Milling Processes with Active Damping: Modeling and Stability

Active dampers are on the verge of appearing in commercial machines as devices that assist the avoidance of machine tool chatter. The adjustment of control parameters in these devices is mostly guided by models that do not consider the dynamics within the control loop of active damper. Therefore, these models neglect the dynamics of actuation, measurement and filtering, which can result in inaccurate stability predictions that hinder the efficient tuning of active dampers. To formulate a more realistic model for milling processes assisted by active damping, this paper derives a novel mathematical model that takes into account the internal dynamics of the actuator, measuring device, and discrete filtering. This study shows that accurate stability prediction requires the incorporation of actuator and filter dynamics into the model, especially at high spindle speeds and large feedback gains.

Modal analysis of spindles while accounting for system decay and its application to machine tool chatter prevention

This paper investigates the vibrational characteristics of a machining spindle over its life span. The experimental investigation was carried out using tap testing, where the fundamental frequencies of the spindle system were recorded for different spindle categories, namely, ‘production’ and ‘prove-out’ spindles. Focussing on production spindles, the system ageing translated through a reduction in the system’s natural frequency is modelled as changes in the bearings’ stiffness. The experimentally evaluated natural frequencies were then used to calculate the equivalent bearings’ stiffness within the spindle by means of a calibrated dynamic stiffness method (CDSM) at various stages of spindle’s life. A comparison between the stability lobes generated for two different instances in time, in a full slotting cuts process, shows that over the life span of a spindle, the stability lobes would shift sufficiently to cause chatter after initially being stable. Therefore, as the spindle ages, the presented methodology can be exploited to predict the updated machining parameters necessary to avoid unstable chatter conditions.<div><br></div><div>This is a post-peer-review, pre-copyedit version of an article published in The International Journal of Advanced Manufacturing Technology. The final authenticated version is available online at: https://doi.org/10.1007/s00170-015-6979-4 <br></div>

Modal analysis of spindles while accounting for system decay and its application to machine tool chatter prevention

This paper investigates the vibrational characteristics of a machining spindle over its life span. The experimental investigation was carried out using tap testing, where the fundamental frequencies of the spindle system were recorded for different spindle categories, namely, ‘production’ and ‘prove-out’ spindles. Focussing on production spindles, the system ageing translated through a reduction in the system’s natural frequency is modelled as changes in the bearings’ stiffness. The experimentally evaluated natural frequencies were then used to calculate the equivalent bearings’ stiffness within the spindle by means of a calibrated dynamic stiffness method (CDSM) at various stages of spindle’s life. A comparison between the stability lobes generated for two different instances in time, in a full slotting cuts process, shows that over the life span of a spindle, the stability lobes would shift sufficiently to cause chatter after initially being stable. Therefore, as the spindle ages, the presented methodology can be exploited to predict the updated machining parameters necessary to avoid unstable chatter conditions.<div><br></div><div>This is a post-peer-review, pre-copyedit version of an article published in The International Journal of Advanced Manufacturing Technology. The final authenticated version is available online at: https://doi.org/10.1007/s00170-015-6979-4 <br></div>

Regenerative Cutting Dynamics for a Periodically Forced Machine-Tool System

In this paper, a nonlinear, regenerative, orthogonal cutting system with a weak periodic oscillation of workpiece is considered. Period-1 motions in such a system are studied through a semi-analytical method, and the corresponding stability and bifurcations of the period-1 motions are analyzed via the eigenvalue analysis. The vibration of machine-tool varying with excitation is studied, and excitation effects on machine-tool chatters are discussed. Numerical simulations of unstable and stable period-1 motions are completed from analytical predictions. The machine-tool chatter can emerge from the saddle-node or Neimark bifurcation of period-1 motions.