An Innovative Machining Strategy for Efficient Peripheral Finishing of Hard Materials with Highly-Varied-Helix End Mill

Grinding is usually applied to the peripheral finishing of hardened steel since the high specific cutting force and low stiffness of slender end mills often causes chatter vibration. On the other hand, varied-helix end mills have suppressed regenerative chatter vibration successfully in the rough machining of flexible workpieces. In this research, varied-helix end mills are applied to the extremely low radial immersion finishing of hardened steel, and the validity of this application is discussed and verified experimentally in terms of suppression of regenerative chatter vibration. A special varied-helix end mill with an extremely large helix angle difference is developed for this new application, and its performance is compared to that of an ordinarily-varied-helix end mill for the low-radial-immersion peripheral finishing of hard materials.

Estimating Cutting Force Coefficients in High-Speed Low-Radial-Immersion Milling Processes

A high-speed milling system is considered, which is prone to chatter vibration, a stability condition dependent on system parameters (e.g., cutting force coefficients). This work is motivated by the need for model parameters which can be used in stability analysis. An Extended Kalman Filter (EKF) is proposed to estimate cutting force coefficients for each tooth in a low-radial-immersion milling process to aid chatter stability prediction. The proposed EKF utilizes tool deflection measurements and no force measurements. The model used in the EKF is found to be observable, a quality required to achieve valid state estimations. Running the EKF with experimental tool deflection measurements produces estimates of cutting force coefficients that result in good correlation between simulation (using the estimated coefficients) and experiment. Such an EKF may help customize chatter stability analysis to any particular tool-workpiece system.

Stability Prediction and Dynamics Analysis of Milling with Variable Pitch Cutter in Low Radial Immersion

By disrupting the regenerative effect, milling cutters with variable-pitch are usually used to improve stability. Since high interrupted cutting processes, such as low radial immersion case, will result in more nonlinear phenomena and some consequent differences of stability chart, in this paper, an improved semi-discretization algorithm is utilized to predict the stability lobes for variable-pitch milling under low radial immersion ratios, the focus of the current manuscript is to investigate the stability trend caused by tool geometries. In addition, the chart differences in the cases of low and high radial immersion milling are also discussed by comparisons. Under certain combinations of parameters, some phenomena, like bunch of isolated island and flip bifurcation region are described, and some influences of tool geometries on stability trends are shown and explained.

A Comparison of the Simulated Dynamics of Various Models Used to Predict Undeformed Chip Thickness in High-Speed Low-Radial-Immersion Milling Processes

Various models have been proposed to estimate the undeformed thickness of chips produced by a CNC milling tool, in order to calculate the forces acting on the tool. The choice of model significantly affects the simulated dynamics of the tool, thereby affecting the dynamic stability of the simulated process and whether or not chatter occurs in a given cutting scenario. Simulations of the dynamics of the milling process can be used to determine the conditions at which chatter occurs, which can lead to poor surface finish and tool damage. The dynamics of a traditional model and a more detailed numerical model are simulated here with particular emphasis on the differences in their chatter bifurcation points. High-speed, low-radial-immersion milling processes are simulated because of their application in industrial high-precision machining.

On a Numerical Method for Simultaneous Prediction of Stability and Surface Location Error in Low Radial Immersion Milling

This brief proposes a numerical approach for simultaneous prediction of stability lobe diagrams and surface location error in low radial immersion milling based on the direct integration scheme and the precise time-integration method. First, the mathematical model of the milling dynamics considering the regenerative effect is presented in a state space form. With the cutter tooth passing period being divided equally into a finite number of elements, the response of the system is formulated on the basis of the direct integration scheme. Then, the four involved time-variant items, i.e., the time-periodic coefficient item, system state item, time delay item, and static force item in the integration terms of the response, are discretized via linear approximations, respectively. The corresponding matrix exponential related functions are all calculated by using the precise time-integration method. After the state transition expression on one small time interval being constructed, an explicit form for the discrete dynamic map of the system on one tooth passing period is established. Thereafter, the milling stability is predicted via Floquet theory and the surface location error is calculated from the fixed point of the dynamic map. The proposed method is verified by the benchmark theoretical and experimental results in published literature. The high efficiency of the algorithm is also demonstrated.