Validation of variable helix milling instability islands

During machining, it is well-known that unstable self-excited vibrations known as regenerative chatter can limit productivity. There has been a great deal of research that has sought to understand regenerative chatter, and to avoid it through modifications to the machining process. One promising approach is the use of variable helix tools. Here, the time delay between successive tooth passes is intentionally modified, in order to improve the boundary of instability. Previous research has predicted that such tools can offer significant performance improvements whereby islands of instability occur in the stability lobe diagram. By avoiding these islands, it is possible to avoid regenerative chatter, at depths of cut that are orders of magnitude higher than for traditional tools. However, to the authors’ knowledge, these predictions have not been experimentally validated, and there is limited understanding of the parameters that can give rise to these improvements. The present study seeks to address this shortfall. A recent approach to analysing regenerative chatter stability is modified, and its numerical convergence is shown to outperform alternative methods. It is then shown that islands of instability only emerge at relatively high levels of structural damping, and that they are particularly susceptible to model convergence effects. The model predictions are validated against detailed experimental data that uses a specially designed configuration to minimise experimental error. To the authors’ knowledge, this provides the first experimentally validated study of unstable islands in variable helix milling, whilst also demonstrating the importance of structural damping and numerical convergence on the prediction accuracy.

Predicting regenerative chatter in milling with hardware-in-the-loop simulation using a dexel-based cutting model

This article proposes a new model for simulating the interaction between cutting process and machine tool in real-time. The purpose of the model is to be coupled with a real CNC (by using hardware-in-the-loop simulation) in order to consider process forces and to predict regenerative chatter vibrations during virtual commissioning. Therefore a dexel-based workpiece model with adaptive resolution is used for the computation of the chip thickness respectively the cutting forces based on the actual machine tool position and the machining progress on the workpiece. Several simulation experiments are performed to validate the model and to analyze its numerical limits, such as computational accuracy and efficiency. The capability of the model to predict chatter is proven by comparing the simulated critical depth of cut with an analytical solution of the stability lobes. Therefore the dynamics of the machine tool were approximated as a single degree of freedom (SDOF) oscillator. A concluding analysis of the real-time factor confirms the model’s ability to be integrated under hard real-time requirements and with cycle times of just a few milliseconds which are typical of CNCs.

An Efficient Third-Order Full-Discretization Method for Prediction of Regenerative Chatter Stability in Milling

The prediction of regenerative chatter stability has long been recognized as an important issue of concern in the field of machining community because it limits metal removal rate below the machine’s capacity and hence reduces the productivity of the machine. Various full-discretization methods have been designed for predicting regenerative chatter stability. The main problem of such methods is that they can predict the regenerative chatter stability but do not efficiently determine stability lobe diagrams (SLDs). Using third-order Newton interpolation and third-order Hermite interpolation techniques, this study proposes a straightforward and effective third-order full-discretization method (called NI-HI-3rdFDM) to predict the regenerative chatter stability in milling operations. Experimental results using simulation show that the proposed NI-HI-3rdFDM can not only efficiently predict the regenerative chatter stability but also accurately identify the SLD. The comparison results also indicate that the proposed NI-HI-3rdFDM is very much more accurate than that of other existing methods for predicting the regenerative chatter stability in milling operations. A demonstrative experimental verification is provided to illustrate the usage of the proposed NI-HI-3rdFDM to regenerative chatter stability prediction. The feature of accurate computing makes the proposed NI-HI-3rdFDM more adaptable to a dynamic milling scenario, in which a computationally efficient and accurate chatter stability method is required.