Machined Surface Quality Research Articles

Design of surface micro texture in the cutter-chip contact area of a cemented carbide cutter

Placing micro textures on a cutter surface can enhance the cutter-chip and cutter-workpiece contact, improving the cutter performance and machined surface quality. However, both the proper placement of micro textures and careful modeling of their density function are required for optimizing the cutting performance. In this study, a ball end milling cutter was utilized to demonstrate how these can be achieved. Initially, the theoretical cutter-chip contact length of the was calculated based on the chip curl theory and the theoretical cutter-chip contact area was obtained by combining the theories of cutter contact area and micro texture placement. Then, a mathematical model describing the micro texture distribution was established using a uniform density function, which allowed the determination of the specific distribution model for the micro textures. Finally, both simulations and experiments were used to validate the accuracy of the theoretical solution for the cutter-chip contact area and the uniformity of the micro texture density function. The results showed that the theoretical cutter-chip contact area and the uniform micro texture density mathematical model are consistent with the simulation and experimental results. The results reported in this paper provide a theoretical basis for the accurate preparation of micro textures.

Investigation on the machined surface quality and removal mechanism of SiCf/SiC composites in ultrasonic-assisted grinding

Investigation on the machined surface quality and removal mechanism of SiCf/SiC composites in ultrasonic-assisted grinding

Analysis of the effect of material thickness during the WEDM processon cutting speed, topography and morphology

Wire electrical discharge machining (WEDM) is an unconventional machining technology that is indispensable in many industries. Machining is performed using the thermoelectric principle, while it is possible to machine all at least minimally electrically conductive materials. Due to the wide range of applications of WEDM, it is necessary to ensure the appropriate quality of machined surfaces, regardless of the thickness of the machined materials, while maintaining an acceptable cutting speed. For this purpose, this study was performed to analyse the effect of material thickness on the cutting speed, morphology and topography of Ampcoloy 35 material. In this study, thicknesses from 5 to 160 mm were analysed in 5 mm increments with the same machine parameter settings. The surface topography and morphology were studied using electron and light microscopy, and cross-sections of the sample were created to examine the condition of the subsurface layer. It was found that the values of Ra at the edge ranged from 1.8 to 3.2 µm and in the middle of the sample from 1.7 to 3 µm, while the trend of increasing Ra with thickness is not visible. It is clear from the morphology analysis that a rugged surface with more craters was created at the edges than in the middle of the sample, while segregated lead needles were also formed at the edges of the samples.

Tool wear and its effect on the surface integrity and fatigue behavior in high-speed ultrasonic peening milling of Inconel 718

Ultrasonic-assisted technology has proven effective for Inconel 718 milling. This study compared tool life and tool wear forms and wear mechanisms at different wear states in high-speed Ultrasonic Peening Milling (UPM) and Conventional Milling (CM). The influence of different tool wear states on the surface integrity index and fatigue performance were systematically analyzed. The results showed 32.5% increase in tool life in UPM owing to lower tool wear compared to that in CM. More important, UPM was significantly more effective in improving machined surface quality and fatigue life with an average increase of 4–17 times.

Prediction modelling of cutting force in rotary ultrasonic end grinding 2.5D woven SiO2f/SiO2 ceramic matrix composite

Rotary ultrasonic end grinding (RUEG) of ceramic matrix composites has advantages in reducing sub-surface damages and improving machined surface quality. The cutting force prediction for RUEG can reveal the mechanic information and machined surface quality in the grinding process. However, the prediction model for grinding force in RUEG of SiO2f/SiO2 ceramic matrix composite has not been reported in literatures. In this paper, the mechanical properties of 2.5D woven SiO2f/SiO2 ceramic matrix composite are estimated using stiffness averaging method. Furthermore, the theoretical prediction model for axial and transverse grinding force component in RUEG of 2.5D SiO2f/SiO2 composite is developed. The development of the prediction model is based on the analysis of a single abrasive grit on the grinding wheel end and side face, including kinematic analysis, penetration depth, and effective machining time in RUEG. The final axial and transverse grinding force component is predicted by considering the force contributions of all end and side abrasive grits involved in RUEG, respectively. The prediction results of the developed model are validated by experimental results. The influence mechanism of process parameters on the axial and transverse grinding force component in RUEG of 2.5D SiO2f/SiO2 composite is explained by the developed prediction model.

Energy-aware sub-regional milling method for free-form surface based on clustering features

Geometric characteristics and curvature of free-form surfaces have a great influence on multi-axis computer numerical control (CNC) machining performances such as processing efficiency, surface quality and energy consumption. The conventional machining methods with the whole tool path generation results in poor low machining efficiency, surface quality, and high energy consumption, due to the repeated and crossing tool path. Therefore, this paper proposes an energy-aware milling method for complex surface based on clustering features. Firstly, the geometric features of free-form surfaces are classified according to their curvature characteristics. Then, free-form surface is discretized and initially divided into different regions, and further divided into patches based on K-means clustering algorithm to generate the same or similar surface type. Moreover, an optimization model for the sub-regional milling tool path is established, and the optimal result is obtained using adaptive dynamic genetic algorithm. Finally, the effectiveness of the proposed method is verified by comparison of traditional milling methods.

Investigation on minimum uncut chip thickness and size effect in micro milling of glow discharge polymer (GDP)

Glow discharge polymer (GDP) is used as the ablator material in inertial confinement fusion (ICF) applications. Its machining surface quality and geometric accuracy directly affect its high energy performance. Minimum uncut chip thickness (MUCT) and size effect are the keys to obtain higher surface quality during micro cutting. In this work, a hybrid analytical model of MUCT using cutting forces and effective rake angle based on minimum energy method (Model A), and micro milling experiment are applied to determine the MUCT of GDP. Furthermore, a specific cutting energy-based model (Model B), considering the mechanical properties, strain evolution, and tool geometry, is established to verify model A. Additionally, the material removal mechanism and size effect are also studied. It is found that the MUCT value of GDP determined by model A is between 0.268 and 0.532 μm, and the normalized MUCT value is between 0.148 and 0.295. These results are verified by the specific cutting energy obtained from model B. It is also proved that model B could preliminarily evaluate the MUCT value of GDP without conducting milling experiment. Moreover, below the MUCT threshold, the severe size effect and ploughing effect cause nonlinearly increase of specific cutting energy, and the random variation of cutting force and surface roughness. When the ratio of feed per tooth to cutting edge radius is larger than 0.61, the size effect could be ignored since the cutting mode is shearing.

Effect of Pre-Existing Micro-Defects on Cutting Force and Machined Surface Quality Involved in the Ball-End Milling Repairing of Flawed KDP Crystal Surfaces

When serving in extremely high-power laser conditions, KH2PO4 (KDP) surfaces are susceptible to incur laser damage points (also known as defects). Using micro-ball end milling cutters to repair and remove the pre-existing damage points on the flawed KDP crystal surface is the most effective method to control the growth of laser damage points on KDP crystal surfaces and prolong their service life. However, there are various forms of micro-defects (such as pits, scratches and brittle fractures) around the laser damage points on KDP crystal surfaces which possess remarkable effects on the micro-milling repair process and consequently deteriorate the repair quality. In this work, combined with nano-indentation experiments, elastic-plastic mechanics and fracture mechanics theory, a constitutive model considering the anisotropic property of KDP crystals and a three-dimensional (3D) finite element model (FEM) were established to simulate the cutting force and surface topography involved in the ball-end milling repairing of flawed KDP crystal surfaces. Besides, the micro-milling experiments were conducted to evaluate the change of cutting force and machined surface quality in the presence of micro-defects with various feed rates. The results show that micro-defects would induce the fluctuation of cutting force and a change of the undeformed cutting thickness (UCT) in the process of repairing the damage points on the crystal surface, which would lead to the brittle-ductile transition (BDT) and affect the machined surface quality. The machined surface quality was found to be deteriorated by the pre-existing micro-defects when the UCT was small (the UCT was less than 375 nm). On the contrary, brittle mode cutting in the local area can be transformed into ductile mode cutting, resulting in an improvement of repaired surface quality that is exhibited by the cutting force and microtopography. This work has great theoretical significance and engineering practical value for the promotion and application of micro-milling repairing technology in the practical manufacturing and operation of KDP optics applied to high-power laser systems.

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

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

The Optimization of Machining Parameters on Cutting Force during Orthogonal Cutting of Graphite/Polymer Composites

Graphite/polymer composites are brittle materials, and tool wear, which has a significant impact on the quality of the machined surface of the material, is very serious during the cutting process. In general, the magnitude of the cutting force directly affects the tool wear; the larger the cutting force, the more severe the tool wear, which in turn affects the machined surface quality of graphite/polymer composites. Therefore, in this study, the effects of machining parameters on cutting forces during orthogonal cutting of graphite/polymer composites were investigated using single-factor and multifactor experiments with cutting speed, cutting thickness, tool rake angle, and rounded edge radius as influencing factors, and the parameters were optimized. The obtained results showed that reducing the cutting thickness and increasing the tool rake angle would significantly reduce the cutting force. During the orthogonal cutting process, when the tool had a small edge radius, the cutting force along the cutting direction was significantly larger than the cutting force along the vertical direction, and as the rounded edge radius increased, the cutting force in the vertical direction exceeded the cutting force in the cutting direction. Finally, the significance of the effect of different machining parameters on the cutting forces was analyzed using analysis of variance (ANOVA). The obtained results showed that the cutting speed, cutting thickness, tool rake angle, and rounded edge radius were extremely significant for the cutting forces along the cutting direction as well as in the vertical direction during orthogonal cutting of graphite/polymer composites.

Effect of cold metal transfer mode on the microstructure and machinability of Ti–6Al–4V alloy fabricated by wire and arc additive manufacturing in ultra-precision machining

The effect of cold metal transfer (CMT) mode on ultra-precision machining (UPM) was investigated to explore phase transformation and potential improvements of surface integrity for a wire and arc additive manufactured (WAAMed)Ti–6Al–4V alloy part. Both CMT and CMT + Pulse (CMT + P) modes are taken into account, which is proved to be the stable methods to fabricate Ti–6Al–4V components. The calculation result shows that the heat input of the CMT + P mode is higher than that of the CMT mode. In the reheated conditions of the CMT + P mode, the α′ → α+β phase transformation occurs, which leads to a decrease in the number of acicular α′ martensite phases decreased and an increase in the size of the lamellar α+β phases. The UPM results showed that the cutting force increase with the cutting speed increases. In addition, the cutting force of the specimens fabricated in CMT mode is a little larger than that in CMT + P mode due to the higher strength and more α′ martensite phases. UPM can create a better surface finish with a reduction of surface roughness up to ∼90%. Moreover, the machine surface quality of the specimens in CMT mode is much better with the surface roughness of ∼0.1 μm at a low cutting speed.

Assessment of machined surface for SiCf/SiC ceramic matrix composite during ultrasonic vibration-assisted milling-grinding

As a new advanced material, silicon carbon fiber-reinforced silicon carbon ceramic matrix composite has attracted increasing attention and would have a promising application in prospect owing to its superior properties such as low density, high strength, and high-temperature resistance. However, there is still no such standard criterion for machined surface quality evaluation of fiber-reinforced ceramic matrix composites or fiber-reinforced composites. Thus, in this paper, investigations have been performed on ultrasonic vibration-assisted milling-grinding SiC fiber-reinforced SiC-based ceramic matrix composite. It aims to propose a primary criterion for the machined surface assessment of fiber-reinforced composites. This study considered and tested fiber-reinforced composites such as SiCf/SiC CMC, Cf/SiC CMC, and CFRP. Due to the large size of pits and voids in the machined surface, it was recommended to use non-contact measuring equipment for surface quality measurement. The 2D surface roughness value of every single machined surface differed much along different measuring directions and regions. Then, the 3D surface roughness was suggested for describing the machined surface quality as its data fluctuation was much more minor than the 2D roughness. The comparative analysis found that the measuring area size significantly influenced the 3D roughness result. This study used a measuring area of 13 × 13 mm2, 11 × 11 mm2, 7 × 7 mm2 for SiCf/SiC CMC, Cf/SiC CMC, and CFRP, respectively, taking into consideration the measuring time consumption and the reliability of measured data.

On modelling coolant penetration into the microchannels at the tool-workpiece interface

A network of microchannels is formed at the interface between the cutting tool and the workpiece during machining due to their rough surface structures. The penetration of coolant into these microchannels has a great effect on the machined surface quality by changing the frictional behaviour and heat transfer characteristics of the interfaces. Most of the present studies focus on the macroscopic coolant delivery process but the mechanism of the microscopic penetration phenomena in the channels at the interface remains unclear. Thus, this study proposes a multi-scale and multi-phase comprehensive theoretical model by combining the machining process, coolant flowing in microchannels and lubrication process, to analyse the coolant transportation in the microchannels and investigate its lubrication effects and the friction mechanisms at the interface between the tool and the workpiece. Then, an orthogonal turning testing of Inconel 718 is designed to verify the proposed model. The cooling and lubrication effect of coolant in the microchannels on the reduction of tool wear, contact length, cutting force and temperature has been demonstrated. And the improvement of chip fragment and microstructure has also been revealed.

Magneto-plasticity in micro-cutting of single-crystal copper

• Magneto-plasticity can be induced by a weak magnetic field in micro-cutting. • Cutting force, chip and machined surface are affected by magneto-plasticity. • Magneto-plasticity shows anisotropy with magnetic field direction. • Directional dislocation flow results in anisotropic magneto-plasticity. The plasticity of metals can be significantly affected by the application of a magnetic field, otherwise known as the magneto-plastic effect. This paper investigates the magneto-plastic effect in the micro-cutting of a non-magnetic ductile material, single-crystal copper, under a weak magnetic field and reports the influence of the phenomenon on the cutting forces and machined surface quality. A softening effect was observed from the large reduction in cutting forces from 3.2 N to 1.5 N under the magnetic field. As compared to the magnetic field intensity and polarity, the variation in magnetic field orientations with respect to the cutting direction exhibited a stronger influence on the cutting force, chip morphology, machined surface texture, subsurface microstructure, surface roughness, and machined surface microhardness of the copper sample. An analytical model was developed based on the geometry of the cutting chips to correlate the orientation-dependent influence of the magnetic field on the cutting forces. On the surface quality, excessive folds with four different types of morphology produced under magnetic-free cutting were suppressed after applying the magnetic field with the most significant improvement achieved with the 90° magnetic field direction. The magnetic-assisted changes in machined surface morphology also led to the reduction in machined surface roughness and microhardness. The optimistic micro-cutting outcomes in this work establish a greater understanding of the magneto-plastic effect and demonstrate the applicability of magneto-plasticity in ultraprecision manufacturing.

Mechatronic stiffness of cable-driven mechanisms: a study on production machine model

Position control of the mechanical structure with naturally limited stiffness is a common problem. Moreover, the system is usually exposed to random exciting by the external force effects and yet it is needed to hold the system in the desired position. Such an example in engineering practice can be the machine tool quill slim structure, which determines the machining accuracy and the machined surface quality. The limited structure stiffness can be overcome by suitable support structure solution. In principle, it is a matter of introducing the necessary force effect in the place where it is necessary to ensure the required position. A promising means how to apply control force to the flexible structure tip can be a thin cable structure with the force actuation and proper force control. The resulting system is characterized by increased stiffness achieved in a mechatronic manner. Therefore, the introduced concept is called mechatronic stiffness. The article describes selected mechanical arrangement of the mechatronic stiffness concept, its features, behaviour and control results. The proposed approach offers a solution for precise position control of the flexible structure. An experimental device was created in parallel with the simulation experiment and preliminary simulation results are obtained. The described concept is transferable to other flexible structures such as various manipulators.

Effects of minimum uncut chip thickness on tungsten nano-cutting mechanism

Tungsten is considered as the preferred material for plasma-oriented materials in fusion reactors, and its ultra-precision machining is a concern at present. However, the critical cutting depth for chip generation is an important parameter affecting the surface formation and removal behavior of materials in ultra-precision machining. Herein, the simulation analysis of nano-cutting tungsten using molecular dynamics (MD) was performed, and a new method to determine the minimum uncut chip thickness hmin by identifying the atomic trajectory and calculating the relative coordinate value was proposed. Then, the effects rarely reported of the chip formation on the removal mechanism of tungsten were studied. The research showed that when the cutting depth h was 0.253 times of the cutting edge radius r, the critical cutting depth without chip was reached, namely hmin. The increasing cutting temperature and stress led to the decrease of machined surface quality and the increase of subsurface damage with the rising in h/r. Meanwhile, the dislocation slip dominated by the direction of 1/2<111> was the main plastic deformation mechanism during cutting without chip, but the dislocation slip dominated by direction of <100> was main plastic deformation mechanism during processing with chip. The plastic deformation dominated by dislocation slip was also accompanied by the amorphization. The dislocation reaction from 1/2<111> dislocation to <100> dislocation occurred under both energy and geometry conditions. Moreover, the dislocations had more energy to expand and evolve as a result of thermal-activated effect induced by elevated cutting force and temperature, which increased the dislocation density, depth of subsurface damage layer, and the stress variation degree of the machined area with the enhancement of hmin during cutting. This research provides a reference method for determining the minimum uncut chip thickness during material removal and is of significance for mastering the microscopic removal mechanism of tungsten.

Linear Tool Path-Smoothing Method in High-Speed Machining Based on Error Feasible Area and Curvature Optimization

Linear tool path is widely used in high-speed NC machining. However, the geometrical discontinuity of the corner between the linear tool paths will lead to fluctuations in speed, acceleration and jerk, which can excite machinery vibration and reduce the machining efficiency and surface quality. To solve these problems, a novel corner smoothing method based on error feasible area and curvature optimization is proposed in this paper. Compared with most traditional corner smoothing methods using higher-order curves with all control points lying in the straight segment and inside of the tool path, the proposed method constructs B-spline transition curves with smaller curvatures to smooth the corners by reasonably distributing the curve control points inside and outside the straight line segment of the tool path (i.e., error feasible area). Furthermore, the corner transition curve is optimized by the minimum curve curvature extreme to improve the smoothness of the corner transition curve and reduce fluctuation in the kinematic profiles while respecting the G3 continuity (i.e., curvature-smooth), transition length limits and the uniqueness of curvature extremum. Finally, the simulation results show that the proposed method can reduce the curvature value and improve the smoothness of the curve and the minimum transitional velocity of the corner, which means that it can enhance machining efficiency and weaken machining vibration. The feasibility and effectiveness of the method are also verified.

Investigation of tool flank wear effect on system stability prediction in the milling of Ti-6AI-4 V thin-walled workpiece

Excessive tool wear can shorten tool life and cause low machining surface quality and efficiency in the milling of Ti-6AI-4 V thin-walled workpieces. The influence mechanism between the tool flank wear and stability predication is of significant for its further development in milling Ti-6Al-4 V. However, the relationship between tool flank wear and stability predication needs to be further investigated as the effect of time-varying tool flank wear is ignored in conventional methods. In this work, a system stability prediction model considering time-varying tool flank wear effect in milling of Ti-6AI-4 V thin-walled workpiece is proposed. The tool flank wear region is discretized into differential elements, and then, the friction effect and process damping effect caused by extruding function between tool and workpiece are analyzed, and a time-varying milling force model is established. In this process, the relationship of cutting tool flank wear band width VB and section radius difference NB is determined, and the indentation volume between tool and workpiece is iteratively calculated, which is used to investigate process damping. After, the time-varying milling force coefficients are derived considering different tool flank wear status. Then, in modal space, the evolutionary process of stability lobe diagrams considering tool flank wear effect is determined. Subsequently, to effectively predict system stability, tool flank wear curves and dynamic cutting force coefficients are calibrated by slot milling, and the average errors between cutting force prediction values considering tool flank wear effect and experimental values in feed direction and normal direction are 7.3% and 12.1%, respectively. Finally, a series of machining tests are conducted to verify the effectiveness of tool flank wear on the machining stability in the milling of Ti-6AI-4 V thin-walled workpieces to some extent, and the experimental results show that the system stability prediction accuracy of the proposed method is improved by 23.8% compared with that using conventional method.

Magnetic field effects on electrochemical dissolution behavior and surface quality of electrochemical machining of Ti-48Al-2Cr-2Nb alloy

Magnetic field effects on electrochemical dissolution behavior and surface quality of electrochemical machining of Ti-48Al-2Cr-2Nb alloy

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.