Uncut Chip Thickness Research Articles

Analysis and modeling of cutting force considering the tool runout effect in longitudinal-torsional ultrasonic vibration-assisted 5 axis ball end milling

The longitudinal-torsional ultrasonic vibration-assisted milling (LTUVAM) can improve the machining quality compared with conventional milling (CM), and the machining quality is closely related to cutting force which is affected by the tool runout effect. In this study, a mechanistic force model for longitudinal-torsional ultrasonic vibration-assisted 5 axis ball end milling considering the tool runout effect is proposed. First, the cutting edge motion model considering the effects of tool runout and ultrasonic vibration is established. Then, based on the analysis of the cutting edge motion relative to the workpiece, the uncut chip thickness (UCT) is computed by means of the geometrical calculation method, and the engaged cutting edge elements are determined according to the Z-map method, the ultrasonic milling force model is established after determining the cutting force coefficients and tool runout parameters through calibration tests. Moreover, the impact of different parameters on dynamic separating characteristics of LTUVAM is studied by the tool-workpiece contact rate. Finally, the verification tests are carried out, and the experimental results show the effectiveness of the ultrasonic milling force model.

Micro geometry analysis of WC-Co carbide blades modified with PVD coating subjected to blunting during milling in chipboard

ABSTRACT In this work, a number of geometrical parameters related to the microgeometry of the cutting edge, such as rounding radius r, maximum clearance wear VBmax and a number of others, were determined. Tools for processing wood materials made of WC-Co cemented carbide with a PVD coating based on Ti and Al. were blunted until the wear indicator VBmax reached the value of 0.2 mm. The cutting blades prepared in this way were tested using a contact profilometer. Based on the data obtained in this way, an analysis of the blade contour over a length of 10 mm was carried out in the AutoCad environment. This section included both the blunted part and the part not involved in the cutting process. Correlations obtained using a procedure written in Visual Basic for Applications using elements of statistical data analysis built into the Excel program, between the selected rounding asymmetry coefficients denoted with letter K and the blade wear indicators proposed in the tests were analyzed. Geometric indicators were also determined, taking into account the working conditions of the tool, affecting the process of its blunting, such as e.g. h (uncut chip thicknesses).

Selection of the numerical formulation for modeling the effect of tool cutting edge microgeometries in machining of Ti6Al4V titanium alloy

The Finite-element method (FEM) is often used to simulate the metal machining process. Currently, several formulations are used in metal cutting simulation, such as Lagrangian (LAG), Eulerian (EUL), Arbitrary Lagrangian–Eulerian (ALE), and Coupled Eulerian–Lagrangian (CEL). The selection of the numerical formulation that better reproduces the material separation in machining to form the chip is a critical issue. This is quite important when the ratio between the uncut chip thickness and the cutting edge microgeometry, often represented by a cutting edge radius, is very low. In this study, orthogonal cutting of Ti6Al4V titanium alloy using different tool edge microgeometries is investigated using LAG and CEL approaches. In the case of LAG approach, two types of cutting models were develop: one using a sacrificial layer (hereby called LAG-SL), and another without sacrificial layer (hereby called LAG-nSL). The cutting models have included a constitutive model (both plasticity and damage) considering the effects of strain, strain rate, temperature, and state of stress, which have proved to be accurate enough to represent the mechanical behavior of the Ti6Al4V alloy in machining. Comprehensive comparisons between CEL and LAG-based cutting models and experimental results are carried out in terms of chip morphology, forces, and residual stresses. When compared to the experimental results, LAG-nSL model gives the best predictions of the maximum chip compression ratio (CCRm) with an error of 9.5%, cutting force (12.8%) and thrust force (9.3%) than the other two models. In the case of the of residual stress profiles, LAG-SL offers good predictions of the maximum compressive residual stress by a preference ratio of 75%. Although CEL yields the worst predictions of chip morphology and forces, it is preferred from the perspective of the thickness of the layer affected by residual stress.

Nonlinearity and periodicity in machining carbon fiber reinforced polymer

In the machining of carbon fiber reinforced polymer (CFRP) materials, the relationship between the cutting forces and uncut chip thickness can be linear or nonlinear, depending on the fiber orientation ranges. In addition, the variation of cutting forces in two consecutive cuts is periodic at a certain fiber orientation of 120 deg and the uncut chip thicknesses of 30μm60μm. It is suggested that the material removal rate changes even for the same uncut chip thickness in consecutive cuts, such as milling and drilling. Through orthogonal experiments for unidirectional CFRP at various fiber orientations and uncut chip thicknesses, this study aims to analyze the observed nonlinearity and periodicity of cutting forces which occur at certain fiber orientation and uncut chip thickness. It is found that both the nonlinearity and periodicity are primarily dependent on the machined surface damage left by the previous cut. This damage state varies with the uncut chip thickness and is influenced by the interaction between the material deformations at the tool rake face and flank face. Thus, the reported findings in the chip formation and cutting force generation reported by this study can guide the tooling and process planning to enhance the machining performance for CFRP.

Modulated orthogonal cutting system realized by piezo stack actuation and linear guide coupling

Modulation-assisted machining (MAM) employs low-frequency feed-direction tool vibration to enhance conventionally continuous cutting processes such as turning, drilling, and boring operations. The development of a suitable tool vibration system is critical for the success of MAM. In this paper, a tool vibration system realized by using piezo stack actuation and linear guide coupling was designed and constructed. The performance of the system was evaluated by conducting orthogonal tube face turning tests on AISI 1045 steel using a range of cutting and modulation conditions. Cutting forces and tool vibration displacement were measured and analyzed. A mechanistic force model based on the variable uncut chip thickness was used to predict the variable primary cutting and feed forces in MAM which showed good agreement with the measured forces. Furthermore, It was found that the vibration amplitude with cutting was reduced when compared to the vibration amplitude without cutting. The reduction in vibration amplitude was predictable as it depends on the feed force and the stiffness of the system. The results indicate good control ability of the tool vibration system across a wide range of cutting and modulation conditions.

A simulation-based analysis on the cooling effect and the tool temperature distribution in modulated turning (MT)

Modulation assistance is widely used for breaking chips of highly ductile materials in turning. This process is also known as the “modulated turning” (MT) or the “modulation assisted machining (MAM)”. The process transforms continuous single point turning into an intermittent cutting process where the tool tip briefly disengages from the workpiece generating discrete short chips. Resembling the milling process mechanics, periodic tool disengagements in modulated turning introduces cyclic cool-down periods and thus it can potentially attenuate tool-tip temperatures. It has been hypothesized that this effect can greatly lower maximum and the average tool-tip temperatures improving wear resistance of the process. This paper presents a thermal model to predict the tool-tip temperature distribution in modulated turning. A generalized chip formation formulation is first developed to predict the uncut chip thickness variation, cutting forces and the tool engagement/disengagement phase durations. Thermomechanical behavior of tool-chip contact is then modeled to predict the rake face temperature distribution. Simulation studies are presented to analyze the temperature distribution and understand peak temperature (hot) spots on the rake face. It has shown that the phase angle parameter – the frequency ratio between workpiece rotation and tool modulation– of MT plays a key role in controlling the hot spots on the rake and can be used as a tool to influence the wear regime.

Constitutive modelling of Ti6Al4V alloy fabricated by laser powder bed fusion and its application to micro cutting simulation

Although there is a rising interest in applications of metal additive manufacturing (AM) for fabrication of highly complex metallic components, poor surface quality of AM parts is an inherent limitation of this technology. Therefore, post-process machining operations are frequently performed to meet the necessary requirements for the application. When compared to wrought Ti6Al4V, the distinct metallurgical and mechanical characteristics of Ti6Al4V fabricated by laser powder bed fusion (LPBF) can lead to very different mechanical behavior, thus different machinability. Therefore, to properly perform the numerical modelling of the cutting processes, it is important to accurately define material mechanical behavior of LPBF Ti6Al4V. This study presents the experimental determination of the coefficients of the Johnson-Cook (J-C) constitutive model and the numerical simulation of orthogonal micro cutting process of hot isostatic pressed (HIP) LPBF Ti6Al4V. Besides, the results were also compared to the wrought Ti6Al4V. For that purpose, quasi-static and dynamic behaviors of wrought and LPBF Ti6Al4V were investigated by quasi-static tensile tests at several temperatures (0.001 s−1 at 20, 400, 600 °C) and at high strain rates using the split Hopkinson pressure bar (SHPB) tests (∼1000-5000 s−1 at 20, 400 °C). Orthogonal micro cutting simulations were performed using experimentally obtained J-C model of wrought and LPBF Ti6Al4V. A series of orthogonal micro cutting tests at different cutting speeds (75, 100, 150 m/min) and uncut chip thickness values (2.5, 5, 7.5, 10 μm) were performed to compare the measured forces and chip compression ratio (CCR) with those obtained by simulation. Good predictions of main cutting force and maximum CCR were obtained for both wrought and LPBF Ti6Al4V under different cutting conditions. Several critical points to be considered for better prediction of thrust force and chip geometries in future studies were highlighted.

Determination of fracture toughness and yield strength of Inconel 718 by milling operation

This paper presents an analytical scheme to determine the fracture toughness and the yield strength of Inconel 718 by the widely used milling operation. It is noted that there exists material fracture at the tool tip and the removed layer of material undergoes plastic deformation along the primary shear plane. The fracture toughness is considered at the tool tip and the yield strength is applied on the primary shear plane. Subsequently, the modified Williams’ model and the modified Williams-minimization model are both proposed on the basis of the geometries and mechanics in the oblique cutting process. The modelling framework indicates that milling can be developed as an alternative method to determine the fracture toughness and the yield strength from the serrated chip thicknesses combined with the corresponding cutting forces. This is because the experimental method by milling is a convenient scheme as it only requires the cutting forces per unit width of cut with respect to a series of uncut chip thicknesses. In addition, the normal shear angle is predicted by applying the principle of energy minimization. Finally, milling experiments are conducted to validate the two proposed methods. Results show that the two analytical methods return similar values of fracture toughness and yield strength, respectively. Besides, it is noted that high values of yield strength are achieved based on Tresca criterion, which would indicate work hardening in the primary shear zone during the serrated chip formation. The proposed methods exhibit advantages of convenience and generality to derive the fracture toughness and the yield strength, since it is still a challenge for the traditional standard mechanical tests for Inconel 718 due to its toughness and ductility.

Numerical study of the minimum uncut chip thickness in micro-machining of Inconel 718 based on Johnson–Cook isothermal model

Numerical study of the minimum uncut chip thickness in micro-machining of Inconel 718 based on Johnson–Cook isothermal model

Experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites

A comprehensive experimental investigation on thermomechanical properties and micro-machinability of carbon nanofibre reinforced epoxy nanocomposites (EP/CNF) is presented in this study. The machinability indicators including cutting force and surface roughness have been investigated. Tensile properties, morphology of tensile fracture surfaces, glass transition temperature, machined chip morphology, and machined surface morphology were also characterised. To investigate the effect of both workpiece material properties and operating conditions on the machinability of EP/CNF, three controlled quantitative factors were selected at different levels, namely CNF loading, cutting speed and feed per tooth (FPT). Micromilling experiments were performed on an ultra-precision desktop micro-machine tool using titanium‑carbon-nitride (TiCN) coated micro-end mills. Among all compositions with CNF concentration ranging from 0.3 to 1 wt%, EP/1 wt% CNF exhibited the best machinability among other nanocomposites with its lowest cutting force of approximately 0.5 N and surface roughness of 0.18 μm. Size effect appeared at FPT below minimum uncut chip thickness (MUCT) indicated by the strong deterioration of surface quality owing to the dominant ploughing effect.

Simulation of Work Hardening in Machining Inconel 718 with Multiscale Grain Size.

Machining nickel-based alloys always exhibits significant work-hardening behavior, which may help to improve the part quality by building a hardened layer on the surface, while also causing severe tool wear during machining. Hence, modeling the work-hardening phenomenon plays a critical role in the evaluation of tool wear and part quality. This paper aims to propose a numerical model to estimate the work-hardening layer for a deeper understanding of this behavior, employing both recrystallization-based and dislocation-based models to cover workpieces with multiscale grain sizes. Different user routines are implemented in the finite element method to simulate the increase in hardness in the deformed area due to recrystallization or changes in the dislocation density. The validation of the proposed model is performed with both literature and experimental data for Inconel 718 with small or large grain sizes. It is found that the recrystallization-based model is more suitable for predicting the work-hardening behavior of small-grain-size Inconel 718 and the dislocation-based model is better for that of large-grain-size Inconel 718. Further, as an important type of cutting tool in machining Inconel 718, the chamfered tools with different edge geometries are employed in the simulations of machining-induced work hardening. The results illustrate that the uncut chip thickness and chamfer angle have a significant influence on the work-hardening behavior.

Analysis of burr formation in finish machining of nickel-based superalloy with worn tools using micro-scale in-situ techniques

The formation of burrs is among the most significant factors affecting quality and productivity in machining. Burrs are a negative byproduct of machining processes that are difficult to avoid because of a limited understanding of the complex burr formation mechanisms in relation to cutting conditions, including both process parameters and tool condition. Thus, the objective of this work was to characterize burr formation under finish machining conditions via a high-speed, high-resolution in-situ experimental method. Various parameters pertaining to burr geometry such as height, thickness, and initial negative shear angle were measured both during and after cutting. Results showed that varying the conditions of uncut chip thickness, tool-wear, and cutting speed all have a significant effect on burr formation, although certain burr metrics were found to be insensitive with respect to different process conditions because the difference was statistically insignificant. This study provides new insights into the relationships between the workpiece material's microstructure, machining parameters, and tool condition on both crack formation and propagation/plasticity during burr formation. Using digital image correlation (DIC) and a physics-based process model not previously utilized for burr formation analysis, the displacement and corresponding flow stress were calculated at the exit burr root location. This novel semi-analytical approach revealed that the normalized stress at the exit burr root was approximately equal to the flow stress for a variety of different conditions, indicating the potential for model-based prediction of burr formation mechanics. Finally, this study investigates factors that influence fracture evolution during exit burr formation. It was found that negative exit burrs are a direct result of high strain rate and high uncut chip thickness, which was expected, but also a microstructural size effect and a tool-wear effect, neither of which have been previously reported. By harnessing ultra-high-speed imaging and advanced optical microscopy techniques, this manuscript deals with the fundamentals of burr formation, including new insights into material response at the grain-scale to the loads imposed with both sharp and worn tools.

The curved uncut chip thickness model: A general geometric model for mechanistic cutting force predictions

The curved uncut chip thickness model is introduced to predict the cutting forces for general uncut chip geometries using the mechanistic approach. Classical geometric models assume that the cutting force is distributed along straight elementary sections of the uncut chip area, which has limited physical validity, but makes mathematical treatments easier for simple cases. The new model assumes that the flow of the material on the contact area of the tool is given by a continuous vector field, according to which the curved uncut chip thickness is measured. The cutting force is distributed along these paths, which leads to a mathematically unique and consistent solution for regular and complex cutting edge geometries. These curved paths can be generated by basic mechanical models, which mimic the more realistic motion of the chip segments along the rake face, without the need of explicit time-consuming cutting simulations. The presented computational procedure generalizes cutting force prediction based on geometric parameters, orthogonal cutting data and the orthogonal to oblique transformations only. The effectiveness of the model for various cutting edge geometries (e.g., thread turning inserts) under extreme cutting conditions is presented in case studies, laboratory and industrial experiments.

Episodes of single-crystal material removal mode and machinability in the micro-cutting process of superalloy Inconel-718

The development tendency of miniaturization gives birth to the demand for Inconel-718 miniature parts, and micro-cutting is a promising technique to produce crystalline material miniature parts. Unfortunately, due to the non-negligible edge radius and property anisotropy, the material deformation behaviour and removal mode vary with cutting conditions, resulting in volatile machinability and inferior quality, which hinders the widespread application of Inconel-718 miniature parts. Therefore, it is urgent to clarify the evolution of material removal mode and machinability of Inconel-718 micro-cutting. Existing researches fail to consider the property anisotropy, strain-rate sensitivity and strain-rate inhomogeneity. This paper establishes strain rate-considered single-crystal micro-cutting models and performs simulations under varying cutting conditions, aiming to depict the episodes of material removal mode and machinability of Inconel-718 micro-cutting. Results show that, uncut chip thickness and cutter rake angle influence the material removal mode, and in turn influence machinability. For Inconel-718 single-crystal with [0° 0° 0°] orientation, the transition from the scratching mode to the ploughing mode is in the uncut chip thickness range of 0.2Re∼0.4Re, and the transition from the ploughing mode to the cutting mode is in the uncut chip thickness range of 0.6Re∼0.8Re or the rake angle range of −30°∼-15°. In comparison, cutting speed and clearance angle have little effect on material removal mode. Cutting speed influences the strain rate of the cutting-affected zone, and in turn influences machinability. The maximum strain rate of the cutting-affected zone is respectively around 0.45 s−1 and 10,960 s−1 under the cutting speeds of 2Re/s and 20000Re/s.

Investigation on the machinability of copper-coated monocrystalline silicon by applying elliptical vibration diamond cutting

Although monocrystalline silicon exhibits promising microstructural properties for enhancing the performance of optical systems, precise and efficient silicon fabrication is still challenging due to its brittleness. In the present work, we demonstrate the ductile machinability of copper-coated silicon by applying elliptical vibration diamond cutting. Firstly, the influence of the copper coating on the ductile machinability of silicon was experimentally investigated by applying ordinary diamond cutting, showing that the critical depth of cut (DOC) for the ductile-to-brittle transition increased from 118 to 418 nm due to the advantages of the copper coating. This provided hydrostatic pressure in the microcutting regime and the compressive force on the subsurface defects to prevent crack propagation. Subsequently, grooving tests on coated and uncoated silicon were performed by applying elliptical vibration cutting (EVC). Due to the small instantaneous uncut chip thickness in each vibration cycle, the critical DOC by EVC for uncoated silicon increased to 459 nm. Applying the favorable copper coating process, the critical DOC by EVC for coated silicon increased to about 988 nm. The ductile machinability of silicon was efficiently improved by combining EVC and the coating process. Furthermore, the influence of vibration amplitude and cutting speed on the ductile machinability of silicon was explored in detail, leading to the optimized machining conditions for the ductile machining of silicon. Finally, based on these fundamental results, ultra-precision microlenses on silicon were efficiently fabricated, which is appropriate for further industrial applications.

Investigation of the cutting-edge radius size effect on dynamic forces in micro end milling of brass260

Rapid increases in the number of industries that need a sub-micron surface finish on 3D micro/meso technological innovation parts are being seen. Due to the high rotating speeds of activities, chatter behaviour also occurs during micro-cutting. Changes in bearing stiffness, gyroscopic effects, and misalignment are only some of the factors that need to be considered while studying the spindle's dynamics. By regulating the movement of the cutting edge, the cutting action may be made more consistent. The structure of the tools also affects the machine's dynamics. Since the modes of different machine-tool components interact with one another, the dynamics of the tool tip are affected. In this research, we consider a model of a micro spindle system to enhance machining accuracy. Timoshenko beam theory is used to account for shear deformation and rotational inertia effects due to the short and thick beam-type designs of each component of the micro tool. A detailed dynamical model of the moving micro end mill is created using an extended form of Hamilton's Principle. A two-degree-of-freedom model of the micro-milling process considers the modal dynamic properties of the tool-holder-spindle assembly and the micro-milling cutting forces. A three-dimensional finite element model of the system is built, and its dynamic response is gathered, to validate the full order finite element approach. Cutting force coefficients are then modelled as a function of instantaneous uncut chip thickness, which is principally affected by the tool edge radius and feed per tooth at different radial immersion depths.

Prediction of cutting forces in flexible micro milling processes by considering the change of instantaneous cutting direction

The characteristic of low stiffness of the micro mills usually leads to instantaneously changeable tool deflections, and as a result, actual in-process cutting parameters such as the instantaneous cutting directions and trajectories of the cutting edges will deviate from their designed values. Existing works seldom studied the influence of this fact on the cutting forces. This article presents a theoretical method to predict the micro milling forces by introducing the effects of the tool’s deflections on the process. Analytical formulae for determining the actual in-process cutting edge position and instantaneous cutting direction are originally derived based on the tool deflections, which are calculated by using the cantilever beam theory. At the same time, other actual in-process parameters such as rake angle, shearing angle and uncut chip thickness are subsequently formulated to detect their influences on the cutting. Based on the obtained actual in-process parameters and tool deflections, the shearing and ploughing forces, which constitute the total micro milling forces, are then formulated separately. An iterative algorithm, which integrates the above derivations with the material piling up effect, is developed to capture the coupling effect between the cutting forces and tool deflections during the micro milling process. A series of experimental tests are conducted, and the results prove that the accuracies of cutting forces and the cutting edge’s moving trajectories predicted by the proposed method are better than those obtained from the classic models without including the changes of cutting directions caused by the tool deflections.

Modeling of laser-assisted micro-milling

The difficulties in the mechanical micro-machining process regarding the tool deflection, size effect and tool wear are much more pronounced than those in mechanical macro-machining. However, the role of mechanical micro-machining cannot be overestimated as an alternative against the non-traditional machining methods in manufacturing complex micro-parts with high dimensional and geometrical accuracies. Laser-Assisted Machining (LAM) can be used as a promising method by reducing the cutting forces and providing the possibility of increasing the Material Removal Rate (MRR) to enhance the productivity of mechanical micro-machining of difficult-to-cut material. Unlike common LAM, the parts are structured before machining using the ultra-short pulsed laser, followed by removing some parts of the material and consequently changing uncut chip thicknesses at the machining process. A model has been developed to understand how the structuring of the parts before micro-milling affects the uncut chip thicknesses at the micro-milling process. The model includes a variety of input parameters in terms of structure, tool, and process parameters to investigate this novel LAM comprehensively. Based on model results, the uncut chip thicknesses are reduced through the structuring of the part. The structure density was found as the most crucial parameter in uncut chip thickness reduction. The influence of structure density on cutting force reduction was also studied in the experiment. Workpieces made of titanium alloy (Ti6Al4V) were structured using a pico-second laser at different structure densities. The structured parts were machined at two feeds of 10 and 50 µm/tooth with a 35 m/min constant cutting speed. Structure density's role in reducing cutting force was also considerable. In addition, the contribution of different factors affecting cutting force reduction in this novel LAM process is discussed.

Study on ploughing phenomena in tool flank face – workpiece interface including tool wear effect during ball-end milling

Ploughing phenomena occurring during precise machining processes affect the formation of surface finish and progress of tool wear. Therefore, this study presents an evaluation of ploughing phenomenon by studying the ploughing forces in tool flank face – workpiece interface during precise ball-end milling of AISI L6 alloy steel. Developed original model of ploughing forces involved the effect of minimum uncut chip thickness hmin and ploughing volume during ball-end milling. Estimated ploughing forces are sensitive to variations of surface inclinations and progressing tool wear. Moreover, intense growth of ploughing force during milling with worn tool is affected by irregular and non-circular profile of worn cutting edge below the stagnant point, and by the appearance of attrition and micro-grooves on tool flank face.

Influence of Size Effect in Milling of a Single-Crystal Nickel-Based Superalloy

This paper details an experimental investigation on the influence of the size effect when slot-milling a CMSX-4 single-crystal nickel-based superalloy using 1 mm- and 4 mm-diameter TiAlN-coated tungsten carbide (WC) end-mills. With all tools having similar cutting-edge radii (re) of ~6 µm, the feed rate was varied between 25-250 mm/min while the cutting speed and axial depth of cut were kept constant at 126 m/min and 100 µm, respectively. Tests involving the Ø 4 mm end-mills exhibited a considerable elevation in specific cutting forces exceeding 500 GPa, as well as irregular chip morphology and a significant increase in burr size, when operating at the lowest feed rate of 25 mm/min. Correspondingly for the Ø 1 mm micro-end-mills, high levels of specific cutting forces up to ~1000 GPa together with severe material ploughing and grooving at the base of the machined slots were observed. This suggests the prevalence of the size effect in the chip formation mechanism as feed per tooth/uncut chip thickness decreases. The minimum uncut chip thickness (hmin) when micromilling was subsequently estimated to be less than 0.10 re, while this increased to between 0.10-0.42 re when machining with the larger Ø 4 mm tools.