Multibody contact dynamics on mechanisms with deep groove ball bearing joints

The dynamics of ball bearings are important to fatigue breakage, dynamic performance and motion precision of mechanisms connected by ball bearings joints with multi-clearances. In this study, a new method is proposed for multibody dynamics analysis on mechanisms under the effects of radial internal clearances and impact of balls/cage pockets interactions of ball bearings. Including balls/rings interactions and balls/cage pockets interactions, the three dimensional dynamics models of the crank slider mechanism are established and calculated by generalized-α algorithms on the basis of Hertzian contact theory and penalty function method. The rules of eccentric trajectories of inner and outer ring center for one ball bearing joint are verified with the results calculated by XU’s references. The results of dynamic errors, motion trajectories, dynamic forces are achieved under different speeds, radial clearances and number of ball bearings. The speeds and radial clearances are critical to the dynamic performances and motion precision of mechanisms, especially the number of ball bearings. The number of ball bearings is important to impact force and motion stability of the mechanism.     

Chatter prediction utilizing stability lobes with process damping in finish milling of titanium alloy thin-walled workpiece

Machining chatter often becomes a big hindrance to high productivity and surface quality in actual milling process, especially for the thin-walled workpiece made of titanium alloy due to poor structural stiffness. Aiming at this issue, the stability lobes are usually employed to predict if chatter may occur in advance. For obtaining the stability lobes in milling to avoid chatter, this article introduces an extended dynamic model of milling system considering regeneration, helix angle, and process damping into the high-order time domain algorithm which can guarantee both high computational efficiency and accuracy. Via stability lobes, the reasonability and accuracy of the proposed method are verified globally utilizing specific examples in literature. More convincingly, the time-domain numerical simulation is also implemented to predict vibration displacement for partial stability verification. In this extended model, process damping is well-known as an effective approach to improve the stability at low spindle speeds, and particularly, titanium alloy as typical difficult-to-machine material is generally machined at low spindle speeds as well due to its poor machinability. Therefore, the proposed method can be employed to obtain the 3D stability lobes in finish milling of the thin-walled workpiece made of titanium alloy, Ti-6Al-4V. Verification experiments are also conducted and the results show a close agreement between the stability lobes and experiments.     

ESPRIT- and HMM-based real-time monitoring and suppression of machining chatter in smart CNC milling system

Suppression of machining chatter during milling processes is of great significance for surface finish and tool life. In this paper, a smart CNC milling system integrating the function of signal processing, monitoring, and intelligent control is presented with the aim of real-time chatter monitoring and suppression. The algorithm of estimation of signal parameters via rotational invariance techniques (ESPRIT) is adopted to extract the frequency characteristics of acceleration signals, and then, cutting state is categorized as stable state, chatter germination state, and chatter state based on amplitude-frequency characteristics of identified acceleration signals. The model of chatter identification is acquired by training a hidden Markov model (HMM), which combines acceleration signals and labeled cutting state. To implement real-time chatter suppression, the algorithm of fuzzy control is integrated into a smart CNC kernel to determine the relationship between cutting force and spindle speed. Furthermore, spindle speed of machine tool could be adjusted timely in the presented system once the chatter is identified. Finally, the effectiveness of the proposed real-time chatter monitoring and suppression system is experimentally validated.     

Fast prediction of chatter stability lobe diagram for milling process using frequency response function or modal parameters

Prediction of chatter stability is important for planning and optimization of machining process in order to improve machining efficiency and reduce machining damage. Based on the classical analytical solution of chatter stability for milling process and in-depth analysis of the impact of modal parameters on the stability lobe diagram, a straight forward procedure for fast predicting stability lobe diagram directly using modal parameters of machining system was put forward. In consideration of the fact that the modal parameters of milling system can be estimated directly from the frequency response function using single DOF modal parameter estimation method, stability lobe diagram can be plotted directly using the tool tip’s frequency response function. The machining performances of a machining center with three different cutting tools were evaluated and the corresponding optimized cutting conditions were determined. The correctness of the proposed method was validated by good agreement of the predicted stability lobe diagram with that using the classical analytical method, and simulation results show that its calculation speed had been improved by 2–3 orders of magnitude. As a result, the proposed method of plotting stability lobe diagram using frequency response function can be utilized as an effective tool to select chatter-free cutting conditions in shop floor applications.     

An investigation into the inhomogeneity of the microstructure and mechanical properties of explosive welded H62-brass/Q235B-steel clad plates

Owing to excellent corrosion resistance, antifriction property, and economic efficiency, H62/Q235B explosive clad plates have been widely used in various fields. Therefore, detailed and systematic study into the microstructure and mechanical properties of this clad material is necessary. In this paper, the composition, microstructure, and mechanical properties of H62/Q235B explosive clad plates were analyzed by optical metallographic observation, mechanical tests, scanning electron microscope (SEM) observation, and energy dispersive spectrometer (EDS) analysis. The results showed that the bonding interface of this clad plate was periodical wavy, and the interface was bonded in two ways, one through wide transition layer with a width up to 280 μm while the other by narrow transition layer with a width less than 20 μm. The major structural components in the transition layer were supersaturated solid solutions. The microhardness of the transition layer was higher than that of base metals, and the microhardness of the base plates in the region near the bonding interface was affected by both force and heat. The shear strength of the H62/Q235B clad plate showed an obvious characteristic of anisotropy. Furthermore, the clad plate tended to crack along the transition layer when it was stretched because of the discontinuity of plastic deformation across H62/Q235B interface.     

Cutting deflection control of the blade based on real-time feedrate scheduling in open modular architecture CNC system

Machining accuracy of thin-walled parts which have low-rigidity is greatly influenced by cutting deflection in flank milling. In this paper, cutting deflection of aero-engine blade during processing is controlled within a required dimensional accuracy based on the strategy of real-time feedrate scheduling which is integrated into an open modular architecture CNC system (OMACS) of five-axis milling machine. The maximum deflection position of blade is determined through combining analytical cutting force model in flank milling and finite element analysis (FEA)-based transient dynamic analysis. Then, the numerical model of blade deflection is established to obtain the numerical relationship among feedrate, cutting force, and blade deflection, which is usually used to get optimized cutting force and feedrate by setting allowable value of blade deflection. To implement blade deflection control during machining, a real-time control strategy of feedrate scheduling based on nonlinear root-finding algorithm of Brent-Dekker and principle of feedrate smooth transition is developed and integrated into OMACS which has functions of real-time cutting force signal processing and real-time feedrate adjustment. Experimental results show that blade deflection is effectively controlled by proposed strategies, machining accuracy, and efficiency are improved.     

Electrochemical micro-machining of high aspect ratio micro-tools using quasi-solid electrolyte

With the outbreak of product miniaturization, there is an increasing demand for the fabrication of micro-tools in recent years. However, fabrication of accurate micro-tools with high aspect ratio is a great challenge for traditional processes due to their mechanical-thermal effects. Electrochemical micro-machining (EMM) has many advantages over other machining processes, which makes it a potential method to manufacture micro-tools. This paper proposes a novel EMM fabrication method of micro-tools with high aspect ratio, in which an agarose hydrogel of high intensity is employed as quasi-solid electrolyte. During the machining process, a tungsten rod is inserted into the quasi-solid electrolyte which is partially immersed into working electrolyte (2 mol/L NaOH solution) to maintain mass balance. The shapes of micro-tools fabricated in liquid electrolyte and quasi-solid electrolyte under same machining conditions are analyzed. Compared to liquid electrolyte, quasi-solid electrolyte has the advantages of improved precision and ability to manufacture high aspect ratio micro-tools. Besides, effects of main parameters, including vertical distance, duty factor, and pulse peak voltage, on the machining accuracy and efficiency are investigated experimentally. Finally, optimum parameters of 12 mm vertical distance, 50% duty factor, and 5 V pulse peak voltage are selected based on experiments. Using these parameters, a cylindrical micro-tool with an average diameter of 12 μm and aspect ratio of 408.33 is successfully fabricated.     

Influence of axial magnetic field on shape and microstructure of stainless steel laser welding joint

The morphology and microstructure of the weld joint have significant influence on mechanical properties of welded specimens. In this paper, the mechanism on how the external magnetic field affected weld profile and microstructure was discussed by applying the longitudinal steady magnetic field to laser welding for SUS301 stainless steel. The optimal and scanning electron microscopes were used to measure the shape of the cross section and observe the microstructure after welding. The results showed that the shape of the cross section and microstructure could be significantly changed using the external magnetic field. Moreover, joint shape changed distinctly with the magnetic field intensity changing. With the increasing of magnetic flux density, the weld profile of the full penetration model changed from funnel to X type; meanwhile, the bottom weld width increased by 40%. In addition, the partial fusion zone occurred, and the weld width decreased by 20% while penetration increased by 18% when magnetic flux density turned into 380 mT. As far as microstructure of weld joint was concerned, it appeared that application of axial magnetic field led to indistinct fusion line and blocky austenite in big size rather than columnar grain in the center of the cross section. This phenomenon could be explained by numerical simulation results.     

An effective multi-objective genetic algorithm based on immune principle and external archive for multi-objective integrated process planning and scheduling

Process planning and scheduling are two major sub-systems in a modern manufacturing system. In traditional manufacturing system, they were regarded as the separate tasks to perform sequentially. However, considering their complementarity, integrating process planning and scheduling can further improve the performance of a manufacturing system. Meanwhile, the multiple objectives are needed to be considered during the realistic decision-making process in a manufacturing system. Based on the above requirements from the real manufacturing system, developing effective methods to deal with the multi-objective integrated process planning and scheduling (MOIPPS) problem becomes more and more important. Therefore, this research proposes a multi-objective genetic algorithm based on immune principle and external archive (MOGA-IE) to solve the MOIPPS problem. In MOGA-IE, the fast non-dominated sorting approach used in NSGA-II is utilized as the fitness assignment scheme and the immune principle is exploited to maintain the diversity of the population and prevent the premature condition. Moreover, the external archive is employed to store and maintain the Pareto solutions during the evolutionary process. Effective genetic operators are also designed for MOIPPS. To test the performance of the proposed algorithm, three different scale instances have been employed. And the proposed method is also compared with other previous algorithms in literature. The results show that the proposed algorithm has achieved good improvement and outperforms the other algorithms.     

An improved cutting force model for micro milling considering machining dynamics

Micro milling, as a versatile micro machining process, is kinematically similar to conventional milling; however, it is significantly different from conventional milling with respect to chip formation mechanisms and uncut chip thickness modelling, due to the comparable size of the edge radius to the chip thickness, and the small per-tooth feeding. Considering tool runout and dynamic displacement between the tool and the workpiece, the contour of the workpiece left by previous tool paths is typically in a wavy form, and the wavy surface provides a feedback mechanism to cutting force generation because the instantaneous uncut chip thickness changes with both the vibration during the current tool path and the surface left by the previous tool paths. In this study, a more accurate uncut chip thickness model was established including the precise trochoidal trajectory of the cutting edge, tool runout and dynamic modulation caused by the machine tool system vibration. The dynamic regenerative effect is taken into account by considering the influence of all the previous cutting trajectories using numerical iteration; thus, the multiple time delays (MTD) are considered in this model. It is found that transient separation of the tool-workpiece occurring at a low feed per tooth, caused by MTD and the existing cutting force models, is no longer applicable when transient tool-workpiece separation occurs. Based on the proposed uncut chip thickness model, an improved cutting force model of micro milling is developed by full consideration of the ploughing effect and elastic recovery of the workpiece material. The proposed cutting force model is verified by micro end milling experiments, and the results show that the proposed model is capable of producing more accurate cutting force prediction than other existing models, particularly at small feed per tooth.