Milling Tool Research Articles

In-Process Chatter Detection in Milling: Comparison of the Robustness of Selected Entropy Methods

This article deals with the issue of online chatter detection during milling. The aim is to achieve a verification of the reliability and robustness of selected methods for the detection of chatter that can be evaluated on the machine tool in real time by using the accelerometer signal. In the introductory part of the paper, an overview of the current state of the art in the field of chatter detection is summarized. Entropic methods have been selected that evaluate the presence of chatter from the qualitative behavior of the signal rather than from the magnitude of its amplitude, because the latter can be affected by the transmission of vibrations to the accelerometer position. Another criterion for selection was the potential for practical implementation in a real-time evaluation of the accelerometer signal, which is nowadays quite commonly installed on machine tools. The robustness of the methods was tested with respect to tool compliance, which affects both chatter occurrence and vibration transfer to the accelerometer location. Therefore, the study was carried out on a slender milling tool with two different overhangs and on a rigid roughing tool. The reference stability assessment for each measurement was based on samples of the machined surface. The signals obtained from the accelerometer were then post-processed and used to calculate the chatter indicators. In this way, it was possible to compare different methods in terms of their ability to achieve reliable in-process detection of chatter and in terms of the computational complexity of the indicator.

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

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

Simulating ion flux to 3D parts in vacuum arc coating: Investigating effect of part size using novel particle-based model

This work presents a novel particle-based computational model capable of predicting coating distribution and composition on real parts in ion-based physical vapor deposition (PVD) processes. The model treats ions as particles (using test particle Monte Carlo method) and electrons as a fluid (using Boltzmann's relation). This combination makes it possible to simulate a realistic plasma sheath around a coated part and locally enhanced electric field around the part's edges. This simulation tool is instrumental especially in predicting the so-called “antenna-effect” - i.e. changes in coating thickness and composition near sharp edges of coated parts. It can also be used for quantifying the effect of coater loading or fixture design, as illustrated by simulating the effect of a top-plate on the coating distribution. The model is validated by two experimental data sets - a milling tool and a drill bit, both coated by hard nitride coatings in a vacuum arc process.

Analysis of cutting forces at different spindle speeds with straight and helical-flute tools for conventional-speed milling incorporating the effect of tool runout

This paper is concerned with the experimental investigation and mechanistic prediction of cutting forces for flat-end milling with tool runout. The effects of the tool geometry, the workpiece material and cutting parameters such as spindle speed, tool engagement and cutting direction are investigated. The mechanistic force model uses the trochoidal flute path to calculate the undeformed chip thickness. Average cutting force and linear regression model are applied for identifying the coefficients of the force model, and tool runout parameters are established from the radii of all flutes at the free end of the cutting tool. A series of milling processes are conducted on AZ31 Magnesium (Mg) alloy and titanium alloy (Ti6Al4V) to analyze the instantaneous cutting force curves, amplitudes of cutting forces and peak forces over a wide range of conventional spindle speeds, namely 600, 1200 and 3000 rev/min. It is found that the values of the cutting force coefficients are higher at lower spindle speed and decrease with an increase in spindle speed, especially when machining Ti6Al4V alloy. For the edge force coefficients, it is observed a slight variation when using cutting tools with different helix angles. Besides, the cutting force amplitudes strongly depend upon the workpiece material. The helix angle has a significant influence on the transverse force amplitude at the spindle speed of 600 rev/min. The cutting forces obtained mechanistically are also substantiated by comparison with measurements.

Vibration features for indirect monitoring of end micromilling process

Monitoring a micromachining process is essential to ensure machined surface quality. Vibration signal has been used to monitor conventional process, however, to apply this signal to monitor a micromachining process is not a simple task. One of the main difficulties is to identify which signal features is correlated to changes in the micromachining process, such as roughness. This work investigates the correlation between vibration signals for an end micromilling operation. Different features were extracted from time and frequency domain aiming to analyze their correlation to the roughness and identify those better correlated to surface roughness. Six carbide end mill tools with a diameter of 0.4 mm were tested under different parameters. Vibration signals were collected, and roughness was measured for each machined channel. Through the analysis of the vibration signals and Pearson Correlation, features that are directly correlated with the roughness were identified, with emphasis on 2 x IPF (Insert Passing Frequency). The results show that the proposed features can be used for indirect monitoring of the end micromilling process, avoiding unnecessary stops and fabrication of rejected parts.

Effect of milling parameters on the surface quality of a flax fiber-reinforced polymer composite

Flax fiber-reinforced polymer composites are an interesting alternative to synthetic fiber-reinforced polymer composites for many engineering applications. When machining flax fiber-reinforced composite materials that are by definition heterogeneous, the matrix and the fibers react differently and hence many sorts of damage may occur such as poor surface roughness, delamination, and fluffing. The novelty of the current work lies in identifying the major factors that affect the quality of the milled surface of composites reinforced with flax fibers and provides recommendations and collaborative solutions to the composite machining community. In this study, the impact of cutting conditions (cutting speed, feed rate, and fiber orientation) on the cutting forces and surface roughness during milling of the flax/epoxy composite is investigated. For this purpose, slotting tests are performed on flax fiber-reinforced polymer plates using a carbide end mill tool based on a full factorial design of experiment. Furthermore, a randomization in the order in which experimental runs are done is used to reduce bias by balancing the effect of uncontrolled variables that have not been accounted for in the experimental design. It is concluded that the feed rate has the most influence on the cutting forces and roughness parameters. Moreover, the fibers orientation also has a significant effect on the outputs, and the cutting speed has less effect but it remains significant.

Structural design and mathematical cutting model of hydraulic section milling for high-strength casings in small wellbores

The section milling of casings is widely applied in the side-tracking and abandonment operations of old wells. In particular, the cutting of casings is a crucial task during section milling; however, this process is associated with problems such as low cutting speeds, especially for the high-strength casings used in small wellbores. Furthermore, theoretical support and experimental investigations related to this operation remain scarce. Accordingly, based on the working principle and structural characteristics of a hydraulic cutter in this work, a geometric model of the interactions between the piston rod and blade was established. A high-performance hydraulic section milling tool suitable for 177.8 mm casings was also developed. According to the motion characteristics of the blade, a dynamic contact force model was established for the cutting process of casings, considering changes in the cutting depth and a mathematical model of the cutting depth, considering the effect of time, was also derived according to the grinding mechanism. The critical parameters of this cutting depth model for the P110 casing were determined through ground field tests and finite-element simulations, and the changes in the trajectory were inverted. The corresponding inversion results for the cutting depth exhibited a rapid increase in the significantly short primary stage, followed by a gradual increase. The theoretical torque and cutting time were also highly consistent with the field-measured values; this further verifies the rationality and correctness of the models used in this study. The results of this study are expected to serve as a theoretical basis for optimising the structural and hydraulic parameters of hydraulic cutters and making further improvements in the cutting efficiency for the side-tracking and abandonment operations of old wells.

Effects of fiber orientation on tool wear evolution and wear mechanism when cutting carbon fiber reinforced plastics

The aim of the present paper is to reveal the influence of different fiber orientations on the tool wear evolution and wear mechanism. Side-milling experiments with large-diameter milling tools are conducted. A finite element (FE) cutting model of carbon fiber reinforced plastics (CFRP) is established to get insight into the cutting stress status at different wear stages. The results show that different fiber orientations bring about distinct differences in the extent, profile and mechanism of tool wear. Severer wear occurs when cutting 45° and 90° plies, followed by 0°, correspondingly, the least wear is obtained when θ = 135° (θ represents the orientation of fibers). Moreover, the worn profiles of cutting tools when θ = 0° and 45° are waterfall edge, while round edge occurs when θ = 135° and a combined shape of waterfall and round edge is obtained when θ = 90°. The wear mechanisms under different fiber orientations are strongly dependent on the cutting stress distributions. The evolution of tool wear profile is basically consistent with the stress distribution on the tool surface at different wear stages, and the extent of tool wear is determined by the magnitude of stress on the tool surface. Besides, the worn edges produce an actual negative clearance angle, which decreases the actual cutting thickness and leads to compressing and bending failure of fibers beneath the cutting region as well as low surface qualities.

Stability enhancement and chatter suppression in continuous radial immersion milling

For continuous radial immersion milling operations, the dominant mode shape becomes difficult to determine when stiffness of the tool and workpiece are comparable, and this can pose a great challenge for ensuring machining processes stability. In this paper, we propose a rapid method to obtain time-varying modal parameters of the workpiece by combining experimental measurements with the receptance coupling method. Firstly, the contact parameters between the workpiece and vise were identified by a so-called dynamic coupling matrix. Then the mode shapes and the time-varying natural frequency of the workpiece were determined using the modal parameters of workpiece. Finally, the stability lobe diagrams (SLDs) were computed using the modal parameters and then were validated by undertaking immersion milling experiments. The experiments showed a more conservative and practical SLD for general workpiece under continuous radial immersion, where the workpiece mode had not always dominated the machining process. Based on the proposed method, we also explored two modifications in form of additional cylinder masses and passive support, to suppress chatter. Both modifications were effective in enhancing the minimum boundary of the conservative SLD, and the modification of passive support worked better. Although the modification of the workpiece could improve the stability boundary, it indirectly affected the dynamics of the milling tool through the interaction area between the workpiece and milling tool.

A Parametric Study on Milling Process of Multi-directional Carbon Fiber Reinforced Plastic (MD-CFRP) Composite Using Nano-solid Dry Lubrication

A carbon fiber reinforced plastic (CFRP) composite has been utilized for high-tech industries owing to their superior mechanical properties such as a high strength-to-weight ratio. However, a CFRP composite has anisotropy and non-homogeneity, and these properties can cause defects during machining processes. The objective of this study is to comprehensively analyze influences of process variables on the milling performances of multi-directional carbon fiber reinforced plastic (MD-CFRP) composite. In the process, the nano-solid dry lubrication technology is applied, considering moisture-sensitivity of CFRP. For a parametric study, a series of experiments are conducted based on a design of experiment (DOE) approach. Twenty-seven experimental cases are designed considering three process variables such as a tool geometry, a spindle speed, and a feed rate. Then, the resultant cutting forces, surface quality, tool geometry, and cutting temperature are considered as performance indexes. From the experimental results, it is found that the router shows the best milling performances by reducing cutting forces and surface roughness values by 64.8% and 58.8%, respectively, compared to the two-flute milling tool. The five-flute milling tool is also superior to the two-flute one. In addition, the cutting temperature is measured to study its effect on the milling performances.

INSIGHTS ON ABRASIVE WATER JET MILLING OF BLIND POCKETS

Abrasive water jet (AWJ) machining is one of the advanced machining techniques used in the industries for processing materials that are extremely difficult to machine using conventional machining techniques. Based on the flexibility of AWJ, this process is currently employed for milling blind pockets over different materials. The most frequent method for making blind pockets in AWJ is the controlled depth milling mode. This approach was carried out with the raster tool paths. The quality of the blind pocket surface is influenced by different AWJ parameters such as water jet pressure, traverse speed, step-over distance, abrasive flow rate, and abrasive types. Among these, the traverse rate was found to be an influencing factor in most of the AWJ milling operations as it determines the nozzle speed followed by the energy density of the abrasive particle drops while striking across the target material surface, which resulted in a controlled depth of cut. This review paper highlights the performance of the AWJ pocket milling operations with various materials. From these results, it is reported that most of the AWJ milled surfaces were found to be of rough quality even though they were using different milling tool path strategies and parameter conditions. In addition, the milled pocket defects, namely uneven flatness, grit embedment, and undercut were observed. Besides, future research and directions have been addressed in which some of the novel concepts/approaches have been introduced including the scale effect examination in AWJ with the use of different nozzle, orifice, and abrasive sizes. This study will be more helpful to produce blind pockets with tight tolerances and a significant reduction in the process defects. The outcomes of this study will bring new innovations to the AWJ milling technique in order to make a significant footprint in the manufacturing industries for machining quality blind pockets over the target materials.

Generation mechanism of surface micro-texture in axial ultrasonic vibration-assisted milling (AUVAM)

Although axial ultrasonic vibration-assisted milling (AUVAM) is promising for rapid and economical fabrication of 3D micro-texture on various surfaces, it has been a challenge to obtain micro-texture with small feature size and large height simultaneously. The mechanism and major influence factors for the formation of micro-texture by this method were explored in this study. It was found that the generation of micro-texture was mainly caused by cutting and extrusion from the flank face of the milling tool under ultrasonic vibration. Therefore, a novel 3D tool model that considered the relief angle, end cutting edge angle and the blade profile was established in simulation analysis. The good agreement between simulation and experimental results demonstrated that the influence of the flank face on the micro-texture was mainly attributed to the relief angle of the milling tool. It was the first time to report that both the height and the profile shape of the micro-texture unit were affected by the overlap and extrusion between the flank surface and the micro-texture. But this influence diminished with the increase of spindle speed. 3D sinusoid-shaped micro-texture with minimum width of 20 μm and height of 2 μm (fully reproduced the ultrasonic amplitude) was realized with the relief angle of 40° and spindle speed of 4000 rpm. Other typical textures of weave, shell, scale and corrugation were also presented to show the effective regulation of texture patterns in AUVAM. This work provides both theoretical and practical basis for such a low-cost, efficient and controllable 3D micro-texture preparation method.

Computational methods for the detection of wear and damage to milling tools

The current paper presents a new computational approach to detect wear and damage to milling tools' cutting edges. The proposed approach is independent from exact information on tool-workpiece interaction conditions and only requires that they remain constant for compared milling operations. Additionally, the approach was thoroughly tested on time-series data obtained from an industrial-scale milling process, instrumented by commercially available instrumentation equipment, during which 18 identical parts were milled. The time-series data contains the bending moments in the x and y directions as well as the torque and tension acting on the milling tool. Some measures used are systematic in nature, based on shape, rotation and work needed for milling, whereas others are statistical in nature, describing the change in the distribution of the data. All of the measures proposed in the current work are relative and mutually invariant, meaning they address different information content of the data independently. A comparison of the mentioned measures with the real-world damage evolution of the milling tool's cutting edges for multiple produced parts yielded consistent results and suggests a high potential for practical tool damage detection in industrial production.

Dynamic simulation and experimental study of cutting force by rake angle of multi-axis high-speed ball-end milling tool

Dynamic simulation and experimental study of cutting force by rake angle of multi-axis high-speed ball-end milling tool

An Innovative Process Design Model for Machined Surface Error Distribution Consistency in High-Efficiency Milling

In the process of high-efficiency milling, due to the impact of plunge caused by the intermittent cutting of the milling cutter, there is a high frequency and small vibration between the milling cutter and the workpiece. In this paper, we construct a transient cutting model for high-efficiency milling tools under the action of vibration, and analyze the factors influencing the consistency of the machined surface error and its distribution. The design variables for the consistency of error distribution of high-efficiency milling are obtained, the significance analysis of the influencing factors is carried out using the central composite experimental design and the response surface method, the interaction between the influencing factors is studied, the evaluation index of the consistency of error distribution of milling is proposed, and the process design method for the consistency of error distribution of the machined surface is proposed and validated. The results of the verification experiments show that, compared with the original process scheme, the two maximum errors corresponding to the new process scheme are on average 61.3% smaller than those of the old process scheme, and the consistency of the machined surface error distribution of the new process scheme is significantly improved compared with the old process scheme.

Thermal Aspects in Edge Trimming of Bio-Filled GFRP: Influence of Fiber Orientation and Silica Sand Filler in Heat Generation.

The present work aims to determine the influence of Glass Fiber-Reinforced Polymer (GFRP) laminating configuration in heat generation during the dry edge trimming process. Temperature measurement experiments were conducted on pure epoxy matrix, 15% unidirectional glass fiber reinforced epoxy, and 28% silica sand-filled GFRP specimens through eight type-K thermocouples evenly distributed along the trim plans and connected to a data acquisition system. Infrared thermographic measurements were also conducted to investigate the tool temperature evolution while processing. It was found that perpendicular fiber edge milling induces a sharp increase with peak temperature measurements reaching 119 °C, while machining parallel to fiber leads to a maximum temperature history of 41 °C, which is very close to that obtained from the pure epoxy test. It was also found that the addition of silica sand grains in the GFRP matrix reduces both tool and specimen temperature magnitudes up to 67% for 90° plies and 14% for 0° plies compared to silica sand-free composite initial values. The heat partition was calculated from the measured (electric) and estimated energies for the tool, the workpiece, and chips, respectively. It appears from predictions that the addition of silica sand grains increases the heat conductivity of the GFRP materials (with rates of 20% for 0° fiber orientation and 10% for 90° fiber direction), while it reduces that conducted to the milling tool. Scanning Electron Microscopy (SEM) inspections helped detect the dominating machining defects relative to each GFRP configuration and explained the heat generation and dissipation effects in light of peak temperature measurements.

On modelling and analysis of voxel-based force prediction for a 3-axis CNC machining

ABSTRACT Determination of cutting forces is the main requirement for understanding the machining process and optimising its parameters for achieving higher productivity and surface finish. This paper presents an exploratory study and the development of a model to estimate cutting forces for a 3-axis CNC milling process using a voxel-based CAD model. The developed algorithm takes the NC code, workpiece/tool material properties, and the tool geometry data as inputs. The cutting tool engagement with the workpiece is computed using a discretized (voxelized) model. The calculated voxel engagement was finally used to calculate the cutting forces using the analytical method. The algorithm was implemented and tested for various case studies and the in-house experimental data for different types of end mill tools. Finally, the effect of variation in the size of the voxel and the number of flutes was studied. The model showed a good correlation and was found to be accurate (~80%) and robust.

Three-Stage Wiener-Process-Based Model for Remaining Useful Life Prediction of a Cutting Tool in High-Speed Milling.

Tool condition monitoring can be employed to ensure safe and full utilization of the cutting tool. Hence, remaining useful life (RUL) prediction of a cutting tool is an important issue for an effective high-speed milling process-monitoring system. However, it is difficult to establish a mechanism model for the life decreasing process owing to the different wear rates in various stages of cutting tool. This study proposes a three-stage Wiener-process-based degradation model for the cutting tool wear estimation and remaining useful life prediction. Tool wear stages classification and RUL prediction are jointly addressed in this work in order to take full advantage of Wiener process, as this three-stage Wiener process definitely constitutes to describe the degradation processes at different wear stages, based on which the overall useful life can be accurately obtained. The numerical results obtained using extensive experiment indicate that the proposed model can effectively predict the cutting tool’s remaining useful life. Empirical comparisons show that the proposed model performs better than existing models in predicting the cutting tool RUL.

Cutting force and vibration prediction of milling processes regarding the nonlinear behavior of cascade controlled feed drives

Milling processes are characterized by their high degree of flexibility regarding shape creation and thus occur in many value adding processes in the electrical and mechanical engineering industry. To facilitate the systematic prediction and the efficient optimization of the process performance, a holistic simulation model of the milling process with cascade controlled feed drives is developed. The integrated model consists of two subsystems, namely the feed drive model and the process model. In the feed drive model, the dynamic models of the electric motors, the serially connected torsional oscillators of the mechanical elements and the cascade control system are established. In this model, the coordinate point of the milling tool is computed at each discrete time step and assigned to the process model. The cutting forces and the corresponding load torques are calculated in the process model and sent back to the feed drive model. In addition, practical validation of the complete system has been conducted both in the time and the frequency domain. The simulation results yield an accurate prediction of the tooth passing frequency with a variance of less than 1%. Moreover, the influence of the cascade control parameters and the nonlinear behavior of the electric motor on the process vibrations have been experimentally verified.

Improving a rotating steel milling tool cooling by an air jet impingement flow

Turbulent jets impacting flat or curved surfaces have been the subject of several research reports in the literature over the past decades, and it is all about the improvement of localized heat and/or mass transfer in part of a system, to achieve this goal, it is necessary to understand the dynamic behavior of the fluid and its effect on local heat and/or mass transfer. The aim of our work is the numerical investigation of a steel milling tool cooling driven in rotation, emitting a heat flux caused by machining a surface, using an impacting cold air jet; Although the technology of cooling the machining tools exists by using liquid jets, it signaled that it is less economical and efficient then using the impinging air jet. The milling tool is cooled using forced convection produced by the air jet at different Reynolds numbers, this phenomenon is governed by the equations of conservation of mass, momentum, and energy. We adopted a mathematical model based on the finite volume method; the calculation code was validated using experimental results available in the literature. We obtained our results by varying the Reynolds number between 12,000 and 26,000 values for an impact distance ratio h/D = 4, and two configurations: one nozzle and two nozzles. Our results showed that when using two cold air jet inlets to cool the milling tool, the distribution of the Nusselt number on the surface is enhanced compared to the one nozzle case, mainly because of the two impact points of the jet on the surface. We also note that increasing the Reynolds number has a significant influence on the distribution of the averaged Nusselt number.