A simulated investigation on the machining instability and dynamic surface generation

Fabrication of deep micro-holes in reaction-bonded SiC by ultrasonic cavitation assisted micro-EDM

The surface generated by electrical discharge machining (EDM) appears to be generally mat. However, the machined surface in EDM with silicon powder mixed fluid becomes glossier and has a smaller surface roughness than that in conventional EDM with kerosine type fluid, which leads to the omission of finish hand lapping of metal mold. The quality of machined surface significantly depends on the kind of workpiece material, but the surface generation mechanism has not yet been made clear sufficiently.In this study, the effect of silicon powder mixing on the surface generation mechanism is experimentally investigated, analyzing the shape of crater generated by a single pulse discharge, the surface roughness, the machined surface and so on. Main conclusions obtained are as follows:(1) The gap distance in EDM with silicon powder mixed fluid is larger than that with kerosine type fluid, because of the lower resistivity of the former and the influence of silicon powder arrangement in the gap.(2) EDM with silicon powder mixed fluid leads to smaller undulation of a crater, because of the smaller impact force acting on the workpiece due to evaporation and expansion of machining fluid.(3) The more the precipitated carbides in the workpiece are, the larger the surface roughness is, since the carbides come off because of crack propagation due to the frequent heat impact during EDM.(4) EDM with silicon powder mixed fluid results in the stable machining without short circuit between the electrode and the workpiece.

The paper presents a modelling and simulation approach for investigating the generation of precision surfaces in relation to the affecting factors from the machine tool, cutting tool, workpiece materials and operation conditions in precision manufacturing qualitatively and quantitatively. The modelling approach integrates dynamic cutting force model, thermal model, vibration model, chatter model, tool wear model, machining system response model and surface topography model as a whole. Some nonlinearities, such as spindle motion error, generation of built-up-edge, etc. in the machining process are modelled in the light of their physical features. The cutting force and surface texture parameters are the outputs of the modelling approach. The whole modelling and simulation is implemented by MATLAB & Simulink programming. Precision turning trials are conducted to validate the modelling and simulations developed. The simulation results are quite consistent with experimental results to certain extent. They all show that surface texture/topography generation is highly affected by feed rate and some nonlinear factors in the precision diamond turning process.

Prediction of measured surface topography with forced vibration effects

Metrological relations between the spray atomization parameters of a cutting fluid and formation of a surface topography and cutting force

This paper presents the investigation on the cutting mechanism in SiCp/Al turning process. The fracture and removal mechanism of SiC particle and its influence on surface generation are investigated on the basis of a three-dimensional (3D) finite element model. Different depth of cut is adopted in this simulation to study the influence of relative position between the cutting tool and SiC particle on the cutting mechanism. By comparing the topography of the machined surface at different depths of cut, the influence of relative particle position on the surface topography was analysed. The dynamic change process of the stress/strain map was used to reveal the formation mechanism of particle fracture. Different particle behaviours such as slightly fracture at top side, residual fragments embedded within cavity and completely debonding are observed in this model. Various types of defects on the machined surface are also observed such as shallow cavity around embedded particles, severe plastic deformation of matrix materials around cavity due to particle squeezing and cavity caused by particle debonding. In addition, the fragments of particles squeezed by cutting tool during cutting process are found to change the chip formation mechanism. Finally, the simulation results are verified by experimental data from precision turning.

Chatter causes machining instability and reduces productivity in the metal cutting process. It has negative effects on the surface finish, dimensional accuracy, tool life and machine life. Chatter identification is therefore necessary to control, prevent, or eliminate chatter and to determine the stable machining condition. Previous studies of chatter detection used either model-based or signal-based methods, and each of them has its drawback. Model-based methods use cutting dynamics to develop stability lobe diagram to predict the occurrence of chatter, but the off-line stability estimation couldn’t detect chatter in real time. Signal-based methods apply mostly Fourier analysis to the cutting or vibration signals to identify chatter, but they are heuristic methods and do not consider the cutting dynamics. In this study, the model-based and signal-based chatter detection methods were thoroughly investigated. As a result, a hybrid model- and signal-based chatter detection method was proposed. By analyzing the residual between the force measurement and the output of the cutting force model, milling chatter could be detected and identified efficiently during the milling process.

Milling is a kind of interrupted cutting. When the tool cuts into the workpiece, it is often accompanied by instantaneous impact, which results in impact vibration of the milling system. Meanwhile, tool wear will occur gradually on flank face with the cutting progress. The impact vibration and tool wear affect the morphological characteristics of machined surfaces. In the present work, the instantaneous impact force is obtained by introducing the italic impact model of single degree of freedom, and the free vibration in the x and y directions under the impact force is obtained by combining the damped vibration equation. The cutting vibration in the x and y directions is obtained by solving the dynamic equation of the milling system with the fourth-order Runge–Kutta method. The equation of the cutting edge is modified according to the tool flank wear, and the machined surface topography considering the dynamic characteristics of cutting system and tool flank wear is obtained by combining the Z-MAP algorithm. The verification milling experiment was carried out on the hardened steel Cr12MoV workpiece. The simulation results are in good agreement with the experimental results. The research results have important guiding significance for the reasonable selection of processing parameters in actual production.

Ti–6Al–4V alloy has been widely used in biomedical applications such as implants, screws, plates, and so on. The key parameters for implant stability are surface integrity (surface roughness and surface topography). In this study, experimental study on the surface integrity and machining characteristics of Ti–6Al–4V alloy machined by wire electrical discharge machining (W-EDM) has been studied. The research was planned using the methods of Taguchi and the orthogonal range of L-27 was chosen. The effect of W-EDM system parameters, such as peak current, pulse length, pulse frequency, wire feed, and spark gap, sets voltage on the machining and surface characteristics of material removal rate (MRR) and surface roughness (SR) along with machined surface topography. Such parameters have been shown to have a significant influence on the characteristics of the output response, and rises in MRR and SR with peak current.

Earth pressure balanced (EPB) shield machines are large and complex mechanical systems and have been widely applied to tunnel engineering. Tunnel face stability evaluation is very important for EPB shield machines to avoid ground settlement and guarantee safe construction during the tunneling process. In this paper, we propose a novel earth pressure field modeling approach to evaluate the tunnel face stability of large and complex EPB shield machines. Based on the earth pressures measured by the pressure sensors on the clapboard of the chamber, we construct a triangular mesh model for the earth pressure field in the chamber and estimate the normal vector at each measuring point by using optimization solution and projection Delaunay triangulation, which can reflect the change situation of the earth pressures in real time. Furthermore, we analyze the characteristics of the active and passive earth pressure fields in the limit equilibrium states and give a new evaluation criterion of the tunnel face stability based on Rankine's theory of earth pressure. The method validation and analysis demonstrate that the proposed method is effective for modeling the earth pressure field in the chamber and evaluating the tunnel face stability of EPB shield machines.

Machined surface topography and surface roughness are significant factors that directly affect the service performance of the material. A simulation of surface topography in ball-end milling of AISI H13 steel is developed based on the relative motion between cutting tool and workpiece. The developed model was verified by milling experiment and can be used to simulate the machined surface topography accurately. In order to optimize the surface roughness by taking material removal rate (MRR) into account, a simulation trial is conducted by employing the product value (p) and ratio (r) of feed per tooth f z and radial depth of cut a e based on the developed model. The effect of r and p on three dimensional (3D) arithmetic average deviation S ba has been investigated. An optimizing model for S ba with regard to p and r is developed. For a given value of p, which means for constant MRR, the value of r for minimum S ba can be calculated. The validation of the optimizing model was conducted by experiment, and the model was proved to be able to precisely predict S ba within the range of cutting parameters in this research.

In order to choose the appropriate reference surface on the machined surface roughness of SiCp/Al composites, the cutting experiments of SiCp/Al composites were carried out, and the machined surface topography was measured by OLS3000 Confocal laser scanning microscope. The 3D measured data of machined surface topography were analyzed by the area power spectrum density. The result shows that the texture of machined surface topography in milling of SiCp/Al composites is almost isotropic. This is the reason that the values of R q at different locations on the same machined surface are obviously different. Through the comparison of performance of different filtering methods, the robust least squares reference surface can be used to extract the surface roughness of SiCp/Al composites effectively.

Ultrasonic assisted electrochemical discharge milling of complex glass microstructure with high-quality

In astronomy, compound freeform surfaces, including multiple sub-components and sub-apertures, are frequently seen in integral field spectrographs (IFSs). The GIRMOS image slicer unit has 42 toric surfaces, each measuring 270μm in width and 6mm in length, with tilts in both the x and y axes. Achieving the required surface roughness is a crucial factor in assessing the performance of high-precision optical mirror surfaces. This work presents a model for simulating surface topography in ultra-precision diamond machining (UPDM) of toric surfaces resembling image slicers. The surface profile on a single feed constitutes the fundamental unit of surface topography. The existing model incorporates the kinematics of the diamond turning process, defined by machining parameters and the corresponding vibrations between the tool and the workpiece in both the x and y directions. The influences associated with the duplication effect models of the tool edge profile, comprising the nose radius effect, waviness effect, material spring back, plastic side flow, and material defects within the 2D profile, are analyzed. The comparison uses a full-nose radius profile and a half-nose radius profile for a better understanding of tool selection as per the image slicer geometry, including variation with respect to vibrations, feed, tool nose radius, and tool edge waviness.

PurposeTracked hydraulic excavators are versatile and ubiquitous items of off-highway plant and machinery that are utilised throughout the construction industry. Each year, a significant number of excavators overturn whilst conducting a lifting operation, causing damage to property, personnel injury or even fatality. The reasons for the overturn are myriad, including: operational or environmental conditions; machine operator acts or omissions; and/or inadequate site supervision. Furthermore, the safe working load (SWL) figure obtained from manufacturer guidance and utilised in lift plans is based upon undertaking a static load only. The purpose of this paper is to determine whether the SWL is still safe to be used in a lift plan when slewing a freely suspended (dynamic) load, and, if not, whether this may be a further contributory factor to overturn incidents.Design/methodology/approachPrevious research has developed a number of machine stability test regimes but these were largely subjective, impractical to replicate and failed to accurately measure the “dynamic” horizontal centrifugal force resulting from slewing the load. This research contributes towards resolving the stability problem by critically evaluating existing governing standards and legislation, investigating case studies of excavator overturn and simulating the dynamic effects of an excavator when slewing a freely suspended load at high rotations per minute (rpm). To achieve this, both the static load and horizontal centrifugal force from slewing this load were calculated for six randomly selected cases of an excavator, with different arm geometry configurations.FindingsThe results from the six cases are presented and a worked example of one is detailed to demonstrate how the results were derived. The findings reveal that the SWL quoted on an excavator’s lift rating chart considerably underestimates the extra forces experienced by the machine when an additional dynamic load is added to the static load whilst lifting and slewing a freely suspended load.Originality/valueThis work presents the first attempt to accurately model excavator stability by taking consideration of the dynamic forces caused by slewing a freely suspended load and will lead to changes in the way that industry develops and manages lift plans. Future research proposes to vary the weight of load, arm geometry and rpm to predict machine stability characteristics under various operational conditions, and exploit these modelling data to populate pre-programmed sensor-based technology to monitor stability in real time and automatically restrict lift mode operations.