining

This article presents an enhanced mathematical model for transient thermal analysis in machining processes. The proposed mathematical model is able to simulate transient tool, workpiece, and chip temperature fields as a function of time for interrupted processes with time varying chip loads such as milling and continuous machining processes such as turning and drilling. A finite difference technique with implicit time discretization is used for the solution of partial differential equations to simulate the temperature fields on the tool, workpiece, and chip. The model validations are performed with the experimental temperature measurement data available in the literature for the interrupted turning of Ti6Al6V–2Sn, Al2024, gray cast iron and for the milling of Ti6Al4V. The simulation results and experimental measurements agree well. With the newly introduced modeling approach, it is demonstrated that time-dependent dynamic variations of the temperature fields are predicted with maximum 12% difference in the validated cases by the proposed transient thermal model.