Micro-end Milling Research Articles

Evaluation of tool wear during micro-milling of ultrasonically assisted abrasive peened Ti-6Al-4V

When a cutting tool shears through a workpiece, the interaction causes wear of the tool which is based on relative hardness, surface characteristics of the tool and the workpiece, cutting environment and thermal behaviour at the work/tool interface. In this study, wear of a micro-end milling tool is investigated while milling of as-received and peened work material (in this case, Ti-6Al-4V). The peening is performed with the help of an in-house developed ultrasonic cavitation process. The process excites the abrasives suspended in the cavitating medium, which impart on the work surface and induce compressive residual stress and uniform grain refinement. As the cutting process advances, a built-up is formed more rapidly in the as-received workpiece, leading to higher cutting forces and earlier chipping of the cutting edge. Under the peened conditions, it is found that the increase in tool edge radius and flank wear width reduce by 50 % and 54 % respectively compared to the as-received one. Additionally, peening-induced grain refinement leads to a 26 % decrease in surface roughness of the machined surface and inhibits burr formation by 60 %. A tool wear chipping model is employed to predict the tool diameter, cutting edge radius and flank wear width of the worn tool. Results show that the magnitude of flank wear, edge radius of worn tool and tool diameter deviate from experiments by 15.2 %, 14 % and 13 % respectively.

Influence of tool flank wear considering tool edge radius on instantaneous uncut chip thickness and cutting force in micro-end milling

Influence of tool flank wear considering tool edge radius on instantaneous uncut chip thickness and cutting force in micro-end milling

Investigation of the tool flank wear influence on cutter-workpiece engagement and cutting force in micro milling processes

Cutting force modeling is essential for comprehending the dynamic mechanics of the micro-milling process. Accurate calculation of the cutter-workpiece engagement is crucial for predicting cutting force accurately. This study proposes an accurate analytical model of cutter-workpiece engagement that comprehensively incorporates tool flank wear, tool edge radius, and tool runout for predicting cutting forces in the micro-milling process. Based on the actual tool radius model, which takes into account tool flank wear and tool edge radius, a fast real-time online method for accurately calculating the entry and exit angles is presented. This method involves the trochoidal trajectories of the current cutting edge and all passing cutting edges from the previous cycle under the influence of tool runout. Then the cutter-workpiece engagement is determined. Based on this, the cutting force is predicted combined with instantaneous uncut chip thickness and cutting force coefficients considering tool flank wear and tool runout. Through micro-end milling experiments, the validity of the cutting force model is confirmed. The statistical analysis of the predicted cutting forces further demonstrates the effectiveness of accounting for the impact of tool flank wear on the cutting forces throughout the entire micro-milling process. Through case analysis, it is concluded that tool flank wear leads to increased cutter-workpiece engagement and significantly affects the cutting forces in three directions. This work could offer a theoretical foundation and practical guidance for predicting tool life and controlling surface quality during the machining process.

Effect of arc-deposited diamond-like carbon (DLC) coating thickness on friction and size effects in high-speed micromilling of Ti6Al4V

Diamond-like carbon (DLC) can be considered one of the frontline coating materials for high-speed cutting tools owing to both abrasion resistance and solid lubricant properties. However, coating thickness increases the edge radius of the micro tools, which may result in adverse rubbing and plowing that need to be investigated. In this work, DLC coatings of 440, 640, and 1160 nm thicknesses with a ∼250 nm Cr interlayer are deposited on WC micro-end mills using a cathodic-arc system. DLC-coated surfaces exhibited enhanced tribological and mechanical properties compared to uncoated surfaces. Among the DLC-coated micro-end mills, the DLC-640 nm coated tool outperformed, manifesting the best machining performance by reducing friction and size effects together.

Development of automated impact system for modal analysis of micro-end mill

The micro cutting tool used in micromilling process is highly susceptible to instability attributed to its low strength. A catastrophic failure of micro-end mill can occur if the chatter is not controlled during micromilling process, especially for micromilling of difficult-to-machine materials, like Ti and Ni alloys. Consequently, the control of chatter in micromilling process is necessary to manufacture the components with required dimensional and geometrical accuracy. Chatter can be avoided by selecting the stable process parameters from stability lobe diagram or by changing the process parameters during machining process by chatter onset identification. The both ways of chatter avoidance require the reliable identification of the modal parameters of the micro-end mill. Experimental modal analysis is most accurate method for identification of modal parameters owing to its ability of consideration of machine tool compliance. Hence, in the present work, automated impact system has been developed to impact the micro-end mill under both stationary and rotating conditions. The developed impact system has been found to be impacting the micro-end with maximum deviation of 2.6% in force amplitude due to repeated impacts. A maximum variation of 1.7% has been found in the natural frequency due to different impacts of the micro-end mill under both stationary and rotating conditions. The accurate estimation of frequency response function is expected to predict the stability of micromilling process with high reliability.

Effects of tailored residual stress on micro-end milling: numerical modelling and validation

Effects of tailored residual stress on micro-end milling: numerical modelling and validation

Experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites

A comprehensive experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites (EP/CNF) is presented in this study. The machinability indicators including cutting force and surface roughness have been investigated. Tensile properties, morphology of tensile fracture surfaces, glass transition temperature, machined chip morphology, and machined surface morphology were also characterised. To investigate the effect of both workpiece material properties and operating conditions on the machinability of EP/CNF, three controlled quantitative factors were selected at different levels, namely CNF loading, cutting speed and feed per tooth (FPT). Micromilling experiments were performed on an ultra-precision desktop micro-machine tool using titanium‑carbon-nitride (TiCN) coated micro-end mills. Among all compositions with CNF concentration ranging from 0.3 to 1 wt%, EP/1 wt% CNF exhibited the best machinability among other nanocomposites with its lowest cutting force of approximately 0.5 N and surface roughness of 0.18 μm. Size effect appeared at FPT below minimum uncut chip thickness (MUCT) indicated by the strong deterioration of surface quality owing to the dominant ploughing effect.

Generic Cutting Force Modeling with Comprehensively Considering Tool Edge Radius, Tool Flank Wear and Tool Runout in Micro-End Milling.

Accurate cutting force prediction is crucial in improving machining precision and surface quality in the micro-milling process, in which tool wear and runout are essential factors. A generic analytic cutting force model considering the effect of tool edge radius on tool flank wear and tool runout in the micro-end milling process is proposed. Based on the analytic modeling of the cutting part of the cutting edge in the end face of the micro-end mill bottom, the actual radius model of the worn tool is established, considering the tool edge radius and tool flank wear. The tool edge radius, tool wear, tool runout, trochoidal trajectories of the current cutting edge, and all cutting edges in the previous cycle are comprehensively considered in the instantaneous uncut chip thickness calculation and the cutter-workpiece engagement determination. The cutting force coefficient model including tool wear is established. A series of milling experiments are performed to verify the accuracy and effectiveness of the proposed cutting force model. The results show that the predicted cutting forces are in good agreement with the experimental cutting forces, and it is necessary to consider tool wear in the micro-milling force modeling. The results indicate that tool wear has a significant influence on the cutting forces and cutting force coefficients in the three directions, and the influences of tool wear on the axial cutting force and axial force coefficient are the largest, respectively. The proposed cutting force model can contribute to real-time machining process monitoring, cutting parameters optimization and ensuring machining quality.

Optimal tool design in micro-milling of difficult-to-machine materials

The limitations of significant tool wear and tool breakage of commercially available fluted micro-end mill tools often lead to ineffective and inefficient manufacturing, while surface quality and geometric dimensions remain unacceptably poor. This is especially true for machining of difficult-to-machine (DTM) materials, such as super alloys and ceramics. Such conventional fluted micro-tool designs are generally down scaled from the macro-milling tool designs. However, simply scaling such designs from the macro to micro domain leads to inherent design flaws, such as poor tool rigidity, poor tool strength and weak cutting edges, ultimately ending in tool failure. Therefore, in this article a design process is first established to determine optimal micro-end mill tool designs for machining some typical DTM materials commonly used in manufacturing orthopaedic implants and micro-feature moulds. The design process focuses on achieving robust stiffness and mechanical strength to reduce tool wear, avoid tool chipping and tool breakage in order to efficiently machine very hard materials. Then, static stress and deflection finite element analysis (FEA) is carried out to identify stiffness and rigidity of the tool design in relation to the maximum deformations, as well as the Von Mises stress distribution at the cutting edge of the designed tools. Following analysis and further optimisation of the FEA results, a verified optimum tool design is established for micro-milling DTM materials. An experimental study is then carried out to compare the optimum tool design to commercial tools, in regards to cutting forces, tool wear and surface quality.

Machined Surface Image Analysis for Process Instability Identification in Micromilling of Ti6Al4V

The low flexural rigidity of micro-end mill used in micromilling process undergoes catastrophic failure at low amplitude chatter and hence it is critical to identify the low amplitude chatter to produce the chatter free machined surfaces. An unfixing of the workpiece to identify the chatter by analyzing the power spectral density of machined surface topography alters the process set-up and even a minute change in workpiece surface flatness during re-fixing the workpiece lead to variation in depth of cut, which changes the precision of machined surface in micromilling process. Consequently, in the present work, an image analysis method has been proposed for machined surface analysis in micromilling of Ti6Al4V. Different statistical parameters like arithmetic gray level average value of the distribution of pixel intensities and optical parameter have been analyzed to detect the process instability. The estimated statistical parameters of the machined surface images for different process parameters has been compared with the measured surface roughness of the machined surfaces. The frequency spectrum analysis of the machined surface image analysis has also been carried out, which shows the distribution of pixel intensities at different frequencies. Arithmetic gray level average of pixel intensity of the machined surface image has been found to be decreasing with an increases of the roughness of the machined surface. The optical parameter of the machined surface increases with the increases of the roughness. The critical value of arithmetic gray level average of pixel intensity and optical parameter was found to be 148 and 0.36, respectively for chatter onset. The distribution of pixel intensity at different frequencies shows the diffused pattern of brightness of the pixel away from the center location in chatter dominant machining while the distribution of brightness of pixel was found to be circular in nature around the central spot in stable machining. The chatter and stable machining condition has also been verified with the vibration spectrum of the workpiece, where the vibration of the workpiece has been captured using accelerometer. The proposed image analysis method can further be used for online monitoring of machining process.

Metallurgical and machining performance aspects of cryotreated tungsten carbide micro-end mill cutters

Demand for miniaturized parts with tight geometrical tolerances and superior surface finish has been increasing. Development of micromachining processes has enabled adaptation of miniaturized parts and products in mechanical systems. Machining at the microscale level has supplementary challenges like high cutting forces, reduced tool life, and damaged machined surfaces. Catastrophic failure of microtools is notable problem, if tool deflection increases significantly. Therefore, incorporating tool life improvement measures are essential to minimize tool replacement cost and also to achieve efficient cutting at micro-level. Cryotreatment imparts better physical properties to cutting tools. In the present work, effects of different cryogenic treatment parameters on micro tungsten carbide-cobalt (WC-Co) end mill cutters and its performance while machining Ti-6Al-4V have been investigated. The metallurgical changes imparted to cutting tool by cryotreatment are studied via micro-hardness test along with EDS, SEM and XRD methods. It is found that all cryotreated tools have higher hardness compared to untreated tools. Additional η (eta) carbide formation has been clearly observed for the tools soaked for 24 h, moreover a variation in other phase of WC is also observed for different set of cryotreatment. Machining process is significantly affected by the low thermal conductivity of Ti-6Al-4V. In case of untreated micro-milling tool, abrasive wear is dominant, whereas built-up-edge affected the performance of cryotreated tools at shallow cryotreatment temperatures for 8–16 h soaking time. Deep cryotreated tool (at 24 h soaking time) performed better in terms of cutting force and surface roughness due to retainment of sharp cutting edge.

The Effect of Speed-Varying Micro-Cutting Tool Dynamics on Stability During High-Speed Micromilling of Ti6Al4V

Abstract Chatter-free machining is necessary in micromilling to avoid the catastrophic failure of micro-end mill. The accuracy of the prediction of chatter-free machining conditions in high-speed micromilling has been improved in this work by including speed-varying micro-end mill dynamics. An optimum design of exponential window has been devised to remove the unwanted spindle dynamics from the displacement signal to construct the speed-dependent frequency response function (FRF) of micro-end mill. The stiffness of the micro-end mill has been found to be increasing with increase in spindle speed and the natural frequency of the micro-end mill has been found to be changing with change in spindle speeds. The cutting velocity-chip load-dependent cutting coefficients have been included to predict the stability using Nyquist criterion. The predicted stability lobe with speed-varying micro-end mill dynamics has increased chatter-free depth of cut significantly compared to the chatter-free depth of cut predicted with static micro-end mill dynamics. The increase in depth of cut with speed-varying dynamics has been found to be 28% at 20,000 rpm, 150% at 52,000 rpm, and 250% at 70,000 rpm. A critical value of acceleration of the workpiece has been identified for chatter onset detection and it has been validated with machined surface image analysis. The magnitude of acceleration in both feed and normal to feed direction has been characterized to analyze the effect of spindle speed and depth of cut on the vibration of workpiece.

Prediction of micro milling force and surface roughness considering size-dependent vibration of micro-end mill

When the characteristic structure size of the component is at the micron level, the internal crystal grains, grain boundaries, and pore defects of the component material with the same size at the micron level cannot be ignored, so the micro-sized component will show different physical properties from the macro-sized component, which is called size effect. Since the tool diameter of a micro-end mill is in the micron level, the micro-end mill will also show a significant size effect phenomenon. In addition, in the micro milling process, because the surface roughness that affects the performance and service life of micro parts is mainly influenced by the vibration of the micro-end mill, in order to enhance the machined surface quality, it is crucial to research the formation mechanism of surface topography in the micro milling process. In this paper, a comprehensive method is proposed to predict micro-end mill vibration, micro milling force, and surface roughness. At first, a size-dependent dynamic model of micro-end mill is presented based on the strain gradient elasticity theory (SGET). Secondly, considering the feedback of a micro-end mill vibration, the micro milling force model is presented and solved through the iterative method. Then the machined surface topography is simulated through the actual cutting edge trajectory considering the micro-end mill size-dependent vibration and material elastic recovery. The results show that the vibration of the micro-end mill will increase the micro milling force and surface roughness. In order to verify the accuracy and efficiency of the presented method, experiments are performed, and it is found that the predicted results are consistent with the experimental results.

Micro-end-milling of carbon nanotube reinforced epoxy nanocomposites manufactured using three roll mill technique

Carbon nanotubes (CNTs) have been applied as nano-fillers to improve mechanical, thermal and electrical properties of polymers. Despite near net shape techniques could be used to manufacture nanocomposites, micromachining processes are still necessary to attain high surface quality and dimensional accuracy. Besides, micromachining of nanocomposites could be a potential approach to produce micro-features/components, following the miniaturisation trend of modern manufacturing. Therefore, micro-machining of these relatively new materials needs to be investigated. A comprehensive investigation on machinability of nanocomposites will be presented in terms of chip formation, cutting force, tool wear, surface morphology and surface roughness. Three controlled quantitative factors are investigated at different levels, including filler loading, cutting speed and feed per tooth (FPT). Micro-slotting is performed on an ultra-precision desktop micro-machine tool using uncoated carbide micro-end mill. The additions of multi-walled carbon nanotube (MWCNT) have shown significant effects on the machinability of these epoxy-based nanocomposites including a dramatic reduction in cutting force and machined surface roughness with accelerating tool wear compared with a neat polymer. The irregular cutting force variations when micro-milling epoxy/MWCNT nanocomposites at feed rates below minimum uncut chip thickness (MUCT) (lower than 2 μm) indicating by their fluctuations that different from those in higher feed rates. It possibly shows the impact of size effects that are illustrated by the observations of chip formation, surface morphology, cutting force profiles as well as specific cutting energy calculation.

Multilayer Diamond Coatings Applied to Micro-End-Milling of Cemented Carbide

Cobalt-cemented carbide micro-end mills were coated with diamond grown by chemical vapor deposition (CVD), with the purpose of micro-machining cemented carbides. The diamond coatings were designed with a multilayer architecture, alternating between sub-microcrystalline and nanocrystalline diamond layers. The structure of the coatings was studied by transmission electron microscopy. High adhesion to the chemically pre-treated WC-7Co tool substrates was observed by Rockwell C indentation, with the diamond coatings withstanding a critical load of 1250 N. The coated tools were tested for micro-end-milling of WC-15Co under air-cooling conditions, being able to cut more than 6500 m over a period of 120 min, after which a flank wear of 47.8 μm was attained. The machining performance and wear behavior of the micro-cutters was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Crystallographic analysis through cross-sectional selected area electron diffraction patterns, along with characterization in dark-field and HRTEM modes, provided a possible correlation between interfacial stress relaxation and wear properties of the coatings. Overall, this work demonstrates that high adhesion of diamond coatings can be achieved by proper combination of chemical attack and coating architecture. By preventing catastrophic delamination, multilayer CVD diamond coatings are central towards the enhancement of the wear properties and mechanical robustness of carbide tools used for micro-machining of ultra-hard materials.

A Comprehensive Micro-Milling Force Model for a Low-Stiffness Machining System

Abstract Micro-milling is widely used in various crucial fields with the ability of machining micro- and meso-scaled functional structures on various materials efficiently. However, the micro-milling force model is not comprehensively developed yet when tool feature sizes continually decrease to under 200 µm in a low-stiffness system. This paper proposes an analytical force model considering the influence of tool radius, size effect, tool runout, tool deflection, and the actual trochoidal trajectories and the interaction of historical tool teeth trajectories (IHTTT). Different micro-milling status are recognized by analyzing the cutting process of different tool teeth. Conditions of single-tooth cutting status are determined by a proposed numerical algorithm, and entry angle and exit angle are analyzed under various cutting conditions for the low-stiffness system. Three micro-milling status, including single-tooth cutting status, are distinguished based on the instantaneous undeformed chip thickness resulting in three types of material removal mechanisms in predicting micro-milling force components. Discontinuous change rates of undeformed chip thickness are found in the low-stiffness micro-milling system. The proposed micro-milling force model is then verified through experiments of micro slot milling Elgiloy alloy with a 150-μm-diametrical two-teeth micro-end mill. The experimental results show a root-mean-square error (RSME) of 0.092 N in the predicted resultant force, accounting for approximately 5.12% of the measured force, by which the proposed theoretical model is verified to be of good prediction accuracy.

Theoretical study on frequency spectrum characteristics of surface profiles generated in micro-end-milling process

Micro-milling is a flexible and promising fabrication process. Based on the analysis of the surface formation process in micro-end milling, the theoretical surface model was established to investigate the influence of seven identified factors, i.e., minimum cut thickness (MCT), elastic recovery, plastic side flow, radial and axial runout, tool wear (corner radius and unbalanced wear coefficient), and vibration on the frequency spectrum characteristics. The increase of MCT coefficient and plastic side flow height could increase the amplitude of the sft and 2sft frequency, while the scenario for the elastic recovery coefficient witnessed a reverse trend. The amplitude of the 0.5sft frequency decreased with the increase of the radial runout offset, while the amplitude of sft frequency increased as the radial runout offset increased; the scenario for the influence of the radial runout angle was opposite. The increase of the axial runout offset could lead to the increase of the amplitude of both 0.5sft and sft frequency. The increase of corner radius could lead to a slight reduction of the amplitude of the 0.5sft and sft frequency. The occurrence of the vibration with high frequency leaded to a low spatial frequency. The corresponding tool wear experiment was conducted, and the frequency-spectrum characteristics of the micro-milled surface were compared with the simulated results to validate the established profile models. The paper successfully correlated the relative importance of the identified individual factors to the corresponding frequency spectrum characteristics, and it had potential applications in predicting and controlling the spatial content of machined surfaces.

Methodology for prediction of sub-surface residual stress in micro end milling of Ti-6Al-4V alloy

To meet the growing demands of sophisticated component service life and miniaturization, the study of surface integrity such as residual stress after the machining becomes more essential. Compressive residual stress improves wear resistance of topological pairs and inhibits the fatigue crack propagation. To obtain a better understanding of state of residual stress at surface and sub-surface level, continuum-mechanics based Finite Element (FE) modeling is established. Dynamic explicit time incrementation scheme with coupled temperature displacement transient analysis is performed. Critical uncut chip thickness and consequences of tool edge radius, feed per tooth, and axial depth on cutting forces are investigated through FEM modeling. Besides the FEM modeling, the theoretical elastoplastic orthogonal cutting model with coupling of thermal and mechanical field variables is also demonstrated. In present research, Ti-6Al-4 V was chosen as the workpiece material because of its wide range of applications in biomedical, electronics, optics and aerospace industry due to their superior mechanical, chemical and high-temperature properties. X-ray diffraction (XRD) technique was used to measure the residual stress developed during micro-end milling process. Simulated results were validated with the experimental observations. To assess the residual stress at sub-surface level, electro polishing is done to remove the surface layer. It was found that both experimental and simulated results follow a similar trend and gave a good agreement between them.

Burr formation and surface roughness characteristics in micro-milling of microchannels

Microchannels are attractive for their widespread applications in electronics, biological, chemical, and energy areas. Micro-milling provides an ideal method to fabricate microchannels, whereas problematic burr formations and inherent rough surfaces of microchannel bottom and sidewalls are still far from sufficiently known. In this study, micro-milling process of rectangular copper microchannels was conducted by a two-flute tungsten carbide micro-end mill. Burr formation mechanism was studied via 3D finite element simulation together with experiment observations. Surface quality of microchannels, i.e., burr size and surface roughness, was studied in different spindle speeds, feed rates, and depths of cut. It was found that the top burr sizes on the down-milling side of microchannels were much larger than those on the up-milling side. The top burr size on the down-milling side and surface roughness on bottom and both sidewalls of microchannels presented a monotonic decreasing trend when the spindle speed increased, and an increasing trend with increasing depth of cut. Nevertheless, they firstly decreased due to the plowing and extruding effect together with elastic recovery when the feed rate was smaller than the tool cutting edge radius, and then changed to increase due to the shearing and cutting process after the feed rate exceed the tool cutting edge radius. High spindle speed, small depth of cut, and moderate feed rate close to the tool cutting edge radius seem to be favorable to improve surface quality of micro-milled microchannels.

Identification of tool wear status and correlation of chip morphology in micro-end milling of mild steel (SAE 1017) using acoustic emission signal

This study describes the identification of micro-end mill wear by means of acoustic emission (AE) signals received from an AE sensor during the micro-end milling (slot milling) of mild steel. The obtained AE signals were processed in the time-domain to compute root mean square (RMS) and dominant frequency and amplitude are obtained from frequency-domain. The RMS value shows the linear trend with the tool wear, and helps to classify the tool wear regions, such as initial, progressive and accelerated wear regions. The Welch power spectral density and spectrogram (short term Fourier transform) analysis help to identify the tool rotational, tool passing and machining frequencies. The discrete wavelet transformation (DWT) technique is used to discretize the AE signal in to five frequency ranges. AE specific energy was obtained from the discretized AE signals. The AE specific energy indicated that a combined type of material removal mechanism occurred in micro-end milling, similar to the macro-end milling. However, ploughing is also observed from the surface topography. Chip structures are also studied and correlated with the micro-end mill wear for tool wear identification.