Micro-end Milling Research Articles

Modeling of burr thickness in micro-end milling of Ti6Al4V

Mechanical micro-machining of Ti6Al4V is finding great demand because of its wide range of application in various fields such as communication, optics and biomedical devices. Increasing demands on functioning and performance requires components to be free from burrs after the machining process. Presence of burrs on micro-mechanical parts or features significantly affects quality and proper assembly of the parts. Also in micro-machining, the size of burr is comparable to that of micro-features. Since the formation of burr is inevitable in any machining process, generally the deburring operation is performed to remove burrs. Burr thickness is one of the important parameters which describe the time and method necessary for the deburring operation. Burrs on micro-parts are generally characterized using scanning electron microscope, which is a time-consuming, costly and non-value-added activity. However, a proper mathematical model will help predict burr thickness easily. In this article, a mathematical model to predict burr thickness during micro-end milling of Ti6Al4V is presented. The proposed model was developed based on the principle of continuity of work at the transition from chip formation to burr formation. Ti6Al4V titanium alloy is one of the materials which generates segmented (saw-tooth) chips at low cutting speeds. Hence, initially an appropriate material constitutive model was selected based on better prediction of burr thickness. Then, to reduce the prediction error, machining temperature was evaluated for all experimental conditions and included in the model. From the initial study, it was found that Hyperbolic TANgent material model gives a better prediction compared to Johnson–Cook material model. Later, after including machining temperature into the model it was observed that the prediction error was reduced. The proposed model was validated with the experimental results.

Tool Condition Monitoring in Micro-End Milling using wavelets

In this work, Tool Condition Monitoring (TCM) strategy is developed for micro-end milling of titanium alloy and mild steel work-pieces. Full immersion slot milling experiments are conducted using a solid tungsten carbide end mill for more than 1900 s to have reasonable amount of tool wear. During the micro-end milling process, cutting force and vibration signals are acquired using Kistler piezo-electric 3-component force dynamometer (9256C2) and accelerometer (NI cDAQ-9188) respectively. The force components and the vibration signals are processed using Discrete Wavelet Transformation (DWT) in both time and frequency window. 5-level wavelet packet decomposition using Db-8 wavelet is carried out and the detailed coefficients D1 to D5 for each of the signals are obtained. The results of the wavelet transformation are correlated with the tool wear. In case of vibration signals, de-noising is done for higher frequency components (D1) and force signals were de-noised for lower frequency components (D5). Increasing value of MAD (Mean Absolute Deviation) of the detail coefficients for successive channels depicted tool wear. The predictions of the tool wear are confirmed from the actual wear observed in the SEM of the worn tool.

Experimental and multiscale numerical investigation of wear mechanism and cutting performance of polycrystalline diamond tools in micro-end milling of titanium alloy Ti-6Al-4V

The present study reports on experiments and numerical simulations carried out to determine the wear and cutting performance of different polycrystalline diamond (PCD) tools in micro-end milling titanium alloy. The influence of tool geometrical parameters on wear resistance and machined surface precision was discussed. Furthermore, subsurface microstructure alteration was employed as an important index for evaluating the cutting performance of PCD tools. A dislocation dynamics-based multiscale framework, which is capable of promulgating the potential mechanism of above alteration, was adopted to quantitatively predict the evolution behavior of subsurface damages layer during micro-cutting process. The results demonstrated that the tool nose, flank and rake wear were of major wear forms and inappropriate tool structural changes can further accelerate tool failure. A PCD tool with rake angle of 5°, clearance angle of 15°, tool cutting edge radius of 20 μm and PCD granularity of 10 μm has highest cutting performance among the tested tools. Using this cutting tool, a surface roughness of Ra = 75 nm better than most previously reported value on titanium alloy Ti-6Al-4V was achieved. A mass of subsurface damages consisted of discrete dislocation configuration, parallel glide lines and persistent slip bands were found after machining. Particularly, small tool cutting edge radius, large rake as well as clearance angle contributed to reducing defects quantity and decreasing the thickness of subsurface damages layer.

Chatter stability prediction in high-speed micromilling of Ti6Al4V via finite element based microend mill dynamics

High-speed micromilling (spindle speeds 100 000 r/min) can create complex three-dimensional microfeatures in difficult-to-machine materials. The micromachined surface must be of high quality, to meet functional requirements. However, chatter-induced dynamic instability deteriorates the surface quality and can be detrimental to tool life. Chatter-free machining can be accomplished by identifying stable process parameters via stability lobe diagram. To generate accurate stability lobe diagram, it is essential to determine the microend mill dynamics. Frequency response function is required to determine the tool-tip dynamics obtained by experimental impact analysis. Note that application of impact load at the microend mill tip (typically 100 – 500 μm) is not feasible as it would invariably end with tool failure. Consequently, alternative methods need to be developed to identify the microend mill dynamics. In the present work, the frequency response function for the microend mill is obtained by finite element method modal analysis. The frequency response function obtained from modal analysis has been verified from the experimentally obtained frequency response function. The experimental frequency response function was obtained by impacting the microend mill near the taper portion with an impact hammer and measuring the vibration of the tool-tip with a laser displacement sensor. The fundamental frequency obtained from finite element method modal analysis shows a difference of 6.6% from the experimental fundamental frequency. Microend mill dynamics obtained from the finite element method is used for chatter prediction in high-speed micromilling operations. The stability lobe diagram predicts the stability boundary accurately at 60 000 r · min–1 and 80 000 r/min; however, a slight deviation is observed at 100 000 r/min.

Tool runout and single-edge cutting in micro-milling

Micro-milling is a direct and effective manufacturing method for complex three-dimensional miniaturized parts. Since tool runout has a great influence on micro-milling, this paper proposes a simple and effective method for measuring tool runout parameters. The method was based on the displacement measurement, which obtained the tool runout length and tool runout angle by solving some equations related to the cutter parameters. After analyzing the cutting trajectories of the cutter, a single-edge cutting criterion in micro-milling was proposed. According to the single-edge cutting criterion, the influencing factors of single-edge cutting both in micro-side milling and micro-end milling were analyzed respectively. Finally, the influences of single-edge cutting on micro-milling were analyzed and the single-edge cutting phenomenon was verified by the experiment of cutting forces.

An experimental investigation of micro-milling brass considering run out by carbide micro-end mills

This paper presented the effect of run out on the experimental characteristic of micro-milling brass using carbide micro-end mills. A method of calculation and measurement for the run out of tool-holder-spindle assembly in micro-end mill was developed. A series of micro-milling process experiments were carried out under varying cutting parameters. The effect of run out on cutting forces, effect of cutting parameters on surface roughness, and size effect were analyzed. It was seen that the cutting force signature was seriously affected by run out in the micro-milling process. When the feed per tooth is less than the run out, the cutting force signals showed that only one cutter flute engaged in cutting process due to the effect of run out. It was also seen that the cutting force signature showed erratic variations due to the effect of tool–workpiece and the run out of tool tip at higher spindle speed. Surface roughness was affected by both cutting speed and feed per tooth. For lower cutting speed, there was increase in the surface roughness with the decrease in the cutting speed due to the effect of built-up edge. For higher cutting speed, there was increase in the surface roughness with the increase in the cutting speed due to dominance of the shearing effects. When the feed per tooth was less than the minimum chip thickness, due to the indentation and ploughing-dominated process, nonlinear increase of specific shear energy can be obtained. At lower feed per tooth, the specific energy increases with increased cutting speed. These results are used to provide strategies to optimize cutting parameters and achieve better surface quality in micro-milling brass process.

Determination of minimum uncut chip thickness under various machining conditions during micro-milling of Ti-6Al-4V

To optimize the machining process, finding the minimum uncut chip thickness is of paramount importance in micro-scale machining. However, strong dependency of the minimum uncut chip thickness to the tool geometry, workpiece material, tool-work friction, and process condition makes its evaluation complicated. The paper focuses on determination of the minimum uncut chip thickness experimentally during micro-end milling of titanium alloy Ti-6Al-4V with respect to influences of cutting parameters and lubricating systems. Experiments were carried out on a CNC machining center Kern Evo with two flute end mills of 0.8 and 2 mm diameters being used in the tests for micro- and macro-milling, respectively. It was found that the micro-milling caused more size effect than macro-milling due to higher surface micro-hardness and specific cutting forces. The specific cutting force depended strongly on feed rate (f z) and lubricating system, followed by depth of cut (a p) and cutting speed (v c), mainly in the micro-scale. All output parameters were inversely proportional to the specific cutting force. Finally, depending on different process parameters during micro-milling of Ti-6Al-4V, the minimum uncut chip thickness was found to vary between 0.15 and 0.49 of the tool edge radius.

Floor surface roughness model considering tool vibration in the process of micro-milling

The high quality of the micro-machined surface is the basic for promoting micro-milling technology, so the understanding of surface forming mechanism and the relationship between micro-milling parameters and the surface roughness are important. In this paper, we establish a comprehensive floor surface model which predicts the surface roughness of the grooves under different cutting parameters and tool geometrical parameters in micro-milling. This model is based on the movement and vibration of micro-end mill and the forming of workpiece surface governed by elastic recovery and tool geometry. The kinematics of micro-milling operations directly affects the topography and is calculated by the trochoidal trajectory formula. The relative vibration of the tool to the workpiece, which affects the ultimate movement trajectory, is measured through experiments. Besides, the elastic recovery and the minimum cutting thickness included in the surface generation procedure are calculated based on the microscopic mechanics of strain gradient theory. Finally, experiments of slot micro-milling under different cutting parameters validate the accuracy of the surface prediction model.

Micro ball-end milling—an emerging manufacturing technology for micro-feature patterns

The demand for complex micro-feature-based miniaturised products has grown over the last few decades due to their higher utility in many fields such as micro-fluidic devices, biomedical devices, orthopaedic implants, automotive industries, aerospace industries, etc. To fulfil the growing demand of these micro-feature-based products, several micromanufacturing techniques have been developed over the years. Micro-end milling is considered to be one of the favourable types among these techniques due to its higher flexibility and applicability over a wide range of materials. This technique is even more prominent when ball-end mill is used as a cutting tool due to its additional ability of finishing complicated surfaces and producing various 3D micro-textured surfaces. The present article provides an inclusive review of the tool fabrication, cutting mechanism, performance analysis, modelling and simulation and industrial applications of micro ball-end milling process. Furthermore, comments on future research scope have been emphasised in the direction of micro ball-end milling of different materials and their applications for producing new products with micro-features.

Instantaneous uncut chip thickness modeling for micro-end milling process

ABSTRACTThe prediction model of instantaneous uncut chip thickness is critical for micro-end milling process, which can directly affect the cutting forces, surface accuracy, and process stability of the micro-end milling process. This paper presents an instantaneous uncut chip thickness model systematically based on the actual trochoidal trajectory of tooth and the tool run-out in micro-end milling process. The variable entry and exit angles of tool, which are affected by the tool run-out, are concerned in the model. The related instantaneous uncut chip thickness is evaluated by considering the theoretical instantaneous uncut chip thickness and the minimum uncut chip thickness, which is formulated by two types of material removal mechanisms, in the elastic-plastic deformation region and the complete chip formation region, respectively. In comparison with the instantaneous chip thickness obtained from the conventional model, the feasibility of the proposed model can be proved by the related simulation results with variable process parameters including feed per tooth, radial depth of cut, and tool run-out. In addition, the predicted and measured cutting forces are compared with validate the accuracy of the proposed instantaneous uncut chip thickness model for the micro-end milling process.

Determination of minimum uncut chip thickness during micro-end milling Inconel 718 with acoustic emission signals and FEM simulation

Micro-machining has gained increased application to produce miniaturized parts in various industries. However, the uncut chip thickness in micro-machining is comparable to cutting edge radius. The relationship between the cutting edge radius and uncut chip thickness has been a subject matter of increasing interest. The acoustic emission (AE) signal can reflect the stress wave caused by the sudden release of the energy of the deformed materials. To improve the precision of machining system, determination of the minimum uncut chip thickness was investigated in this paper. The AE signal generated during micro-cutting experiments was used to analyze the chip formation in micro-end milling of Inconel 718. The finite element method (FEM) simulation was used to analyze the results of the experiments. The results showed that the cutting tool geometry and material properties affected the minimum uncut chip thickness. The estimation of the minimum uncut chip thickness based on AE signals can produce quite satisfactory results. The research on the minimum uncut chip thickness can provide theoretical basis for analysis of surface quality and optimal choice of cutting parameters.

Experimental characterization on micro-end milling of titanium alloy using nanofluid minimum quantity lubrication with chilly gas

The objective of this research is to experimentally characterize the micro-end-milling process of titanium alloy (Ti-6Al-4V) using both nanofluid minimum quantity lubrication (MQL) and chilly gas. A series of micro-end-milling experiments are conducted by considering nanodiamond particles for nanofluid MQL. In the case of chilly gas cooling, the chilly CO2 gas having the temperature of −25 °C is provided. During the experiments, the milling forces, coefficients of friction, surface roughness and tool wear are analysed according to types of nanofluids and their weight concentrations. Overall, the experimental results show that the nanofluid MQL with chilly CO2 gas cooling is effective for reducing milling forces, coefficients of friction, tool wear and surface roughness. It is also demonstrated that the lower concentration (0.1 wt.%) of nanodiamond nanofluid is more effective to reduce the milling force, coefficient of friction and tool wear. On the other hand, it is found that the higher concentration (1.0 wt.%) is more advantageous for reducing the surface roughness.

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.

Component Mode Synthesis Approach for Micro End Mill Dynamics Considering Machine Tool Compliance

Chatter in machining can be avoided by machining at stable process parameters obtained from the stability lobe diagram. The accuracy of the stability lobe diagram depends on the accurate determination of micro-endmill dynamics. It is not trivial to experimentally determine the tool tip dynamics of a micro-endmill due to relatively small diameters (10μm to 500μm). There are different approaches to estimate the micro-endmill dynamics, however, one of the most widely used techniques is the receptance coupling method. It combines the frequency response functions for the two substructures (machine tool with a portion of shank and the micro-endmill) to determine the tool-tip dynamics. There are certain issues with the receptance coupling which uses only two locations for determining the FRF one at the coupling and other at tip unlike the component mode synthesis which allows use of mode shapes at multiple points in the substructures. Another limitation for receptance coupling method is that the FRF for both substructures must be sampled at same frequency for coupling but the first non-rigid modal frequency for free-free substructure of micro-endmill can exceed 50kHz which would require a high bandwidth sensor with a sampling frequency in excess of 100kHz. Consequently, the component mode synthesis approach has been used to couple the substructures. The mode synthesis approach adopted in the present work contains the position dependent modes along the length of substructures. In order to ensure that the component mode synthesis yields accurate results, a test case of a micro-endmill as a fixed cantilever beam has been analyzed. Note that the actual micro-end mill dynamics is affected by the machine tool compliance which needs to be accounted for. Hence, a component mode synthesis of the two substructures, machine-tool/shank and the micro-endmill is carried out. The mode shapes of machine-tool shank substructure are determined experimentally whereas the micro-endmill mode shapes are determined via finite element method (FEM). The predicted micro-endmill dynamics with machine tool compliance has been experimentally validated.

Improved machinability of SiC/SiC ceramic matrix composite via laser-assisted micromachining

This study is focused on numerical modeling and experimental evaluation of laser-assisted micromachining (LAMM) of SiC/SiC ceramic matrix composite (CMC) materials. A novel experimental setup consisting of a fiber laser and the necessary optics, a three-axis CNC linear stage, and a high-speed spindle was used to implement the LAMM process. The laser-assisted micro-milling system provides unique micro-milling capabilities for flexible machining of very difficult-to-machine materials, such as ceramics, composites, and high-temperature alloys. The integrated high-power laser beam is flexibly focused on arbitrary positions around the cutting tool at a low incidence angle with a very small focused spot, thus making the system suitable for a wide range of micro-milling methods. Micro-end mills of cubic boron nitride (CBN) tools were used to perform slotting operations with and without laser preheating on CMC materials for comparative assessment. A three-dimensional (3D) transient finite-element-based thermal model was used to analytically predict appropriate process parameters on the basis of material removal temperature (Tmr). The effects of LAMM on the tool wear and tool life were evaluated experimentally. In addition, an economic analysis was carried out to compare LAMM of CMC materials with conventional micromachining methods.

Special Theme: Modelling of Manufacturing Processes

Manufacturing sector is the backbone of the economic development of our nation. There is an increasing emphasis on this sector through the ‘‘Make in India’’ program of Indian Government. Two major challenges for the industry are to achieve the goal of sustainability and face stiff global competition. It is pertinent, therefore, to apply scientific methods in manufacturing with a focus on reducing energy consumption, pollution, resources and cost. This needs proper attention for entire manufacturing system including supply chain. However, the most difficult task is to optimize the manufacturing processes. This requires proper modelling of the processes. The modelling of manufacturing processes requires the application of material science, plasticity, heat transfer, fluid mechanics, thermodynamics and several other areas of physics and chemistry. The governing differential equations of a manufacturing process are solved using some numerical techniques such as Finite Difference Method (FDM), Finite Element Method (FEM), and Finite Volume Method (FVM). The research in the modelling of manufacturing process comprises the development of suitable governing equations of the process as well as their solution. It invariably requires the experimental validation. It is important to understand the validity of assumptions and the constraints of the shopfloor. It is encouraging to note that manufacturing fraternity is recognizing the importance of modelling of manufacturing processes. This is reflected from a large number of papers submitted in the prestigious 5 International & 26 All India Manufacturing Technology, Design and Research (AIMTDR) Conference held at Indian Institute of Technology Guwahati during December 12–14, 2014. This special issue entitled ‘‘Modelling of Manufacturing Processes’’ contains eight revised and enhanced papers presented in the conference. The papers have been further peer reviewed for publication in the special issue. The papers in the special issue are from casting, traditional and non-traditional machining, forming and welding areas. The paper ‘‘Mathematical Modelling of Solidification in a Curved Strand during Continuous Casting of Steel’’ describes transport phenomena based heat transfer and fluid flow model of continuous casting process using finite volume method. The turbulent flow of molten materials and metallurgical effect are considered in this study. Through simulation, the authors identified the upper limit of casting speed for avoiding the defects. The paper ‘‘Finite Element Simulations of Micro Turning of Ti–6Al–4V using PCD and Coated Carbide Tools’’ describes the simulation of micro-turning process over Lagrangian frame where chip formation is modelled by adaptive re-meshing technique. The material model includes strain gradient effect to represent the flow stress of the work material. Polycrystalline diamond (PCD) tool shows better performance than coated carbide in terms of low interface temperature and low power consumption at high cutting speed. The error in the prediction of cutting forces between numerical model and experimental results is less than 15 %. The paper ‘‘Finite Element Method based Modeling for Prediction of Cutting Forces in Micro-end Milling’’ describes the prediction of cutting forces in micro-end milling process by considering the effect of frictional & Uday S. Dixit usd1008@yahoo.com

An experimental investigation on the machining characteristics of Nimonic 75 using uncoated and TiAlN coated tungsten carbide micro-end mills

We report the machining characteristics and machinability of a nickel based superalloy in this study. A micro-milling operation is loaded on Nimonic 75 using uncoated and TiAlN coated tungsten carbide micro-end mills. A full factorial design of experiments was devised to optimize the machining conditions to reduce the flank wear on the tool surface. The optimized machining conditions for uncoated micro-tools were found to be a cutting speed (vc) of 13m/min and a feed rate (fz) of 6mm/min. Following this, the tools were coated with TiAlN using a semi-industrial four-cathode reactive pulsed direct current unbalanced magnetron sputtering system. Further experiments were then performed using these optimized machining conditions using both uncoated and TiAlN coated micro-tools in order to ascertain the tool wear and surface integrity. The change in geometry of the machined slot was estimated based on the variation in tool radius of the micro-end mill with progression of the operation. A direct comparison was made between the results observed using both uncoated and TiAlN coated tungsten carbide to illustrate the effect of the nanocomposite TiAlN coating. It was seen that TiAlN coated micro-tools exhibited a superior performance as compared to the uncoated ones with respect to tool life and micro-burr formation.

Tool-wear monitoring during micro-end milling using wavelet packet transform and Fisher’s linear discriminant

Tool wear is one of the most important parameters in micro-end milling, and can be used to monitor the condition of the machine and the tool. A micro-end mill has different characteristics from a macro-scale end mill; in particular, shank run-out (which is negligible in the macro-scale tool due to the low aspect ratio) is significant in micro-end milling, inducing excessive tool wear and reduced tool life and leading to sudden, premature failure. In this paper, a novel tool-wear monitoring method is described for determining the state of a micro-end mill using wavelet packet transforms and Fisher’s linear discriminant. Force and torque signals were measured using a dynamometer and were used to reflect geometric changes in the micro-end mill due to wear. Because of the small signal-to-noise ratio, sensor signals measured during the milling process were periodically averaged, and the resulting single-period signals provided improved efficiency of feature extraction using wavelet packet transforms. The extracted features were classified in the wavelet domain and used to determine the tool state employing a hidden Markov model. The recognition results were compared with those of an energy-based monitoring technique, and we found that our method could determine the tool state more accurately for both normal wear and premature failure of micro-end mills.

Finite Element Method Based Modeling for Prediction of Cutting Forces in Micro-end Milling

Micro-end milling is one of the widely used processes for producing micro features/components in micro-fluidic systems, biomedical applications, aerospace applications, electronics and many more fields. However in these applications, the forces generated in the micro-end milling process can cause tool vibration, process instability and even cause tool breakage if not minimized. Therefore, an accurate prediction of cutting forces in micro-end milling is essential. In this work, a finite element method based model is developed using ABAQUS/Explicit 6.12 software for prediction of cutting forces in micro-end milling with due consideration of tool edge radius effect, thermo-mechanical properties and failure parameters of the workpiece material including friction behaviour at tool-chip interface. Experiments have been performed for manufacturing of microchannels on copper plate using 500 µm diameter tungsten carbide micro-end mill and cutting forces are acquired through a dynamometer. Predicted cutting forces in feed and cross feed directions are compared with experimental results and are found to be in good agreements. Results also show that FEM based simulations can be applied to analyze size effects of specific cutting forces in micro-end milling process.

Micro-machinability of nanoparticle-reinforced Mg-based MMCs: an experimental investigation

As a composite material with combination of low weight and high engineering strength, metal matrix composites (MMCs) have been utilised in numerous applications such as aerospace, automobile, and bioengineering. However, MMCs are recognised as difficult-to-cut materials due to their improved strength and high hardness of the reinforcing particles. This paper presents an experimental investigation on micro-machinability of Mg-based MMCs reinforced with Ti and TiB2 nano-sized particles. The tool wear of AlTiN-coated micro-end mills was investigated. Both abrasive and chip adhesion effect were observed on the main cutting edges, whilst the reinforcement materials and volume fraction play an important role in determining the wear type and severity. The influence of cutting parameters on the surface morphology and cutting force was studied. According to analysis of variance (ANOVA), depth of cut and spindle speed have significant effect on the surface roughness. The specific cutting energy, surface morphology and the minimum chip thickness was obtained and characterised with the aim of examining the size effect. Furthermore, higher cutting force and worse machined surface quality were obtained at the small feed per tooth ranging from 0.15 to 0.5 μm/tooth indicating a strong size effect. Overall, Mg/TiB2 MMCs exhibit better machinability.