Micro-milling Process Research Articles

Evaluation of Tool Path Strategy and Cooling Condition Effects on the Cutting Force and Surface Quality in Micromilling Operations

Compared to milling on a macro scale, the micromilling process has several cumbersome points that need to be addressed. Rapid tool wear and fracture, severe burr formation, and poor surface quality are the major problems encountered in the micromilling process. This study aimed to reveal the effect of cutting path strategies on the cutting force and surface quality in the micromilling of a pocket. The hatch zigzag tool path strategy and the contour climb tool path strategy under different cooling conditions (e.g., dry, air blow, and flood coolant) at fixed cutting parameters. The micromilling tests revealed that better results were obtained with the use of the contour tool path strategy in terms of cutting forces (by up to ~43% compared to the dry condition) and surface quality (by up to ~44% compared to the air blow condition) when compared to the hatch tool path strategy. In addition, the flood coolant reduces the cutting temperature and eliminates chips to significantly enhance the quality of the micro milled surface.

Investigation of Machining Tool Path on Surface Roughness and Dimensional Accuracy for High-Speed Micro Milling

This paper investigates the effects of machining tool path and cutting layer strategies on machining efficiency and accuracy in micro-milling of linear and circular micro-geometric features. Although micro-milling includes many characteristics of the conventional machining process, detrimental an effect in downscaling the process can be excessive tool wear which could, in turn, increase the machining forces and hence affect the geometrical accuracy and surface roughness. Most of the research in micro milling reported in the literature has focused on optimising machining parameters, such as feed rate and depth of cut to achieve lower cutting forces, better surface roughness, and better machining efficiency. However, is there yet little known about the effect and stability of machining tool paths and cutting layers strategies for the micro-milling process. Various tool path strategy, including lace(0°), lace(45°), lace(90°), concentric and waveform in producing linear and circular micro geometric features were compared and analysed. The effect of various cutting layer strategies in producing thin walled structure was investigated. The optimisation method with respect to surface roughness and dimensional accuracy is proposed for selection of optimum machining strategies experimentally tested. Experimental results show that the most commonly used strategy lace(0°) and concentric, reported in the literature have provided the least satisfactory machining performance, while the waveform strategy provides the best balance of machining performance for both linear and circular geometries. Adopting an optimum sequence of material removal layer in micromachining of thin walls has proven to improve the overall accuracy. This paper concludes that an optimal choice of machining strategies in process planning is as important as balancing machining parameters to achieve desired machining performance.

Model for the prediction of 3D surface topography and surface roughness in micro-milling Inconel 718

Nickel-based superalloy Inconel 718 retains high strength at high temperature, which meets the requirements of micro-parts in the fields of aerospace, energy, and power. However, Inconel 718 is a kind of difficult-to-machine material. During the micro-milling process, scale effect, multiple regenerative effect, and dynamic response all affect its surface roughness, causing the prediction of surface roughness of micro-milled parts difficult. To solve this problem, we study the predictive modeling of surface roughness of micro-milled Inconel 718. Based on the previously built instantaneous cutting thickness model, cutting force model, and the dynamic characteristics of micro-milling system, the authors establish a flexible deformation model of micro-milling cutter generated by cutting force. Since the machined surface is generated by duplicating the tool profile on the workpiece surface, and based on the actual cutting trajectory as well as flexible deformation of micro-milling cutter, the authors build the surface topography simulation model to predict surface roughness and conduct experiments to verify the accuracy of the model. The research realizes the prediction of surface roughness of micro-milled Inconel 718 parts and partially reveals the machining mechanism of micro-milling.

Minimum chip thickness determination by means of cutting force signal in micro endmilling

Issues related to ploughing affecting the performance of the micromilling process have recently been reported in literature. It is well known that there is a minimum chip thickness (hmin) below which ploughing is the main material removal mechanism and no shear occurs. This leads to a non-effective material removal, resulting in a poor surface quality. In order to solve this problem, the minimum chip thickness has been predicted by measuring the cutting forces. However, the determination of hmin by means of the cutting force signal, at the instant the chip is being formed, has not been approached. In this article, a method of determining hmin, based upon the signal variation of the cutting forces and the effect of tool radial runout during chip formation is proposed. Carbide micro-endmills without coating were used to cut an aluminium alloy (RSA 6061-T6) sample and the cutting forces were measured using a micro-dynamometer. The microtopography of a microchannel wall was assessed using an optical profiler in order to establish the approximate position where the chip starts to form (hmin). As the cut progresses, the force component normal to the feed (FfN) reverses when the undeformed chip thickness is equal to the cutting edge radius (re). Simultaneously, the thrust force increases rapidly, and continues to grow but at a lower rate as FfN increase. The main cutting force and the active force present significant differences to each other. The minimum chip thickness was estimated as 0.3re by means of the behavior of the active force. A small quantity of material left on the wall of the microchannel could be observed in align with the cutting movement together with a deterioration of the surface finish attributed to the increase of re. Results show that the size of the material left is 2 to 4 times greater than hmin. Conversely, the quality of the microchannel floor improves as re increase. This shows that there is a balance between hmin and re and the effect upon the finish of the channel wall and floor. That should be important for microchannel fabrication in terms of performance of micro-scale heat exchangers depending on fluid viscosity. The topographic analyses of the wall and the images of the chips show an agreement with estimated hmin under different cutting conditions and cutting edge radii. The proposed method in this paper not only permits the determination of the minimum chip thickness but also has the advantage of making it possible to estimate the cutting edge radius and to monitor the cutting edge wear.

Novel dry metal electrode with tilted microstructure fabricated with laser micromilling process

A novel dry metal electrodes with tilted microstructure arrays was fabricated with laser micromilling process by adjusting the incident angle of the laser beam. After discussing the laser fabrication process for dry metal electrodes, the effects of the laser incident angle, width of unscanned area, laser output power, and scanning times on the shape and size of the microstructures are further discussed. Our experimental results show that the tilted angle of the surface microstructures of the dry metal electrodes depended on the laser incident angle. The heights of the surface microstructures of dry metal electrodes were greatly increased by increases of the laser output power and scanning times. Compared with vertical microstructure arrays, the developed dry metal electrodes with 60° tilted angle microstructure arrays demonstrated much lower impedances.

Investigation of the micro-milling process of thin-wall features of aluminum alloy 1100

Thin-wall geometrical features are observed in many mechanical components, including micro-components such as blades of a micro-impeller and the walls of a micro-channel. Although several papers have already presented research studies regarding thin-wall development, there is still a lack of information regarding how thin a wall could be produced via micro-milling processes. This paper investigates thin-wall quality in terms of the shape and dimension, the problems faced by micro-milling technology in producing thin-wall features, and the feasibility of producing thin-wall components using a micro-milling process aluminum alloy 1100 (AA110). The minimum wall thickness of 11.71 μm was successfully machined in good condition. The actual deviation of the wall thickness, including the tool dimension incompatibility, was in the range of −4.69 to 3.48 μm. A 16-blade micro-impeller with average blade thickness of 11.96 μm was manufactured. Among the 16 blades, four cloven blades, nine deflected blades, and three blades were in good condition.

Contact angle hysteresis analysis on superhydrophobic surface based on the design of channel and pillar models

Contact Angle Hysteresis (CAH) is critical to the hydrophobicity of a surface, which describes the dynamic characteristic of droplets. In this paper, two different micro-structured surfaces respectively with micro-channel and micro-pillar structures (20 samples for each structure, with a range of channel and pillar widths between 25 and 250μm) were fabricated by mechanical micro-milling process to investigate the effect of structural parameters on hydrophobicity of surfaces. It was found that the solid fraction plays a decisive role for a surface in the transition from being hydrophilic to hydrophobic. Quantitative interpretation was conducted and a dynamic methodology was established based on the physical nature of the controllable motion of a droplet. The five key states of a droplet including the initial, pre-forward, forward, pre-backward and backward were the main focus of this research. The prediction results based on the established model showed good consistency with experiments. The proposed model can estimate the advancing and receding angles very well. The outcome of this research will lead to new methodologies for preparing hydrophobic surfaces with micro-machining technology and play an important theoretical guiding role in fabrication of superhydrophobic surfaces.

Uncertainty analysis of force coefficients during micromilling of titanium alloy

Predicting process forces in micromilling is difficult due to complex interaction between the cutting edge and the work material, size effect, and process dynamics. This study describes the application of Bayesian inference to identify force coefficients in the micromilling process. The Metropolis-Hastings (MH) algorithm Markov chain Monte Carlo (MCMC) approach has been used to identify probability distributions of cutting, edge, and ploughing force coefficients based on experimental measurements and a mechanistic model of micromilling. The Bayesian inference scheme allows for predicting the upper and lower limits of micromilling forces, providing useful information about stability boundary calculations and robust process optimization. In the first part of the paper, micromilling experiments are performed to investigate the influence of micromilling process parameters on machining forces, tool edge condition, and surface texture. Under the experimental conditions used in this study, built-up edge formation is observed to have a significant influence on the process outputs in micromilling of titanium alloy Ti6Al4V. In the second part, Bayesian inference was explained in detail and applied to model micromilling force prediction. The force predictions are validated with the experimental measurements. The paper concludes with a discussion of the effectiveness of employing Bayesian inference in micromilling force modeling considering special machining cases.

Effect of lubrication on machining response and dynamic instability in high-speed micromilling of Ti-6Al-4V

The micromilling process is increasingly being used in biomedical, defence, electronics and aerospace industries for fabrication of miniaturized products of complex shapes. For difficult-to-cut materials like Ti alloys, limited stiffness of the micro-tool is a major impediment. This limitation can be overcome by using high rotational speeds which leads to reduction in chip load and, therefore, the cutting forces. However, high spindle speeds and low tool stiffness can render the process unstable due to dynamic variation in cutting forces. High speed micromilling of Ti6Al4V generates high temperature in cutting zone due to low thermal conductivity of Ti alloys which can also lead to a variation in cutting force and, hence, dynamic instability. Cutting fluid can play an important role because of its capacity to reduce friction and its ability to dissipate the heat generated between the micro tools and the workpiece. This study is focused on studying the effect of lubrication on the cutting forces and dynamic instability called chatter in high speed micromilling of Ti6Al4V. Experiments were carried out at different spindle speeds and feeds in both dry and lubricated condition. A significant reduction up to 38% in the cutting forces has been found in lubricated machining as compared with the dry machining. Two distinct regimes, lubrication sensitive (at rotational speeds >47,000rpm) and lubrication insensitive (at rotational speeds <47,000rpm) have been observed in this study. A critical spindle speed of 47,000rpm has been identified which corresponds to the onset of lubrication dominant regime. To capture the effect of process mechanics, mechanistic force model using velocity-chip load dependent coefficients has been used to predict the forces and stability boundary. The predicted stability boundary limits have been compared with the experimental onset of chatter at four different spindle speeds. FFT spectrum analysis of acceleration of workpiece has been done to detect the process instability.

Fabrication of porous copper surfaces by laser micromilling and their wetting properties

Porous copper surfaces show their great merits in the applications of chemical reaction, sound absorption and heat transfer. In this study, a laser micromilling method is proposed to fabricate porous surfaces with homogeneous micro-holes and cavities of the size about 1–15μm on pure copper plates in a one-step process. The laser micromilling was performed by a pulsed fiber laser via the multiple–pass reciprocating scanning strategy. Based on the measurement of scanning electron microscope (SEM) and 3D laser scanning confocal microscope, the formation of surface structures was investigated together with the laser ablation mechanisms. The effects of laser processing parameters, i.e., laser fluence, scanning speed, number of scanning cycles and scanning interval, on the formation and surface morphology of porous surfaces were systematically assessed. Furthermore, the wettability of the porous copper surfaces was also evaluated by measuring the static contact angle of water. The results showed that the laser fluence played the most significant role on the formation of porous copper surfaces. The average depth and surface roughness of porous copper surfaces increased with increasing the laser fluence and number of scanning cycles while decreased with the increase in scanning interval. The scanning speed played little influence on the formation of porous copper surfaces. These results can be closely related to the variation of energy density and re-melting process during the laser micromilling process. Moreover, all the copper porous surfaces were found to be hydrophobic. The contact angle of porous copper surfaces was significantly dependent on laser fluence, but weakly affected by the scanning speed and number of scanning cycles.

Construction and characterization of micro/nano-topography on titanium alloy formed by micro-milling and anodic oxidation

The surface micro/nano-topographical modifications have been widely used in improving the biocompatibility of biomedical implants. In this paper, the feasibility of the micro-milling process for construction of micro-grooves without burrs on titanium alloy materials was investigated. Low melting point alloy was selected as supporting material to extend the boundary of the workpiece, so the burrs produced on the surface of supporting material. A novel hierarchical micro/nano-topography with micro-grooves and TiO2 nanotubes was fabricated on titanium alloy surface combining micro-milling and anodic oxidation. The scanning electron microscope, energy-dispersive spectroscopy, laser scanning microscope, and contact angle tester were used in characterizing surface features of machined workpieces. Results showed that the micro-grooves with dimension of 40 μm in depth were machined using V-shaped cutting tool, and the TiO2 nanotubes with dimension of 70 nm in diameter were superimposed on the surfaces of micro-grooves by anodic oxidation. In addition, the hydrophilicity of micro/nano-structured surface was significantly enhanced with the water contact angle decreasing from 114.8° to 60.3°. It is concluded that this hybrid method combining micro-milling technology and anodic oxidation can be used in improving the biological activity of biomedical implants through changing surface topographies.

Application of linear and nonlinear vibration absorbers in micro-milling process in order to suppress regenerative chatter

In this paper, linear and nonlinear vibration absorbers are designed to suppress regenerative chatter in micro-milling process. Suppressing regenerative chatter leads to improve surface finish. Micro-milling process is considered as a two degree of freedom system, and the run-out effects of cutting tool are also taken into account. Linear and nonlinear absorbers in two directions are composed of mass, spring and dashpot elements. The optimum values of the absorber parameters are determined by means of an optimization algorithm in order to minimize the cutting tool vibration. The effectiveness of different types of the absorbers in micro-milling process is illustrated. The optimum parameters for each type of absorbers are presented.

Accurate measurement of micromachining forces through dynamic compensation of dynamometers

In this paper, we present a comprehensive approach for accurate measurement of high-bandwidth three-dimensional (3D) micromachining forces through dynamic compensation of dynamometers. Accurate measurement of micromachining forces is paramount to gaining fundamental understanding on process mechanics and dynamics of micromachining. Multi-axis dynamometers are used to measure 3D machining forces. However, specified bandwidth of these devices is below the frequencies arising during micromachining while using ultra-high-speed (UHS) spindles. This limitation arises from the effects of the dynamometer's structural dynamics on the measured forces. Therefore, accurate measurement of micromachining forces entails high frequency correction of the signals acquired by the dynamometer by removing the influence of those effects. The presented approach involves: (1) accurate identification of 3D force measurement characteristics of the dynamometer within a 25kHz bandwidth to capture the effects of the dynamometer dynamics; (2) design of a pseudo-inverse filter-based compensation technique to remove the influence of the dynamic response in 3D; and (3) validation of the compensation technique through custom-devised experiments. Subsequently, the compensation method is applied to the micromilling process to obtain accurate broadband 3D micromachining forces using a miniature multi-axis dynamometer. It is concluded that the presented approach enables accurate determination of 3D micromachining forces. The presented compensation technique is also readily applicable for expanding the bandwidth of large dynamometers.

Experimental investigation of porous metal materials in high-speed micro-milling process

Porous stainless steels, which have recently extended to the microscale domain, have been widely used for biomedical materials because they have good strength-to-weight ratio and superior corrosion resistance. Most components fabricated from porous materials still require secondary machining despite being produced in near-net shapes. However, they have poor machinability. High cutting force, high cutting temperature, poor surface integrity, and severe tool wear are encountered in the machining process of the difficult-to-cut porous materials. This article focuses on the micro-cutting performances and effects of cutting parameters in the micro-milling of porous stainless steel materials, including tool wear patterns and mechanism, effects of tool wear and cutting parameters on surface topography, cutting force, and cutting temperature. Comparative experiments were performed to investigate surface burr and cutting performances of porous materials in the micro-milling process. The effects of machining parameters on porous stainless steels in micro-milling are studied. Furthermore, a preliminary relationship of cutting parameters with micro-milling force and milling temperature is also established. The tool wear patterns and mechanisms were observed. Based on the findings, this article concluded that tool wear influences the surface morphology of the machined part and the effects of structural porosity on the cutting process during the micromachining of porous materials.

A multi-objective algorithm for optimization of modern machining processes

Multi-objective optimization aspects of four modern machining processes namely wire-electro discharge machining process, laser cutting process, electrochemical machining process and focused ion beam micro-milling process are considered in this work. In WEDM process cutting velocity and surface quality are important objectives which are mutually conflicting in nature. Minimization of kerf taper is vital in the laser cutting process which increases with the increase in material removal rate. The ECM process is characterized by high material removal rate, but poor dimensional accuracy, high tool wear rate and high over cut. FIB micro-milling process is useful in applications where a nano-level surface finish is desired but this process is characterized by a very low material removal rate. All the above mentioned objectives are vital as they closely govern the performance of the machining processes considered in this work. Therefore, the aim of this work is to achieve these objectives through process parameter optimization. In order to handle multiple objectives simultaneously a new posteriori multi-objective optimization algorithm named as multi-objective Jaya (MO-Jaya) algorithm is proposed which can provide multiple optimal solutions in a single simulation run. The regression models for the above mentioned machining processes which were developed by previous researchers are used as fitness function for MO-Jaya algorithm.In the case of WEDM process the optimization problem is an unconstrained, linear and parameter bounded. In the case of laser cutting process the optimization problem is a non-linear, unconstrained, quadratic and parameter bounded. In the ECM process the optimization problem is a non-linear, unconstrained, quadratic and parameter bounded. The second case study of ECM process the optimization problem is a non-linear, constrained, non-quadratic and parameter bounded. In the case of FIB micro-milling process, the optimization problem is a non-linear, unconstrained, quadratic and parameter bounded. In addition, the performance of MO-Jaya algorithm is also tested on a non-linear, non-quadratic unconstrained multi-objective benchmark function of CEC2009. In order to handle the constraints effectively a heuristic approach for handling constraints known as the constrained-dominance concept is used in MO-Jaya algorithm. In order to ensure that the newly generated solutions are within the parameter bounds a parameter-bounding strategy is used in MO-Jaya algorithm. The results of MO-Jaya algorithm are compared with the results of GA, NSGA, NSGA-II, BBO, NSTLBO, PSO, SQP and Monte Carlo simulations. The results have shown the better performance of the proposed algorithm.

Tool Point Frequency Response Prediction for Micromilling by Receptance Coupling Substructure Analysis

One of the challenges in micromilling processing is chatter, an unstable phenomenon which has a larger impact on the microdomain compared to macro one. The minimization of tool chatter is the key to good surface quality in the micromilling process, which is also related to the milling tool and the milling structure system dynamics. Frequency response function (FRF) at micromilling tool point describes dynamic behavior of the whole micromilling machine-spindle-tool system. In this paper, based on receptance coupling substructure analysis (RCSA) and the consideration of rotational degree-of-freedom, tool point frequency response function of micromilling dynamic system is obtained by combining two functions calculated from beam theory and obtained by hammer testing. And frequency response functions solved by Timoshenko's and Euler's beam theories are compared. Finally, the frequency response function is identified as the modal parameters, and the modal parameters are transformed into equivalent structural parameters of the physical system. The research work considers the difference of theoretical modeling between the micromilling and end-milling tool and provides a base for the dynamic study of the micromilling system.

A runout measuring method using modeling and simulation cutting force in micro end-milling

In the process of micro end-milling, the micro tool axis is not the same line of the spindle axis due to the eccentricity of the tool-holder-spindle assembly, which is called tool runout. Tool runout has significant effects on cutting force variation, which can lead to higher peak forces and uneven tool wear of the cutter. Hence, a runout model must be included in a cutting force modeling to simulate accurate cutting force during micro-milling process. In this paper, the method for modeling and simulation to measure a runout of tool-holder-spindle in micro end-mill was developed. The simulated cutting force with regard to runout was compared with the measured cutting force. It is noted that they had similar variation pattern and closely matched amplitude levels. The result indicated that the effects of tool runout were predominant for the 0.9 mm diameter and at low feed per tooth.

Swept Mechanism of Micro-Milling Tool Geometry Effect on Machined Oxygen Free High Conductivity Copper (OFHC) Surface Roughness.

Cutting tool geometry should be very much considered in micro-cutting because it has a significant effect on the topography and accuracy of the machined surface, particularly considering the uncut chip thickness is comparable to the cutting edge radius. The objective of this paper was to clarify the influence of the mechanism of the cutting tool geometry on the surface topography in the micro-milling process. Four different cutting tools including two two-fluted end milling tools with different helix angles of 15° and 30° cutting tools, as well as two three-fluted end milling tools with different helix angles of 15° and 30° were investigated by combining theoretical modeling analysis with experimental research. The tool geometry was mathematically modeled through coordinate translation and transformation to make all three cutting edges at the cutting tool tip into the same coordinate system. Swept mechanisms, minimum uncut chip thickness, and cutting tool run-out were considered on modeling surface roughness parameters (the height of surface roughness Rz and average surface roughness Ra) based on the established mathematical model. A set of cutting experiments was carried out using four different shaped cutting tools. It was found that the sweeping volume of the cutting tool increases with the decrease of both the cutting tool helix angle and the flute number. Great coarse machined surface roughness and more non-uniform surface topography are generated when the sweeping volume increases. The outcome of this research should bring about new methodologies for micro-end milling tool design and manufacturing. The machined surface roughness can be improved by appropriately selecting the tool geometrical parameters.

Burr Reduction of Micro-milled Microfluidic Channels Mould Using a Tapered Tool

Moulds with micro-sizes features needed for many applications, such as for hot embossing, can be manufactured using micro-milling process. However, the burrs formed in the micro-milling process are a challenge that needs to be addressed. The burr sizes are comparable to the micro-milled feature sizes and the common types of burr seen being the top/side and exit burrs. The use of a tapered geometry micro-milling tool is investigated in this paper that enables reduction in both the top and exit burrs. The straight and tapered micro-milling tools of various angles are used and the burrs formed are observed. Micro-milling experiments are conducted on an aluminium alloy by producing common positive features seen in the mould for the production of polymer microfluidic devices. The results show that the burr reduction can be attributed due to the increase of the taper angle. It is seen that the tapered tool not only substantially reduces the top burrs, but also leaves behind inclined walls which further help in reducing exit burrs formed during the subsequent finish face milling. Furthermore, embossing trials performed with the micro-milled tapered geometry moulds show improved performance not only because burrs are reduced and also because the taper eases mould release.

Ultraprecision Machining of Silicon Wafer by Micromilling Process

Silicon, being a material which allows light transmission beyond visible spectrum is a good candidate for the integration of microfluidic and optical devices. The lack of rapid prototyping method in particular mechanical-based micromachining process for silicon device processing is due to the brittleness property of silicon. Silicon fails or breaks without significant deformation when subjected to tension or stress. This study presents an experimental investigation on surface of micro-machined single-crystal silicon with (111) orientation. Full immersion slot milling was conducted using solid cubic boron nitride (CBN) micro-end mills. Influences of cutting parameters including spindle speed, feed rate and axial depth of cut on surface roughness were analyzed. Surface and subsurface characterization studies show that the primary material removal mode is ductile or partial ductile using lower feed rate.