Micro-end Milling Research Articles

Experimental correlation between acoustic emission and stability in micromilling of different grain-sized materials

Unstable cutting in micromilling could severely damage the tool and hinder the possibility to a usable machined part. This study proposes an experimental correlation between cutting stability, acoustic emission (AE) signal and roughness profile in micro-end milling operations. The monitoring of cutting stability occurred over the micromilling of two different grain-sized mid-carbon steel. The results have shown that the grain size influences the amplitude of AE signal. The root mean square of AE (AErms) has an expressive rise when performing unstable cutting in both workpiece materials, influencing the roughness surface levels. The index proposed to correlate the AErms data to the cutting stability is experimentally validated and a new threshold for chatter and chatter-free cut is proposed. Although AE in-process monitoring is feasible for recognizing chatter occurrence in micromilling operations and general profile patterns, an overall prediction of surface roughness depends on more than AE data.

Micro-end milling of biomedical Tz54 magnesium alloy produced through powder metallurgy

In this study, micro machinability of Mg5Sn4Zn (TZ54) alloy produced by a hot-pressing process (one of the powder metallurgy methods) is investigated. For this purpose, micro-milling experiments were carried out, considering different cutting parameters. In order to determine the appropriate cutting parameters, the variation of cutting forces, surface quality and burr width were investigated. The cutting forces obtained in micro-milling of the TZ54 alloy are relatively small compared to other biomedical materials. In terms of both cutting forces and surface roughness, the minimum chip thickness is between 7% and 34% of the tool edge radius. However, compared to alloys such as Ti6Al4V and 316 L, the burr width is about 80% smaller. An important contribution of the study is that the minimum chip thickness (0.1 μm) has been confirmed in terms of both cutting force and surface roughness. In addition, even at high material removal rates, the minimum cutting forces are important findings of the study. The predominant wear mechanism in the cutting tool is abrasive wear. In view of tool life, surface roughness and burr formation, it is envisaged that the TZ54 alloy can be an easy machinable, alternative biomedical material.

Conduction-Based Thermally Assisted Micromilling Process for Cutting Difficult-to-Machine Materials

The increasing demand for complex and wear-resistant forming tools made of difficult-to-machine materials requires efficient manufacturing processes. In terms of high-strength materials; highly suitable processes such as micromilling are limited in their potential due to the increased tool loads and the resulting tool wear. This promotes hybrid manufacturing processes that offer approaches to increase the performance. In this paper; conduction-based thermally assisted micromilling using a prototype device to homogeneously heat the entire workpiece is investigated. By varying the workpiece temperature by 20 °C < TW < 500 °C; a highly durable high-speed steel (HSS) AISI M3:2 (63 HRC) and a hot-work steel (HWS) AISI H11 (53 HRC) were machined using PVD-TiAlN coated micro-end milling tools (d = 1 mm). The influence of the workpiece temperature on central process conditions; such as tool wear and achievable surface quality; are determined. As expected; the temporary thermal softening of the materials leads to a reduction in the cutting forces and; thus; in the resulting tool wear for specific configurations of the thermal assistance. While only minor effects are detected regarding the surface topography; a significant reduction in the burr height is achieved.

An Approach to Improve Cutting Performance in Micromilling of Titanium Alloy

Abstract Application of liquid carbon dioxide to improve cutting performance in micro-end milling of Ti-6Al-4V titanium alloy was proposed in this study. It was found that the machined roughness decreased with the cutting speed as observed in the conventional cutting, when a 0.5 mm diameter end milling cutter was used in dry cutting. But, the tiny and shattered chips produced by the use of 0.3 mm diameter cutter could adhere on the machined surface and deteriorate surface finish, if the cutting speed was higher than 40 m/min. Cutting temperature was effectively decreased by applying liquid carbon dioxide during micromilling, which in turn reduced the amount of chips adhering on the machined surface and lowered flank wear. The surface roughness Ra at a cutting speed of 70 m/min was improved from 0.09 μm under dry cutting to 0.04 μm under the liquid carbon dioxide assisted cutting condition. And there were no flank wear and very few burrs left on the machined surface for the condition used in the experiment. The height of the burrs was only 25% of that under dry cutting. More, minimum quantity lubrication (MQL) was proposed to be applied together with the liquid carbon dioxide to enhance lubrication effect. It was noted that the machined surface roughness was further decreased by 15% as compared with that when the liquid carbon dioxide was applied alone. The height of burrs was reduced from 32 μm to 16 μm.

Investigation on laser-induced oxidation assisted micro-milling of Inconel 718

Poor surface quality and rapid tool wear are the main problems in micro-cutting of Inconel 718. In this study, a novel hybrid machining method named laser-induced oxidation assisted micro-milling is proposed to solve the aforementioned problems. A loose oxide layer and a relatively flat sublayer are formed on the material after laser irradiation. Under optimized laser parameters with a scanning speed of 1 mm/s and an average laser power of 4.5 W, the thicknesses of the oxide layer and the sublayer are 24 and 18 μm, respectively. The influence of cutting parameters on milling force, surface roughness, surface quality, and top burr size is studied in detail. Cutting force and thrust force in the proposed hybrid machining process are lower than those in the conventional micro-milling. Results show that for the investigated range of parameters, the optimal feed per tooth and depth of cut in the hybrid process are 3 μm/z and 3 μm, respectively. When using the optimal parameters, the surface roughness of the machined slot bottom is 108.5 nm. The top burr size on the up-milling side and the down-milling side is 26.8 and 36.2 μm, respectively. In addition, the tool wear mechanism is coating delamination in hybrid process, whereas chipping, coating delamination, tool nose breakage, and adhesion are the main tool wear mechanism in the conventional micro-milling. For the same amount of material removal, the proposed hybrid process can decrease the tool wear and enhance the service life of the micro-end mill as compared to conventional micro-milling.

Cutting forces analysis of micro-milling process on Inconel 718 – a finite element approach

This paper presents a modeling and simulation of micro-milling process with finite element modeling (FEM) analysis to predict cutting forces. The micro-milling of Inconel 718 is conducted using high-speed steel (HSS) micro-end mill cutter of 1mm diameter. The machining parameters considered for simulation are feed rate, cutting speed and depth of cut which are varied at three levels. The FEM analysis of machining process is divided into three parts, i.e., pre-processer, simulation and post-processor. In pre-processor, the input data are provided for simulation. The machining process is further simulated with the pre-processor data. For data extraction and viewing the simulated results, post-processor is used. A set of experiments are conducted for validation of simulated process. The simulated and experimental results are compared and the results are found to be having a good agreement.

Experimental investigation on CO2 laser-assisted micro-slot milling characteristics of borosilicate glass

Demands for micro-machining on glass have been increasing in various industries due to the unique properties of glass, such as transparency or biocompatibility. However, micro-channel fabrication on glass with high precision has been challenging due to its brittle characteristics. This research presents the CO2 laser-assisted micro-milling process and investigates the machining characteristics experimentally. Micro-channels without cracks were fabricated using micro-end mill and CO2 laser irradiation as an assisting heat source. Compared to the process without laser heating in the same matching conditions, the average surface roughness was reduced by 96%, and cutting force was reduced by 28% and 66% for the feed and thrust direction, respectively. Continuous and sheared chips were observed with laser heating, indicating the process is in ductile-regime machining. Through the investigation of machining parameters, it was found that micro-channels with low average surface roughness can be achieved at the proper laser power when the workpiece is heated up to the strain point at tool position, at low feed rate, and at high axial depth of cut, as long as the tool withstands the cutting forces. Consequently, it can be concluded that it is possible to increase the material removal rate in micro-milling of borosilicate glass with high quality by using the CO2 laser, which was found to be an effective and suitable heating method.

Investigation on surface quality and burr generation of high aspect ratio (HAR) micro-milled grooves

With a widespread application of micro machinery, high-quality surface demand of high aspect ratio (HAR) micro-grooves is increasing gradually. However, problems of imperfect inside-wall surface quality and massive burrs are particularly prominent in the generation process of HAR micro-grooves with the large length-diameter ratio micro-end mills. In this paper, a preliminary research on surface quality and burr generation mechanism is conducted to achieve the desired HAR micro-grooves. The top-burr generation rule of HAR grooves is discussed by establishing a FE model. The effects of cutting fluids and the wear of micro-milling tools on surface quality and micro-burr generation were analyzed. Results have shown that the burr generation and surface quality were strongly dependent on the tool wear and the groove depth. As the cutting cycles increased, the top-burr dimensions kept level with the variation in the edge-radius. Furthermore, the water-free alcohol based lubrication was demonstrated to play a positive role in improving surface quality and reducing burr generation. Finally, a desired HAR micro-groove with a nanometer level areal surface roughness (Sa = 77.4 nm) was obtained under the optimized conditions.

Process Mechanics Based Uncertainty Modeling for Cutting Force Prediction in High Speed Micromilling of Ti6Al4V

High speed machining is essential to counter the low flexural rigidity of micro-end mill in micromilling process. High speed machining can excite the higher frequency modes and in addition, run-out is also amplified. These effects in high speed micromachining induces the uncertainty in cutting forces. The estimation of cutting coefficients without inclusion of uncertainty cannot give accurate values and hence, the predicted cutting force and machining stability using these cutting coefficient may not be accurate. In the present work, cutting coefficients have been determined using the Bayesian inference which includes the uncertainty in estimated cutting coefficients. Note that, estimated cutting coefficient independent of chip load and cutting velocity does not include the high speed micromachining mechanics like size effect. The cutting coefficients have been estimated as a function of cutting velocity and chip load in the present work. The segmented cutting coefficients for different ranges of cutting speed varying from 10000 rpm to 110000 rpm has been obtained as a non-liner function of chip load. Bayesian inference modelling for cutting coefficients has been carried out using the Metropolis-Hastings (MH) algorithm Markov Chain Monte Carlo (MCMC) method. The convergence of prior and posterior distribution has been verified using correlation and trace of samples used for sampling. The posterior distribution shows that there is a good fitting, which accurately predicts the mean of the cutting coefficients. Finally, predicted cutting coefficients have been compared with cutting coefficients obtained from the deterministic approach using least square method. The experimentally estimated cutting coefficients are found to be lying within the upper and lower limit of predicted cutting coefficients with Bayesian inference approach. The predicted cutting coefficients using Bayesian inference shows the deviation of 0.89% and 8.4% at 40000 rpm from experimentally obtained cutting coefficients for tangential and radial cutting coefficients, respectively. The experimental cutting force is found to be lying within the upper and lower limit of predicted cutting force with Bayesian inference based cutting coefficients fitting at 105000 rpm.

Fabrication of Micro-end Milling Cutter Based on WEDG

The effect of unbalance caused by installation error is the one of the main reasons for tool runout in micro milling processes, especially at high-speed on the spindle. As tool runout contributes significant impact to the machining accuracy in micro milling processes, an on-machine fabrication method which could avoid the existence of installation error of spindle deviation and tool tilt caused by off-line of micro-end milling cutter has been proposed. To verify the possibility of an on-machine fabrication method based on wire electro-discharge grinding, a micro-EDM machine with an ultra-high-speed air turbine spindle and a geometrically optimized tool design were developed. Ultra-fine grain carbide end mills with the target diameter equal to 50 μm were fabricated by wire electro-discharge grinding based on the on-machine fabrication method. And some micro slot milling experiments were conducted in pure copper. The experimental results indicate that the micro cutting process using the on-machine fabricated tool by wire electro-discharge grinding is possible. Moreover, this method can improve the reliability of the cutter and the processing accuracy of the parts, avoids the error caused by the second clamping of the micro-milling cutter and improves the overall quality of micro-milling.

Investigation on Micro-milling of Ti–6Al–4V Alloy by PCD Slotting-Tools

Ti–6Al–4V is characterized by various protruding characteristics and has been found extensive applications in aerospace, biomedical device industries and other fields. However, these properties in turn bring about the increase in cutting forces, make the chips intractable to control and also shorten the tool service life, etc. A preliminary exploration of micro-milling of Ti–6Al–4V by PCD slotting-tool (MST-PCD) was launched in this paper. The most prominent advantage was the PCD slotting-tools could reach a higher linear velocity even the spindle speed was lower, which can strongly ensure the strength and stiffness of micro cutters and suppress the vibration occurrence due to the high spindle speed. The machining processes of using PCD slotting-tool and carbide micro-end mill (MEM-WC) was compared in detail. The wear experiments were conducted to confirm the effectiveness and advantage of MST-PCD. The cutting forces and surface quality, as well as the effects of tool wear on these output responses were investigated. Then model establishment and multi-objective parameter optimization were performed based on the orthogonal experiment results. Finally, verification experiments were also carried out to confirm the effectiveness of the developed models and MST-PCD. The findings in this paper can also be applied to other difficult-to-machine materials.

Modeling of Top Burr Formation in Micro-End Milling of Zr-Based Bulk Metallic Glass

Abstract In this study, an analysis is presented for the formulation of a model in top burr formation to estimate the width of the top burrs formed in micro-end milling process. In this analysis, the width of the top burrs, considered to be the measure of burr size of top burrs, represents the extent of burr formation. The model is applied to quantify the burr formation in Zr-based bulk metallic glass, an amorphous metallic alloy. In this study, micromilling experiments were carried out on the bulk metallic glass work material at different levels of feed rate, axial depth of cut, and cutting speed to compare the experimental values of top burr width with the predicted values. Analysis is carried out to characterize top burr formation based on the experimental results of burr sizes in the up-milling and down-milling side edges, and the influential effects of the cutting parameters on the burr formation are analyzed.

Effect of Progressive Tool Wear on the Evolution of the Dynamic Stability Limits in High-Speed Micromilling of Ti-6Al-4V

Abstract Micromilling can fabricate complex features in a wide range of engineering materials with an excellent finish but the limited flexural stiffness of the micro-end mill can result in catastrophic tool failure. This issue can be overcome by using high rotational speeds. Note that the combination of high rotational speeds and low flexural stiffness can induce process instability which is aggravated by the accelerated wear of the micro-tools at high speeds, specifically, for Ti-alloys. The effect of progressive tool wear on the stability has been investigated in micromilling of Ti-6Al-4V. For incorporating tool wear, the cutting force coefficients are modeled as a function of initial and instantaneous cutting edge radius (CER) and feed per tooth. The initial CER of the micro-tool is considered due to the inherent variability in the tool grinding process. A significant increase (85–114%) in the instantaneous CER is observed with an increase in the length of cut. A 2DOF time-domain model based on semi-discretization method has been used to characterize the evolution of stability limits with an increase in the length of cut. The progressive tool wear affects the stability limits along with the initial CER and the feed per tooth. At higher speeds (90,000–110,000 rpm), the effect of progressive tool wear is pronounced and the stability limits reduce by ∼30% in that range.

Fabrication of micro-end mill tool by EDM and its performance evaluation

Micro-milling is a fast, cheap and controlled process compared to other micro-fabrication processes such as lithography, laser/electron/ion beam machining, etc. However, scarcity of cutting tools of very small dimensions often results in limited application of micro-milling. In the present study, electro discharge machining (EDM) is used for fabrication of micro-end mill tool. To ensure high dimensional accuracy of the tool, a parametric study is conducted by replicating the a tungsten carbide block to a tungsten carbide (WC) block. The relationships between the drilled cavities on the block and the features on the micro-tool are established. The influence of machining parameters (voltage, capacitance and spindle speed) on the response variables (entrance diameter, hole depth, material removal rate (MRR) and surface roughness) is reported. Capacitance is found more dominant as compared to other selected process parameters. Using optimized parameters from the parametric study, a WC micro-end mill tool of 100 µm diameter is fabricated. Channel of around 110 µm width, 40 µm depth and surface roughness of 70 nm is successfully fabricated on aluminum. The performance of the fabricated tool is compared with a commercial end mill tool by milling micro channels on stainless steel.

Fabrication of large aspect ratio (LAR) PCD micro-end mill with a hybrid method and performance verification

Micro-milling technology is a well-developed method in machining the increasingly required micro-grooves. High wear-resistant PCD micro-milling cutters have become the desirable substitute for traditional carbide tools since micro-milling is a highly tool-dependent process. However, the fabrication of PCD micro-milling cutters is difficult due to its high hardness, particularly for those with a larger aspect ratio and a smaller diameter. For these reasons, a hybrid fabrication method combining the laser and precision grinding is proposed to improve the performance and quality of PCD micro-milling cutters. The laser investigations were carried out first to explore the machining mechanism and influence of laser parameters on the initial forming quality. The grinding mechanism of laser-treated PCD was analyzed profoundly. Verification experiments were conducted to compare the cutting performance of the self-fabricated PCD and carbide tools by considering the surface quality and tool wear. Results showed the laser parameters used were directly related to the surface roughness and edge quality of the laser-treated PCD. After the precision grinding, the laser-treated PCD surface became smoother with the areal roughness (Sa) of 0.3 μm. A PCD diamond micro-end mill with an aspect ratio of more than 2 was achieved by using the hybrid method. Micro-milling experiments demonstrated that the self-fabricated PCD showed remarkable superiority in terms of surface roughness, burr formation, and tool wear.

Modified Mechanistic Model Based on Gaussian Process Adjusting Technique for Cutting Force Prediction in Micro-End Milling

Micro-end milling is in common use of machining micro- and mesoscale products and is superior to other micro-machining processes in the manufacture of complex structures. Cutting force is the most direct factor reflecting the processing state, the change of which is related to the workpiece surface quality, tool wear and machine vibration, and so on, which indicates that it is important to analyze and predict cutting forces during machining process. In such problems, mechanistic models are frequently used for predicting machining forces and studying the effects of various process variables. However, these mechanistic models are derived based on various engineering assumptions and approximations (such as the slip-line field theory). As a result, the mechanistic models are generally less accurate. To accurately predict cutting forces, the paper proposes two modified mechanistic models, modified mechanistic models I and II. The modified mechanistic models are the integration of mathematical model based on Gaussian process (GP) adjustment model and mechanical model. Two different models have been validated on micro-end-milling experimental measurement. The mean absolute percentage errors of models I and II are 7.76% and 6.73%, respectively, while the original mechanistic model’s is 15.14%. It is obvious that the modified models are in better agreement with experiment. And model II performs better between the two modified mechanistic models.

Stability modeling with dynamic run-out in high speed micromilling of Ti6Al4V

The low flexural rigidity of the micro-tool makes it susceptible to failure under high cutting forces and/or unstable process conditions. One way to counter the effect of low flexural rigidity is to use high rotational speed which reduces the chip load and, therefore, the cutting forces acting at the tip of the micro-end mill. However, high rotational speeds can amplify the effects of run-out and misalignment. This speed-dependent tool eccentricity, known as dynamic run-out, can be much larger than the quasi-static (speed independent) run-out. The dynamic run-out is more pronounced at high rotational speeds (∼100,000 RPM) due to the increased centrifugal force. The chip thickness is modified due to the dynamic run-out which affects the cutting forces and the stability. It is necessary to incorporate the dynamic run-out in the chip thickness model for an accurate prediction of the forces and the stability limits in high speed micromilling. Consequently, this paper is focused on characterizing the dynamic run-out and incorporating its effect on the undeformed chip thickness, cutting forces and stability limits. The predicted cutting forces with the modified chip thickness due to dynamic run-out shows a better agreement with the experimentally measured cutting forces as compared to the quasi-static run-out and ideal cycloidal chip thickness based force models. The predicted cutting force with dynamic run-out in the feed-direction gives an error of 11.54% as opposed to a relatively large error of 73.1% with a quasi-static run-out based model at 100,000 rpm. The Nyquist stability criterion has been used to predict the stability for 2-DOF chatter model of micromilling process. At a chip load of 3 µm/flute, the predicted stable depth of cut is higher if the dynamic run-out is included at high rotational speeds (> 51,500 rpm). This increase is attributed to a high dynamic run-out which results in cutting via only one flute. However, if the dynamic run-out is relatively low (between 29,500 rpm and 51,500 rpm), both the flutes are engaged in machining and the predicted limit is lower than that of the quasi-static run-out. The predicted values are in reasonably good agreement with the experimentally determined chatter onset points for most cases.

Size-dependent responses of micro-end mill based on strain gradient elasticity theory

A significant size effect will occur on the tool part of micro-end mill due to its small diameter, which means the internal structure of the tool part material will affect the mechanical properties of the tool part. In view of this, a comprehensive method considering size effect is proposed in this paper to predict both the static and dynamic behaviors of micro-end mill more accurately. Based on the strain gradient elasticity theory (SGET) and Hamilton’s principle, dynamic model of micro-end mill tool is presented, in which the Timoshenko beam model (TBM) considering the shear deformations and rotary inertia effects is employed. Based on the presented model, the static and dynamic behaviors of micro-end mill is obtained utilizing the finite element method (FEM). The influences of size effect on micro-end mill are investigated in detail by contrasting the static and dynamic behaviors of micro-end mill with different tool diameters and different length-to-diameter ratios, respectively. In order to verify the accuracy and efficiency of the presented method, an improved experiment is performed in this paper.

A Free Interface Component Mode Synthesis Approach for Determining the Micro-End Mill Dynamics

The prediction accuracy of the stability boundary in the machining process depends upon accurate estimation of cutting tool-tip dynamics. Note that the experimental modal analysis using direct impact at miniature end mill (typically 50–500 μm in diameter) is not feasible as it can result in tool failure. Consequently, alternative techniques such as experimental modal analysis using reciprocity theory and frequency-based receptance coupling substructure analysis (RCSA) have been used extensively for determining tool-tip dynamics. The experimental approach based on reciprocity theory assumes that the structure is symmetric (cross frequency response functions (FRFs) are same between two points of interest in a structure). RCSA requires a very fine frequency resolution and matrix inversion, which can lead to computational complexities. In addition, RCSA takes into account the FRFs only at the interface and free end, which can induce errors. Owing to these issues with existing approaches, this paper proposes a free-interface component mode synthesis (CMS) approach for estimation of micro-end mill dynamics. The effect of machine tool compliance including the collet–tool interface has been included for estimation of micro-end mill dynamics via a free-interface CMS approach wherein the experimental and analytical mode shapes are coupled. The predicted micro-end mill dynamics have been compared with RCSA and experimental modal analysis using reciprocity theory. Finally, the stability lobe diagrams for high-speed micromilling of Ti6Al4V has been made using the tool-tip dynamics from CMS, RCSA, and experimental technique using reciprocity theory and validated against experimental measurements for onset of instability.

Experimental investigation of top burr formation in high-speed micro-end milling of titanium alloy

ABSTRACTSuperalloys with burr-free parts are most preferably used in biomedical, aerospace, marine and automotive applications. In order to reduce the global pollution content, industries strive to execute stringent green manufacturing technologies. There is a need to investigate the different available tools in high-speed micro-milling process to achieve desired burr free with good surface finish on super alloys without using traditional coolants. In machining of titanium and its alloys, because of low thermal conductivity and reactivity with tool materials instigate the burr formation on work material and lowers the tool performance. The main objective of this article is to investigate the top burr formation in high-speed micro-end milling of alpha + beta-titanium alloy-grade 23 ELI (Ti-6Al-4V) under dry cutting conditions using Uncoated and physical vapor deposition coated AlTiN, TiAlN tungsten carbide end mills. Machining performance of the three cutting tools was compared. From the comparison of cutting tools for machining titanium alloy-grade 23, it is found that coated TiAlN tools produce less burr formation than coated AlTiN and uncoated tungsten carbide tools.